Recent progress in understanding the persistent 
luminescence in SrAl2O4:Eu,Dy 
 
Virginija Vitola , Donats Millers, Ivita Bite , Krisjanis Smits  and Agnese Spustaka 
 
Institute of Solid State Physics, University of Latvia, Riga, Latvia  
 
ABSTRACT 
Ever since the discovery of SrAl2O4:Eu,Dy persistent afterglow material, that can intensively glow 
up to 20 h, the mechanism of long-lasting luminescence has been a popular area of research. The 
research is focused on discovering the mechanism of persistent luminescence in order to prolong 
the duration and intensity of afterglow in a controlled way. Although most researchers  
agree on the general things, there are still many unclarities and ambiguities to discuss 
upon. This review paper briefly sketches in the highlights of past research on the 
luminescence mechanism in SrAl2O4:Eu,Dy, mainly focusing on the research conducted 
in the past decade dedicated to clearing these ambiguities. This paper provides an 
overview of the latest persistent luminescence mechanisms offered by researchers. 
 
 
 
 
 
 
KEYWORDS  
Persistent luminescence; 
long afterglow; 
luminescence mechanism; 
luminescence mechanism; 
strontium aluminate 
 
 
 
Introduction 
 
Long-lasting luminescence (known also as persistent 
luminescence) has many well-known applications – 
for example decorations, toys and emergency signs for 
cases of power shortage during an emergency, many of 
them are based on Eu doped strontium alumi-nates. 
Recently multiple unexplored fields of applica-tion 
have appeared, therefore intensifying the search for 
maximally efficient and long-lasting luminescent 
materials. Road marks with luminescent paint are 
being developed, as well as plasma electrolytic oxi-
dation coatings for protection and decoration [1]. A 
very intriguing application in persistent luminophores 
is the in-vivo medical imaging using luminescent 
nanoparticles [2–4].  
The most commonly commercially used material for 
persistent luminescence applications is SrAl2O4:Eu,Dy. 
This material, if compared to others, is superior with its 
luminescence lifetime that greatly exceeds that of any 
other material known to date (over 20 h). The material is 
chemically stable, exhibits excellent photoresistance and 
a very bright afterglow [3]. Therefore, it is also widely 
studied. The knowledge of the processes contin-uing in 
the material is essential to improve the optical properties 
of the long-lasting aluminate luminophores for different 
applications. However, details of these pro-cesses are 
not clear, especially the nature and role of defects 
involved. Researchers are interested in study of defects 
that are involved in the luminescence process and the 
possibilities to control them in the process of synthesis 
in order to obtain the longest afterglow and  
 
 
 
good thermal stability The scientific literature about 
the luminescence processes continuing in the material 
is not consentaneous – different models exist that 
could be responsible for the long afterglow and the 
defects that are involved are not clear. This article 
highlights the latest discussions about the persistent 
luminescence mechanism in strontium aluminates. 
 
SrAl2O4 as a host material 
 
The long-lasting luminescence was an incompletely 
studied subject until the 1990s, although there were sev-
eral luminescent materials known and used for multiple 
applications, such as glow in the dark watch dials and 
toys. For commercial applications mostly zinc sulphide 
with copper doping was used and, to achieve longer 
luminescence lifetimes and higher afterglow intensi-ties, 
they were sometimes co-doped with radioactive 
elements [5]. The era of intense long-lasting lumines-
cent material research began with the discovery of 
SrAl2O4 doped with Eu. That was a great discovery, as 
the afterglow time and intensity exceeded the known 
compounds greatly. Not long after this discovery an 
improvement was made – by SrAl2O4:Eu co-doping 
with dysprosium, the already intensive and lengthy 
afterglow was enhanced even more and now could reach 
more than 10 h [6]. With this, the study of Eu activated 
alkaline earth materials and other compounds began – 
adding different aluminates, silicates and phos-phates to 
the list of persistent phosphors [7–11]. They all share the 
same blue–green emission colour, as they 
 
 
 all contain Eu
2+
 as the luminescence centre. Other 
emission colours are much less frequent, although there 
are a few materials that are non-Eu doped, but the after-
glow time is significantly smaller [12,13]. The overall 
conclusion is that the number of known efficient com-
pounds that exhibit a long-lasting afterglow is still quite 
low and SrAl2O4:Eu,Dy is still one of the best persistent 
luminescent materials that is available, judging by the 
afterglow duration and intensity.  
Several strontium aluminate compounds is known. 
The SrAl2O4 is a stable compound in SrO-Al2O3 sys-
tem. It has a stable monoclinic phase at room tem-
perature, that transforms into hexagonal when heating 
at temperatures above 650°C and back to monoclinic 
at the same temperature during cooling [14]. SrAl2O4 
has a tridymite structure constructed by corner-sharing 
AlO4 tetrahedra that are tilted with respect to each 
other [15]. The occupation of Al
3+
 ions in the oxy-
gen tetrahedra of the compound leaves a charge defi-
ciency that is then compensated by Sr
2+
 ion incorpo-
ration in the channels created within ‘rings’ of corner 
shared AlO4 tetrahedra, therefore it is called the 
stuffed tridymite structure. There are two possible 
positions for Sr cations in this matrix. The structure of 
the low-temperature phase has a three-dimensional 
network of corner-sharing AlO4 tetrahedra, which has 
channels in the a- and c-directions where the Sr
2+
 
ions are located [16] (Figure 1).  
The two crystallographically different sites for Sr
2+
 
have different coordination numbers (i.e. 6 + 1), sim-ilar 
average Sr-O distances (i.e. 2.695 and 2.667 Å), but 
slightly different in individual Sr-O distances [18]. The 
two environments differ by a slight distortion of their 
‘square planes’ [19]. When doping with Eu ions, it 
incorporates in the position of Sr ion [18,19].  
The Sr
2+
 and Eu
2+
 ions are very similar in their 
ionic size [20] (1.21 and 1.20 Å, respectively) and the 
presence of Sr(I) and Sr(II) sites (6 and 7 coordinated) 
results in different symmetry and orientation around 
incorporated in these sites Eu ion leading to 
deviations in the luminescent properties for both sites. 
It is also important to note the dopant incorporation in 
the matrix, as lattice disorders and substitutional ions 
introduce localised states in the host matrix band gap by 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. The crystal structure of SrAl2O4, drawn with 
VESTA [17]. 
 
 
interrupting the long-range symmetry of the material. 
The examples of lattice imperfections in SrAl2O4 are: 
 
• Sr vacancy (VSr) – a point defect with 2− charge in 
respect to the lattice, as Sr is usually in 2+ form – 
thus its vacancy can be a hole trapping centre itself or 
attract some other defect for charge compensation  
• Oxygen vacancy (VO) – a point defect with 2+ 
charge in respect to the lattice, as O is in 2− form 
thus its vacancy can be an electron trapping cen-tre 
itself or attract some other defect for charge 
compensation 
 
There are other possibilities, like interstitial or substi-
tutional ions, that can either be trapping centres them-
selves or attract other defects for charge compensation. 
For example – if a Dy
3+
 would replace a Sr
2+
 ion, there 
would be 1+ charge incompatibility. This would either 
act as an electron trap or these defects could cooperate  
– 2 Dy
3+
 ions could need 3 sites of Sr
2+
 ions. The 
pos-sibility to trap an electron would also be 
dependent on the reduction and oxidation potential of 
the ion, as well as on the trapped electron energy level 
position in the host. 
We have to note that the method of synthesis, con-
centration of dopants and the reductive atmosphere 
are very important factors in determining the creation 
of defects. Reduction of Eu
2+
 leads to the creation of 
oxygen vacancies and possibly some lattice 
distortions, excess Dy could lead to byproduct 
DyAlO3 creation [21], the heating rate, boric acid 
content as well as other synthesis parameters can lead 
to a different amount of aggregation [22,23].  
The dopant incorporation (Eu and Dy ions or dif-
ferent) in different sites of the SrAl2O4 host is usu-
ally determined by the ionic radii of the dopants and 
the host lattice components. From the literature it is 
known that the radii of the SrAl2O4 components and 
the dopants are as follows – see Table 1.  
The information given in Table 1 gives ground to 
state that Eu
2+
, Eu
3+
 and Dy
3+
 are very likely to be 
found in Sr
2+
 positions. This is also confirmed by 
EPR measurements in [19]. If we were to consider the 
two possible sites of Sr
2+
, we could estimate that they 
are quite similar crystallographically [24] – therefore 
the dopant ions could be expected to occupy the sites 
more or less equally. EPR measurements confirm this 
state-ment in [19].  
Also, one could note that Sr
2+
 and Eu
2+
 ions have 
really very close ionic radii. This can be used to explain 
the reason that Eu
3+
 ions incorporated in the Sr
2+ 
Table 1. ionic radii of the SrAl2O4 and the dopants.  
Ion Ionic radius, A
˙ 
Ion Ionic radius, A
˙ 
Eu
2+ 
1.2 Sr2+ 1.18 
Eu
3+ 0.95 Al3+ 0.53 
Dy
3+ 
0.91 O2− 1.4 
  
sites of SrAl2O4 can be reduced to Eu
2+
 without any 
problems [19].  
Clabau in an article [19] reports that the formation of 
by products Sr3Al2O6 during the synthesis of SrAl2O4 
as well as the thermoluminescence measurements indi-
cate that SrAl2O4 tends to have strontium vacancies and 
oxygen vacancies due to the requirement of charge neu-
trality. Aluminium vacancies might exist as well, but 
would be energetically unfavourable to form because the 
Al–O bonds are short and strong. Owing to the 
requirement of charge neutrality and the absence of 
oxygen in the synthesis atmosphere, the co-doping with 
Dy
3+
 should enhance the cation deficiency [19].  
The generally accepted opinion is that Eu
2+
 and 
Dy
3+
 ions substitute for Sr
2+
 in both sites. The addi-
tional Sr vacancies are introduced for excess charge 
compensation of incorporated trivalent ions. The 
unan-swered question is – what happens when Eu
3+
 
is reduced? We know for sure, that before reduction 
Eu incorporates in the matrix in the trivalent form and 
the reduction leads to the formation of coexisting 
Eu
2+
 and Eu
3+
 species as well as some additional 
defects that are involved in the afterglow mechanism 
[25], but this matter still needs thorough research. 
 
Role of lanthanides in long-
lasting luminescence materials 
 
Lanthanide ions, either in a divalent, trivalent, or, more 
rarely, tetravalent state, take a very important place 
among luminescence centres in crystalline phosphors – 
both long lasting and also fast fading luminescence have 
wide applications, for example, the Nd
3+
 narrow line of 
4f
2
5d – 4f
3
 emission in laser crystals [26], 4f
7
5d
1
-4f
8
 
line transition of Tb
3+
 is used as one of the components 
for tube lighting [27], etc. Lanthanides are commonly 
referred to as rare earth activators, although several of 
them are not very rare. There are two types of lan-thanide 
luminescent transitions that might be of inter-est. 
Transitions between 4f
n
 levels are quite invariant in 
different compounds, and due to this invariance usually 
are observed as quite sharp and line-like absorption as 
well as emission. Transitions between 4f
n−1
5d and 4f
n
 
configurations, however, do depend on the host matrix 
quite strongly, thus shifting the maximum of emission 
quite noticeably and also broadening the absorption and 
emission spectra in different matrices [28]. The 
interesting properties of lanthanide ions arise from the 
fact that they possess a partially filled 4f shell that is 
shielded from the impact of crystal f ield of the host by 
outer shell electrons.  
For real-life applications the colour of emission as 
well as the quantum efficiency and the thermal stabil-
ity of the process play a crucial role. All these factors 
are closely related to the relative and absolute location 
of the lanthanide energy levels. For example, the posi-
tion of the host-sensitive lowest 5d state relative to the 
 
 
host-invariant 4f states is important for the lumines-
cence colour of the obtained strontium aluminate phos-
phor [18]. The absolute position of the 4f and 5d states 
relative to valence band and conduction band states also 
affects luminescence quenching and charge-trapping 
phenomena [29]. For example, if we talk about the 
possibility of trapped charge to be thermally released 
from a trapping centre, the energetical position of the 
trapping centre plays a crucial role. The approach for 
estimation of rare-earth ion levels positions in the host 
band scheme is presented in a number of articles by P. 
Dorenbos [29–31]. The main idea of this approach is 
that, if the positions of ground state and excited state 
levels are known for one rare-earth ion, then the posi-
tions of these levels can be estimated in the same host 
for any other rare-earth ion.  
As for SrAl2O4:Eu,Dy, we are looking at the case of 
Eu
2+
, that acts as a luminescence centre and its lumi-
nescent 4f
6
5d
1
 → 4f
7
 transition [31,32]. All the lumi-
nescent materials from aluminate and silicate groups 
doped with Eu show a similar emission spectrum – a 
broad band with the maximum within blue–green region 
[33–39]. The energetical displacement of an electron 
filled 5d level of Eu depends strongly on the 
configuration of its surroundings. The 5d – 4f transi-tions 
of rare-earth ions depends greatly on the sym-metry, 
coordination, covalence, bond length, site size and 
crystal field strength in which they reside, making it 
possible to tune the excitation and emission wave-length 
over a wide range by varying the composition and local 
crystal structure [39]. It is quite safe to say that exactly 
Eu
2+
 is playing the role of luminescence centre and the 
f-d transition is being shifted in dif-ferent crystal fields 
while still maintaining its shape, as it can be observed in 
Figure 2. The reasoning for this assertion is that this 
luminescence disappears in samples without Eu doping, 
the normalised lumines-cence bands have almost 
identical shapes – only the maximum position shifts due 
to differences in crys-tal field in different materials [39–
41]. For example, Chernov et al. [42] reports, that the 
different crystalline  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2. Eu
2+
 emission in different compounds. 
 structure of the hosts, orthorhombic for the case of 
Sr4Al14O25 and monoclinic for SrAl2O4, gives rise to 
different crystallographic strontium sites for SrAl2O4 
and Sr4Al14O25 responsible for the Eu
2+
 4f
6
5d
1
 → 4f
7
 
emission and for the observed emission shifting to the 
shorter wavelengths. Zhang et al. [43] attribute this 
change in crystal structure and resultant shifting of 
emission maximum to the increase in the Al/Sr ratio. 
The size of the obtained particles can also contribute to 
the local crystal field of the luminescence centre, thus 
slightly shifting the maxima [44–46].  
The co-activation of the trivalent rare earth does not 
change the position of the luminescence emission band 
nor the shape of it – for SrAl2O4:Eu
2+
,RE
3+
 it only 
contributes to the afterglow time and intensity, mak-ing 
the duration of afterglow much longer and much more 
intensive. The afterglow is present also without the 
Dy
3+
 co-doping [47]. Other rare earths have been used 
as co-activators as well, for example, Nd
3+
 [42] and 
Tm
3+
 [48]. This is the reason we can be sure to say that 
the luminescence centre in these compounds is Eu
2+
, but 
Dy
3+
 somehow contributes to the trapping centres of the 
material. In the work of Rezende et al. [25] it is 
proposed, that the co-dopant ion can also influence the 
[Eu
2+
]/[Eu
3+
] ratio obtained after the reduction process 
therefore stating that the co-dopants play an important 
role in the stabilisation of either of the Eu valences in the 
samples.  
The photoluminescence that is characteristic to the 
trivalent rare earth co-activators in most cases is not 
observed, leading to a conclusion that direct excita-
tion of trivalent rare earth is not involved, and nor is 
the energy transfer from Eu
2+
 to the trivalent rare 
earth. However, there are authors who claim to have 
observed photoluminescence of Dy
3+
 [49], however 
some authors claim to observe Dy
3+
 lines only in 
radi-oluminescence spectrum [50,51]. This indicates 
that maybe traces of Dy luminescence are present also 
in the SrAl2O4:Eu
2+
,Dy
3+
 material, just hidden 
under the bright emission of Eu
2+
 and could be 
observed at different dopant concentrations. All these 
assertions do not lead to a clear answer about the 
Dy
3+
 role in the material. 
 
 
Persistent luminescence models 
 
Historical view 
 
During these more than 20 years since the discovery of 
SrAl2O4:Eu many vast studies have been conducted to 
enhance the properties of long afterglow phosphors – 
prolong their persistent luminescence and improve the 
intensity. Despite this, there is still a lack of full under-
standing of the mechanism of this phenomenon. This is 
the reason why the development of new materials and 
the enhancement of the properties of the known 
compounds are often based on trial and error methods. 
 
 
The main models for long afterglow phosphors, that 
have been discussed until 2010 and their evolution is 
very briefly described in subchapters below, because, 
although it has been discussed also in previous research 
and review articles [18,52–54], the understanding of 
different possible mechanisms and the proof for or 
against them is crucial for the research conducted in 
recent past as well as the reader. 
 
Matsuzawa model 
 
The search for the mechanism began with the Mat-
suzawa (the original patent of persistent luminescence in 
SrAl2O4:Eu,Dy) model [55]. In this publication, the 
long-lasting luminescence of SrAl2O4:Eu,Dy with a 
model where Eu
+
 ions are created during excitation is 
explained. The hole from Eu
2+
 is transferred to valence 
band (in other words, an electron from valence band is 
lifted to Eu
2+
) and Eu
+
 is created. Holes are then 
localised on Dy, that acts as a trapping centre, recharg-
ing it to Dy
4+
, and afterglow is determined by the 
thermal release of holes from the trapping centres and 
the following migration and recombination (Figure 3). 
The essence of the mechanism lays in the presumption 
that Eu
2+
 excitation is followed by hole delocalisation 
from it and thus Eu
+
 is formed. The hole is trapped on 
Dy
3+
, recharging it to Dy
4+
. At a temperature that is 
high enough to release the trapped charges, the hole is 
released, migrates back to Eu
+
 and creates excited Eu
2+
. 
The electron then returns to the ground state by the 
release of a photon. The scheme of mechanism proposed 
by Matsuzawa is in Figure 3. 
 
Aitasalo models 
 
It was clear that Matsuzawa model needed improve-
ments due to some physical inaccuracies, for example, 
the creation of Eu
+
, that is energetically unfavourable. 
Aitasalo team consecutively proposed several con-
sequent models for the long afterglow, by adding 
improvements to his previous works.  
In this mechanism, it was assumed that under 
excitation the holes in valence band are created and 
the electron is localised at a trap level located within 
the band gap. The holes created are considered to be 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3. The Matsuzawa model for the long-lasting 
lumines-cence involving hole as free charge carrier. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 4. The Aitasalo model for the long-lasting 
luminescence involving indirect Eu
2+
 excitation. 
 
the only free charge carriers, however the hole trapping 
at alkaline vacancies was assumed (Figure 4). There-fore, 
the proposed mechanism excluded the formation of Eu
+
 
and propose the possible thermally stimu-lated migration 
of electrons between somewhat dif-ferent traps until the 
electron and hole are localised at oxygen and alkaline 
vacancy complex. The elec-tron–hole recombination 
within this complex is fol-lowed by energy transfer 
process, that creates an excited Eu
2+
 ion. The energy 
transfer and oxygen and alkaline earth metal ion vacancy 
complexes play a crucial role in this mechanism. Also, 
the luminescence centre needs to be in close proximity to 
the vacancy complex for an efficient energy transfer 
process. However, it is not clear what the defects could 
be, for electron traps hav-ing the energy levels below to 
that of oxygen vacancy. This mechanism is good in the 
way that it does not limit the role of co-dopants to be 
charge-trapping centres. Aitasalo proposed that trivalent 
rare earth co-doping could possibly be responsible for a 
larger number of lattice defects – due to the charge 
compensation.  
Later on Aitasalo proposed corrections on his previ-
ous work, presenting an improved model [56]. As the 
previous models for mechanism of long-lasting lumi-
nescence [56–58] received quite a lot of criticism on the 
physical accuracy, additional experiments were con-
ducted to clarify the actual role of the co-dopants – this 
led to a conclusion, that Dy does not act as the trapping 
centre and the process could be more com-plex. A 
similar model was proposed by Dorenbos [59] involving 
some more details. The proposed processes are as 
follows – electron is thermally promoted from Eu
2+
 
excited state to the conduction band, electron migrates 
through the conduction band and can then be localised 
on defects – oxygen vacancies as well as trivalent rare-
earth ions. The scheme of this model is in Figure 5. 
 
Charge carriers can also migrate from one trap to 
another and finally migrate back to the luminescence 
centre, where excited Eu
2+
 transition to ground state 
creates the characteristic luminescence. Other authors 
have described a similar model [60].  
The fourth work [61] on the topic of mecha-nism 
of persistent luminescence presented a very sim-ilar 
idea to that described in [56], that was also 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5. The model proposed by Aitasalo for the long-lasting 
luminescence involving Eu
3+
 accumulation during excitation. 
 
verified by XANES (X-ray absorption near edge 
struc-ture) and EXAFS (Extended X-ray absorption 
fine structure) measurements that confirm Eu
3+
 
accumu-lation during excitation. However, when 
explaining the mechanism, Aitasalo emphasises that 
there are doubts about Eu
3+
 presence, but Eu
2+
 and 
hole complexes might be more believable and the 
recombination of these complexes with the electron is 
the source of the long-lasting luminescence. The main 
difference from the previous model and that proposed 
by Dorenbos [59] is the involvement of several kinds 
of electron traps.  
Quite a similar opinion was expressed by Holsa et 
al. – Holsa turned attention to the results of XANES 
measurements of SrAl2O4:Eu,Dy [17] and tried to 
observe any valence changes of dopants in the mate-
rial during excitation and emission. He concluded that 
under excitation only the Eu
2+
/Eu
3+
 oxidation hap-
pens, therefore some changes to the known mod-els 
are proposed. There is an assumption that in 
SrAl2O4:Eu,Dy UV radiation leads to the excitation 
of Eu
2+
. The host matrix conduction band is close to 
the excited Eu
2+
 level therefore some electrons can 
escape from the excited levels of Eu
2+
 by the help of 
sur-rounding thermal energy to the conduction band. 
The electrons migrate through the conduction band 
and are eventually trapped on a defect level. The 
trivalent rare earth co-dopants can also act as electron 
traps. Ther-mal energy can then liberate the charge 
carriers from the trapping centres and they can either 
be de-trapped or return to luminescence centre. The 
defects, that are claimed to be present in this material, 
are cation vacan-cies, oxygen vacancies and 
interstitial ions as well as the dopant ions [17]. The 
same conclusions were drawn by Qiu et al. [60], 
turning attention to the fact, that the trapping and de-
trapping process from the trap levels can be repetitive. 
 
 
Clabau model 
 
Clabau suggested a model that involves electrons as 
the free charge carriers. To present proof for this 
hypothesis, electron paramagnetic resonance exper-
iments were conducted. The experimental results 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6. Clabau model for the long-lasting 
luminescence involving direct electron transition from 
luminescence centre to trapping centre. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 7. Dorenbos model for the long lasting luminescence 
involving electron migration through conduction band. 
 
showed that the concentration of Eu
2+
 ions changes 
during excitation and afterglow – it decreases in the 
excitation phase and increases during afterglow [19]. 
Therefore, Clabau presumed Eu
2+
 could be ionised – 
after Eu
2+
 excited state is formed during UV irradi-
ation, thus Eu
3+
 is created. Electron migration from 
trapping centres to the luminescence centres in this 
model does not happen through the conduction band, 
but with the aid of a direct transition between traps, 
that can occur if the levels are located in close 
proximity to each other. Schematic representation of 
this model is in Figure 6. 
 
Dorenbos model 
 
The study of excitation and luminescence processes of 
SrAl2O4:Eu,Dy completed by Dorenbos led to the new 
version of long-lasting luminescence mechanism [59].  
Dorenbos presented the locations of the Eu
2+
 d-
bands in the band gap of SrAl2O4 matrix. The estima-
tion of positions of dopant energy levels in SrAl2O4 
material is discussed in [29–31]. The Eu
3+
 energy 
levels are located right beneath the bottom of the 
conduction band and Dy
2+
 level is located around 0.9 
eV below the conduction band. In this model, electron 
is thermally promoted from Eu
2+
 excited state to the 
conduction band, as the energy difference is small and 
therefore thermal ionisation at room temperature is 
very prob-able. The electron migrates through the 
conduction band and is trapped by Dy
3+
 thus 
recharging it to Dy
2+
. Electron can be thermally 
released from Dy
2+
 and can contribute to forming the 
excited Eu
2+
 centre. From there excited Eu
2+
 centre 
has two options – either it is again thermally excited 
to the conduction band or it can transit to the ground 
state with photon emission (Figure 7). 
 
 
Latest discussion of luminescence 
processes in SrAl2O4 
 
The extensive work of scientists and researchers start-
ing from the moment of the discovery of persistent 
 
luminescence in rare earth doped strontium aluminates 
had made some progress in the understanding of after-
glow mechanism, but more work needed to be done. 
Last decade was dedicated to more in-depth analysis 
about the material and the processes of charge carrier 
transfer, leading to the persistent luminescence. There-
fore we will now take a look at the research that was 
conducted during the last decade and the conclusions 
that can be drawn from it. 
All of the long afterglow mechanisms described 
above involve the generation of migrating charge car-
riers during excitation that are localised in trapping 
centres afterward [55–59,61,62]. The cause of persis-tent 
afterglow is thermally stimulated gradual charge release 
from trapping centres with resultant recombi-nation. The 
differences in these models concern (I) charge carriers – 
electrons or holes; (II) the charge carrier migration – 
does it happen through valence, conduction band or via 
levels of trapping centres; (III) the excitation of 
luminescence – does the charge car-rier directly transfers 
to the luminescence centre or the luminescence centre is 
excited with the aid of energy transfer process; (IV) the 
nature of trapping centres – intrinsic defects or co-
activator (Figure 8). It is shown by TSL measurements 
that there are several different kinds of trapping centres 
in the material [18] but the exact nature of these trapping 
centres is not clear.  
The analysis of the results that are described in sub-
chapters 4.2. – 4.5. show that the most appropriate 
model for the explanation of persistent luminescence in 
SrAl2O4:Eu,Dy material is as follows: during excitation 
Eu
3+
 centres are created and electrons are localised in 
trapping centres. After the thermal release of electrons 
from trapping centres they recombine with Eu
3+
, cre-
ating excited Eu
2+
 and a radiative Eu
2+
 transition to 
ground state is the cause of luminescence. 
There are some ambiguities in this model – does the 
electron recombination with Eu
3+
 occur only by thermal 
release of electrons from trapping centres; there is no 
evidence if the charge state change of co-activator Dy
3+
 
can or cannot take place; as well as there is hardly any 
data about intrinsic luminescence of SrAl2O4 that could 
allow the identification of intrinsic 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 8. The known and unknown about the persistent luminescence mechanism in SrAl2O4:Eu,Dy. 
 
defects. Therefore, more work had to be done to 
provide strong evidence for the mechanism.  
In the last decade several more authors have put 
forward different ideas about the mechanism in the 
per-sistent SrAl2O4:Eu,Dy phosphor, but mostly 
scientists have invested their effort in finding proof 
that would support one or another of the mechanisms, 
that were offered before. 
 
The mathematical approach 
 
At this point, the mechanism where the electrons are 
considered to be the main charge carriers has become 
the predominant one and one of the reasons to assert 
this is the mathematical calculations of the band struc-
ture and the rare earth levels in the host band 
structure. J. Holsa et al., Nazarov et al., Zhai et al. and 
others have worked on the ab initio calculations of the 
structural and electronic properties of the material 
[15,24,63–70]. Several things have been confirmed by 
these calcula-tions. The most noticeable difference in 
the electronic properties of the material caused by 
doping is the appearance of impurity energy levels 
within the band gap [64].  
The calculations give an estimate for the energy states 
of europium in the band gap, that can help greatly in the 
understanding the nature of charge carriers. Effi-cient 
electron transfer from the Eu
2+
 5d states to the 
conduction band followed by trapping of electrons is 
required for the appearance of the persistent lumines-
cence. The 5d states must be positioned close to or even 
within the conduction band in this material. However, 
the lowest Eu
2+
 5d state has to be located below the 
conduction band, as the Eu
2+
 luminescence is experi-
mentally observed. It has been shown that the lowest 4f 
states of europium are located at about 4.5–5 eV above 
the top of the valence band [15] (we have to note that 
this result is not fully compliant with the experimentally 
determined values). It is also confirmed that the Eu
2+
 
4f
7
 ground state is located in the band gap. Experimen-
tally determined Eu
2+
 excitation spectra are ranging 
from 200 to 450 nm and that would correspond to the 
excited state positions in the band gap [18]. The excited 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 9. Photoluminescence of SrAl2O4:Eu,Dy at 10 
K temperature.  
 
electrons at the Eu
2+
 ions, which can be released to 
the conduction band, act as the primary charge carriers 
responsible for the persistent luminescence [24]. This 
has been accepted as a fact in the scientif ic commu-
nity and holes are no longer regarded as potential main 
charge carriers.  
Another thing that these theoretical calculations 
address, is the second luminescence band that is 
observable in the emission spectrum at the low-
temperature range. [69,71] This band, peaking at 455 nm 
(Figure 9) (the position of the second band in different 
publications is slightly different, ranging from 440–455 
nm) is relatively less intensive if com-pared to the 530 
nm emission band and the intensity decreases with 
increasing temperatures until the band is thermally 
completely quenched at temperatures above 150 K 
[69,71]. The afterglow curves for both bands also differs 
– Bierwagen et al. shows two components in the decay – 
a slow and fast component. In the fast com-ponent, the 
amplitude in the blue maximum is nearly as high as in 
the green maximum whereas it is much weaker for the 
slower component [72].  
There have been several assumptions for the possible 
origin of this second band, for example, charge transfer 
from oxygen to residual Eu
3+
 ion that takes place upon 
UV irradiation and is associated with hole trapping at 
 Sr
2+
 vacancies [19], a preferential orientation of the d 
orbitals of Eu
2+
 on Sr sites that appear to line up [73], 
anomalous low-temperature luminescence that origi-
nates from a higher Eu
2+
 5d state that may be 
observed due to the absence of high energy lattice 
vibrations at low temperatures [74].  
The ab initio calculations together with XRD 
measurements have, however, shown that two crys-
tallographic sites are available for the Sr
2+
 ions. The 
distances between sequential Sr
2+
 ions are alternat-ing 
by 3.9 and 4.6 Å, respectively. These two Sr sites have 
low local symmetry and differ in the coordina-tion 
numbers. The data presented here are in favour of a 
hypothesis that the emission bands originated from the 
Eu
2+
 ions located at two non-equivalent crystallo-
graphic sites, when incorporating instead of Sr
2+
 ions, 
and this opinion is shared by experimental investiga-
tions as well [46]. Calculations show, that both sites 
occur in almost equal amounts in the host lattice [15].  
Separate investigation of bands behaviour at low 
temperatures shows the integrated intensity of one 
luminescence band (450 nm) is decreased while the other 
band’s intensity (512 nm) is increased when the 
temperature is increased. That is why the existence of the 
non-radiative energy transfer between these two bands is 
proposed. This was explained more in-detail by Nazarov 
et al. [40] with the aid of adiabatic potentials of the two 
Sr sites. The Eu
2+
 ions entering the Sr II sites show 
broad emission at higher energy (at 450 nm), and those 
ions at the Sr I sites cause the lower energy band at 520 
nm. Nazarov calculated the crystal field splitting for 5d 
states, obtaining 5 different energies. These ener-gies 
were in a good agreement with the excitation bands and 
also the two dif ferent emission bands. The lowest energy 
level that arises from 5d state crystal field split-ting in Sr 
I position – 3.1 eV is responsible for the green 
luminescence and lowest energy level in Sr II position – 
3.3 eV is responsible for the blue luminescence. One can 
therefore conclude that at relatively high temperature the 
energy can be transferred from Eu
2+
 in the Sr II position 
to Eu
2+
 at the Sr I position, thus explaning the 
redistribution of emission intensities from both sites at 
different temperatures [15,64].  
To summarise – there have been several opinions 
on the origin of the second luminescence band. The 
theoretical calculations that have been conducted in 
the last decade are in favour of the opinion that in 
SrAl2O4:Eu,Dy the 450 nm luminescence band arises 
from the Eu
2+
 incorporated in Sr II position. 
 
Intrinsic defect role in the persistent 
luminescence mechanism 
 
One of the most important tasks in completing the 
model of mechanism responsible for persistent lumi-
nescence is the nature of defects, that act as trapping 
centres and the role of Dy co-doping that enhances 
 
 
the luminescence intensity and afterglow time greatly. 
To tailor the luminescent properties of the material to 
our needs, the trial and error method is not the best 
approach. It is sure to say that for more ef f icient 
commercial applications predictive modelling of the 
possible defect sites, although very difficult, would be 
superior to the empirical one.  
Lately, several new research papers have been pub-
lished that deal with the defects in the material and 
their role in the persistent afterglow.  
Thermally stimulated luminescence glow curve 
analysis is used to study the trap depth and the results 
show similar glow curve maxima in both doped and 
undoped materials [75]. The multiple maxima of the 
TSL curve point to the presence of a number of stable 
electron traps able to retain charge over considerable 
periods, whereas the presence of closely spaced peaks 
implies closely spaced electron traps comprising shal-
low, intermediate-energy and deep traps [76,77]. The 
idea of multiple, closely spaced trap levels has been 
implied in several other discussions on the mechanism of 
afterglow [18]. Hagemann et al. [78] have expressed the 
idea that the trap depth depends on the spatial distance to 
the Eu
2+
 centres, meaning that, depend-ing on the 
excess energy of the electron after reaching the excited 
state, electron can move further away from the emission 
centre and a trapping–de-trapping process can take 
place. It is now conf irmed, that SrAl2O4:Eu,Dy 
possesses a great variety of electron traps. The pop-
ulation of these traps is dependent on the excitation 
wavelength and temperature [78].  
Han et al. [79] have agreed on the wide distri-bution 
of trapping centres and continued the idea that 
distribution by a distance of co-dopant cations around 
luminescence centres can influence lumines-cent 
properties, meaning that for long-lasting lumines-
cence phosphors traps there are more parameters to be 
considered than just energetical depth. He suggests 
that distance between Eu
2+
 and co-dopant should also 
be counted in and proposes that it might be done by 
using Morse potentials and treating the constituents of 
the crystal as potential wells and barriers, that can be 
widened and narrowed under different conditions. For 
example, trapped electrons can be released by climb-
ing over potential barriers or just tunnelling through 
them under thermal stimulation, thus, previous mech-
anisms would work the same. And thermal stimulation 
can be introduced either by direct stimulating electrons 
or by weakening barriers via enhancing lattice vibra-
tion [79]. The idea of taking the dopant and co-dopant 
distances into account is very reasonable – because we 
can also observe, that dopant concentration influ-ences 
the efficiency of the afterglow. It can be seen that the 
PL intensity of emission from Eu
2+
 ions initially 
increases with the increase of dopant and co-dopant 
concentration. After it reaches the maximum value, a 
further increase in concentration makes the intensity 
  
decrease, because of the concentration quenching 
effect [21,72,80,81].  
When considering the possible origin of the trap-ping 
centres, it can not be excluded that similar defects to 
those in α-Al2O3 could be present in the strontium 
aluminate – F centres (oxygen vacancies occupied by 
two electrons or VO
0
), F
+
 centres (oxygen vacancies 
occupied by one electron or VO
+1
 or), their aggregates 
such as F2 centres (two associated oxygen vacancies 
occupied by four electrons), and the F2
+
 centres (two 
associated oxygen vacancies occupied by three elec-
trons). Also, the vacancies of the two other main lat-tice 
constituents Vsr and VAl can be involved in the charge-
trapping mechanism. Zhai et al. have expressed an 
opinion that all three Sr, Al and O vacancies might be 
involved in the charge-trapping process [67], whereas 
Chithambo et al. express the opinion, that the elec-tron 
trapping centres may be oxygen vacancies acting as F-
centres or oxygen aggregates [76].  
Vitola et al. have studied the TSL and X-ray excited 
emission spectra of undoped material to understand what 
defects exist in undoped SrAl2O4, that might be present 
in activated materials as well. The main conclu-sion 
from luminescence and TSL data analysis is that two 
main things contribute to the spectral properties – trace 
impurity metals, namely, Cr
3+
 and Mn
4+
, that is close to 
impossible to get rid of in the synthesis process, and 
intrinsic defects – F- centres and F2– centres lumi-
nescence. Other authors also have noted, that impurity 
metals are present as contaminants in the strontium 
aluminate lattice, regardless of the preparation method 
[82] and that means they can also be responsible for 
trapping centre generation.  
An extensive numerical study was conducted by 
Finley et al. [83] to determine the energetically most 
favourable defects, that might act as electron trapping 
centres. Experimental observations from thermolumi-
nescence and electron paramagnetic resonance data 
support the presence of either cation or anion 
vacancies [19,83,84], therefore first principle 
calculations were conducted on monoclinic SrAl2O4 
and SrAl2O4:Eu
2+
 to explore the presence of point 
defects due to cation and anion vacancies and their 
potential relationship to persistent luminescence. The 
team concludes that anion vacancies are present in 
both compositions, and energetically favourable in the 
crystal structure. Com-paring undoped and Eu doped 
materials it turns out, that when Eu
2+
 is present in the 
crystal structure, the trap depths shift to such 
energetic positions that are responsible for strontium 
aluminates exceptional lumi-nescent lifetime. 
 
 
Dy role in the mechanism of 
persistent luminescence 
 
The suggestions in previous reports of the Dy
3+
 ion role 
are rather different. It was proposed that Dy
3+
 traps a 
 
 
hole [55,78] while others proposed that the Dy
3+
 ions 
induce the formation of traps associated with a charge 
compensating defect [58,85], stabilising the traps [49] or 
act as an electron trapping centre itself [37,48,59,60]. 
While these conflicting opinions still exist, one thing is 
clear – Dy doping changes the environment surround-ing 
the Eu
2+
 ions in the SrAl2O4 host material [86] and 
generates some trapping centres [87].  
First, let’s look into the research of Zhai et al. [67]. 
This research states that in SrAl2O4 material Dy
3+
 ions 
substitute Sr
2+
 cations to form luminescent cen-tres in 
SrAl2O4. This statement comes from the fact, that 
SrAl2O4:Dy (without Eu doping) emits an after-glow. 
This is also stated by other authors, for example, Liepina 
et al. in [88] and Jamalaiah et al. in [84]. Char-acteristic 
Dy emission lines can also be observed in 
SrAl2O4:Eu,Dy under X-ray irradiation [89,90]. The 
other role of Dy
3+
 doping is to create a substitutional 
positive defect in the crystal lattice, that can act as an 
electron trap [91]. Owing to the requirement of charge 
neutrality, when two Dy
3+
 ions substitute for 2 Sr
2+
 
ions additionally one Sr
2+
 vacancy is involved to bal-
ance the charge of the phosphor, leading to the forma-
tion of two positive defects – Dy
3+
 in Sr
2+
 site and one 
negative strontium vacancy [92]. VSr is negative and 
could act as the hole trap. Extra electron and hole traps 
will be created in SrAl2O4 host when VO, VSr and VAl 
are present. The depths of these traps play a crucial role 
in determining the time of the material afterglow. 
Similar conclusions can be drawn from [69]. Moreover, 
Chithambo et al. [76] have assigned the largest TSL 
glow peak to the electron release from Dy trapping cen-
tre, justifying it with the energetical distance ∼ 1 eV 
from conduction band for the Dy
2+
 supposed electron 
trapping state. It is interesting to note that in major-ity 
publications in the photoluminescence spectrum of 
SrAl2O4:Eu
2+
,Dy
3+
 only Eu
2+
 luminescence band is 
visible, not the Dy
3+
 luminescence – since these tran-
sitions are forbidden and require excitation at the exact 
wavelengths of Dy absorption.  
The other opinion is expressed by Liepina et al. [88]. 
In this research SrAl2O4:Dy material is studied through 
TSL, X-ray excited luminescence (Figure 10) and 
kinetics analysis and this gives rise to a claim that Dy 
might act as a hole trapping centre. There is quite a 
strong argument in favour of this claim – Dy
3+
 emission 
spectrum is observable also in afterglow [64] and TSL 
spectrum [84,88,93]. The Dy
3+
 characteris-tic lines, that 
are observable under X-ray irradiation SrAl2O4:Dy
3+
 
are said to be created by electron return-ing to Dy
4+
 ion, 
thus creating an excited Dy
3+
 centre. The grounds for 
this claim is the TSL curves that indi-cate the same 
activation energies for trapped charge carriers that those 
of SrAl2O4:Eu,Dy. Since the persis-tent luminescence of 
SrAl2O4:Eu,Dy arises due to Eu
3+
 and electron 
recombination, it is possible only if dur-ing heating in 
TSL electrons are released. Therefore, 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 10. The SrAl2O4: Dy emission spectrum under 
X-ray irradiation. 
 
 
the Dy
3+
 luminescence during TSL arises via released 
electron recombination with Dy
4+
. Based on TSL and 
time-resolved spectral measurements the authors con-
clude that under X-ray irradiation Eu
2+
 and Dy
3+
 serve 
as hole traps and the Eu
3+
 as well as Dy
4+
 are accu-
mulated. Therefore, it was concluded that under X-ray 
excitation the Eu
2+
 as well as Dy
3+
 each lost an elec-
tron and no evidence was obtained on Dy
2+
 creation. 
Thus, the electrons were trapped at host defects.  
Qiu et al. [60] claim, that the trivalent co-dopant can 
introduce deeper electron trap centres. Other authors also 
state that the role of co-doped Dy
3+
 is to increase the 
number of vacancies and to alter the depth of the existing 
vacancies [49]. The role of Dy in SrAl2O4 is also 
discussed by Delgado et al. [49] also looking at the role 
of boron addition. The author states, that the role of the 
co-dopants, such as Dy
3+
 or B
3+
, is not interfering 
directly in the mechanism by getting reduced or oxi-
dised but stabilising the oxygen vacancies. These oxy-
gen vacancies are located in the near surroundings of the 
photogenerated Eu
3+
 cations and are claimed to be 
responsible for the long phosphorescence by acting as 
electron trapping centres. The electron density of Dy
3+
 
or B
3+
 is delocalised towards these oxygen vacancies, 
thus stabilising them [42]. The replacement of Sr
2+
 by 
Dy
3+
 in the host material induces defects on the crystal 
structure due to the size and charge differences. Fur-
thermore, to compensate the charge dif ference, induced 
by the presence of Dy
3+
, oxygen uptake may take place. 
The incorporation of oxygen into the lattice gives rise to 
more defects. Similar conclusions are drawn by Yoon et 
al. [22] who points out to additional TSL maxima due to 
oxygen vacancies, created by boron addition. We have to 
note that only few publications deal with the B
3+
 
influence on the photoluminescent properties of 
SrAl2O4:Eu,Dy, however, it is quite often used in the 
synthesis process, therefore this is another thing that 
could lead us to a better understanding of the persistent 
luminescence process. 
 
 
The XANES measurements of dopant valence 
 
Laastusaari et al. and Korthout et al. have conducted a 
research on the valences of the dopants of SrAl2O4 
material using XANES method [85,92]. The charging of 
the Eu
2+
 persistent luminescence could involve the 
excitation of an electron from the Eu
2+
 4f ground levels 
via the 5d levels to the conduction band and migra-tion 
to trapping levels. This may create either the Eu
3+
 ion or 
the Eu
2+
–h
+
 pair Therefore the knowledge of the dopant 
valences during excitation might serve as a strong 
evidence for this claim. It was observed, that under 
excitation the ratio for Eu
2+
/Eu
3+
 signal inten-sity 
changes – Eu
3+
 intensity increases, confirming the Eu
2+
 
ionisation to Eu
3+
 with the liberation of an elec-tron 
[92], however there was no correlation observed 
between the Eu
2+/
Eu
3+
 luminescence intensity ratio and 
the persistent luminescence intensity or duration, 
therefore it led the authors of this publication [85] to  
a conclusion, that the Eu
2+
–h+ pair is formed dur-ing 
the charging rather than the Eu
3+
 ion. In this pair, the 
hole may be manifested as an O
−
 ion adjacent to 
Eu
2+
 or could be located in a cation vacancy. As for 
the Dy valence measurements – no formation of Dy
2+
 
or Dy
4+
 ion was detected during excitation, pointing 
out that it might not act as a trapping centre. But it is 
also possible, that Dy
3+
-electron pair could be 
formed, which would allow the electron to become 
trapped without the reduction of the co-dopant ion. 
The pos-sibility that reduction of the Dy
3+
 does 
happen, but only for a minor fraction of the ions, can 
also not be excluded, and that could remain 
undetected by XANES measurement. [85,92]. 
 
Zeng model 
 
As mentioned beforehand, recently several additional 
explanations for the trapping mechanism have been put 
forward. Taking in account the predominant view of 
electrons as free charge carriers, a study of TSL 
(thermally stimulated luminescence) glow curves, decay 
kinetics and OSL (optically stimulated luminiscence) 
was conducted for SrAl2O4:Eu and SrAl2O4:Eu,Dy by 
Zeng et al. [18]. This research team observed the 
presence of multiple electron trapping centres having 
different energetical depths. The elec-tron is promoted to 
the conduction band from Eu
2+
, migrates through the 
conduction band and is trapped, for example, at oxygen 
vacancies. Electron can be thermally released and can 
contribute to forming the excited Eu
2+
 centre that can 
return to the ground state with photon emission. Doping 
of Dy is said to decrease the proportion of deep traps 
(over 0.65 eV). The traps are supposedly created by 
oxygen vacancies and the energies are distributed in a 
wide range. The more shal-low traps (with energetical 
depth less than 0.65 eV) can easily be emptied by the 
energy available in room temperature to conduction 
band by lattice vibrations, 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 11. The mechanism proposed by Zeng for long lasting afterglow involving a distribution of vacancy related trapping centres. 
 
however the thermal excitation of deeper traps at room 
temperature is much less probable. What is interest-ing – 
Dy is considered to be an electron trapping centre as 
well and if Dy introduces more shallow traps and 
decreases the proportion of deeper traps, that is a good 
explanation for the enhancement of luminescent prop-
erties of SrAl2O4:Eu by Dy co-doping. The shallow 
trapping centres contribute to the intensity of after-glow, 
whereas the decrease in the number of deeper traps 
could shorten the duration of the afterglow. The 
schematic representation of this model is in Figure 11.  
This model of long-lasting luminescence proposes 
a wide distribution of traps having different 
energetical depths, though it is not yet clear how the 
oxygen vacan-cies could form this wide distribution 
of traps. There are experimental research works, that 
deal with trap distri-bution and, although several 
different depth trapping centres are observed [68], but 
not an almost continu-ous distribution. Therefore this 
theory still needs to be confirmed. The other note – up 
to this day experimen-tal evidence of Dy
2+
 creation in 
SrAl2O4:Eu,Dy has not been provided. 
 
Qu model 
 
A very similar model has been proposed by Qu et al. 
[94] for CaAl2O4:Eu
2+
,Nd
3+
 material, that is 
 
 
structurally very similar to strontium aluminate mate-
rial, Sr being replaced with another alkaline earth metal 
ion Ca and instead of the trivalent rare earth Dy, triva-
lent Nd is used. It is proposed, that after the excitation 
and promotion to the conduction zone, the electrons can 
be trapped when they encounter the O vacancy or Nd 
dopant. Some trapped electrons can move back Eu, 
leading to persistent luminescence. Trivalent rare earth 
co-doping is considered to increase the lumines-cence 
intensity, because not only this co-dopant can offer 
additional electrons (by forming electron trap-ping 
centres, that can be emptied), but also introduce deeper 
electron trap centres. The authors claim, that it is not the 
wide distribution of oxygen vacancy energy levels, but 
the distribution of the 4f levels of trivalent rare earth, 
that plays a key role (Figure 12). Accord-ing to their 
calculations, trivalent ion introduces sev-eral 4f levels 
into the band gap that are distributed within a medium-
energy range ( > 0.4 eV). The lowest 4f level is located 
more than 1 eV below the conduc-tion band. The 
electron trapped in the lowest 4f level can move to only 
other nearby 4f levels with the assis-tance of thermal 
vibration and phonon coupling but cannot move directly 
to the conduction band with-out radiation. Therefore, the 
electron has to navigate between the 4f levels before 
having a chance to move back into the conduction band 
when it reaches the top 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 12. The mechanism proposed by Qu for long lasting afterglow involving a distribution of trivalent rare 
earth related trapping centres. 
 4f levels or other higher trap centres like oxygen 
vacan-cies. Hence, to prolong the decay time, many 
empty energy levels with different energies 
distributed from shallow to deep positions in the band 
gap must be available. The authors claim, that the 
distribution of the trap centre levels is proportional to 
afterglow dura-tion. We have to note that a disputable 
claim is that Eu
2+
 5d levels (excited Eu
2+
) are in the 
conduction band. 
 
 
centre, then the recombination can also occur through 
tunnelling.  
We have to note that this model has several ambi-
guities, because the predicted energetical position of 
VC is below that of Eu
2+
 and would lead to trapping 
holes rather than electrons and the proposed 
tunnelling of electrons from VC to Eu to form the 
excited cen-tre seems unclear –requiring a 
considerable change in electron energy. 
 
 
Gnidakouong model 
 
Similar to Qu model, Gnidakouong et al. [95] considers Dy 
and oxygen vacancies as electron trapping centres, but also 
introduces interstitial vacancies as another type of trapping 
centre. This research work was devoted to explaining the 
mechanoluminescence of the mate-rial, and also resulted in 
a suggestion for the persistent luminescence model. 
Electrons, after thermally being released into the conduction 
band are subsequently caught by the trivalent rare earth co-
dopant (Dy
3+
, creating a divalent Dy
2+
), the oxygen 
vacancies (VO) and the vacancies resulting from the lattice 
distortion – interstitial vacancies (VC). Upon absorption of 
light from the excitation source, the electronic configuration 
of Eu
2+
 changes from 4f
7
 to 4f
6
5d
1
, after which the elec-
tron can return to the ground state (Figure 13) or escape to 
the conduction band. There are three options from there. 
First, electrons can be caught by oxygen vacancy traps and 
this trap can be emptied in the afterglow pro-cess or through 
a mechanical stimulus. Electrons that are released in this 
way can reach ionised europium ions through the 
conduction band to form excited Eu
2+
, resulting 
recombination will be accompanied by the light emission. 
Second, electrons released by the ionised europium are 
caught by dysprosium ions creating diva-lent ions de-
trapped by the same route with the same effect as in the first 
case. Third, the carriers are caught by interstitial vacancies 
and are thermally released with the help of a mechanical 
stimulus. If an electron trapped in an interstice is very close 
to a luminescent  
 
 
Liepina model 
 
The observation of low-temperature luminescence 
afterglow, that is mentioned in several articles [72,96] 
is not compatible with the idea of only thermal release 
of charge carriers from the trapping centres. 
Therefore, an extensive study was conducted on the 
luminescence processes in the low-temperature range 
[97]. Based on the measurements of decay kinetics, 
and due to the lack of TSL maxima in the low-
temperature range, a persistent luminescence model 
involving electron tun-nelling from trapping centre to 
Eu
3+
 was proposed. It was suggested that the charge 
carriers are electrons and at low temperatures electron 
tunnelling is the domi-nant process in creation of 
excited Eu
2+
 centre, whereas in temperatures that 
offer enough thermal energy for the release of 
electrons, thermally stimulated process becomes 
dominant, however it is not excluded that the last step 
of electron migration could still be tunnelling process.  
The thermally released electron during migration 
can be re-trapped and released multiple times. The last 
step of electron migration could be either its direct 
trapping at Eu
2+
 excited state or trapping at a deeper 
trap associated with Eu
3+
 that is followed by excited 
Eu
2+
 creation via tunnelling. The radiative decay of 
excited Eu
2+
 created via both processes is the ori-gin 
of observed luminescence. The mechanism is sim-plif 
ied – it is clear from the analysis of TSL glow curves 
that several trapping centres are involved. This goes in 
agreement with the above-mentioned Zhang 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 13. The mechanism proposed by Gnidakouong for long lasting afterglow involving interstitial vacancies as trapping centres.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 14. The mechanism for long lasting afterglow by Liepina involving electron tunnelling. 
 
 
model. However, TSL measurements reveal that at 
room temperature and above the number of efficient 
traps involved is limited [97]. The schematic 
represen-tation of this model is in Figure 14. 
Tunnelling luminescence was mentioned as a possi-
ble process for electron transition to luminescence cen-
tre in SrAl2O4 material also by Chithambo et al. [76], 
based on the fact, that the phosphorescence that can be 
observed between irradiation and TSL measurement 
proceeds for periods much longer than required for the 
main peak to fade completely. Tunnelling between trap 
levels and luminescence centre together with thermally 
stimulated electron release has also been mentioned later 
in [98].  
To conclude the discussion about different models, 
that were proposed in the last decade we can say, that a 
consensus has still not been found. However, proof to 
clear up some of the ambiguities has been found. For 
example, holes are no longer considered to be the charge 
carriers in SrAl2O4:Eu,Dy material. Several authors 
have found proof to the distribution of trapping cen-tres 
with different energetical depth. Charge migration from 
one trapping level to another is discussed in all of these 
models. Tunnelling from a closely located trap-ping 
centre to the luminescence centre as an addition to the 
thermally stimulated process has been shown. Therefore 
it is up to the reader to choose the most plau-sible model 
based on the proof and the unclarities for each 
mechanism that are described above. 
 
 
SrAl2O4:Eu,Dy and the charge carrier transfer pro-
cesses in it has been improved during the last decade. A 
lot of attention was turned to the theoretical cal-
culations, thus proving the crystal field splitting of 5d 
states in Eu and the two unidentical crystallographic 
positions that are responsible for the appearance of two 
emission maxima in low temperatures. An energeti-cal 
distribution of trapping centres has been discussed in 
several publications and a supplement to the ther-mally 
stimulated process has been acknowledged – the 
tunnelling process, that explains the afterglow in tem-
peratures as low as 4 K. There still is more work to be 
done – although the role of Dy has been discussed in 
several publications and it is clear that it contributes to 
the creation of electron trapping centres, the exact nature 
of the trapping centres and Dy doping is yet to be 
understood. We also do not know, why Dy
3+
 emission is 
not visible in photoluminescence, but can be seen in 
radioluminescence and TSL measurements. However, it 
seems that we are getting much closer to fully 
understanding the processes that govern the long-lasting 
luminescence in Eu activated aluminate materials. 
 
 
 
Disclosure statement 
 
No potential conflict of interest was reported by the authors. 
 
 
Summary  
Ever since the discovery of SrAl2O4:Eu,Dy long after-
glow material, the model of long-lasting afterglow and 
the possibilities to enhance the intensity has been a pop-
ular area of research due to the vast applications it pro-
vides. During the last decade, researchers have mostly 
focused on finding experimental or theoretical proof that 
would back up or disprove any of the mechanisms, that 
were offered beforehand. The knowledge about 
 
Funding 
 
This work was conducted with the funding of Scientific 
Research Project for Students and Young Researchers 
realised at the Institute of Solid State Physics, University of 
Latvia [SJZ/2018/2]. 
 
 
ORCID 
 
Virginija Vitola  http://orcid.org/0000-0002-2599-4377  
Ivita Bite http://orcid.org/0000-0001-7049-0840  
Krisjanis Smits http://orcid.org/0000-0002-1652-9211 
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Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon
2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²