Structural and electronic properties of β-NaYF4 and β-NaYF4:Ce3+

Abstract

In this work, the density functional theory approach with linear combination of atomic orbitals (LCAO) as implemented in the CRYSTAL17 computer code is applied to hexagonal β-NaYF4, located in three possible space groups of this compound: P6‾, P63/m and P6‾ 2 m. First, the disordered crystalline structure of NaYF4 was modelled in a large supercell containing 108 atoms. In order to obtain better agreement with the experimental data, we used several different exchange-correlation functionals. Basic properties, such as lattice constant, band gap and total energies were calculated and compared for all three space groups and three exchange-correlation functionals - HSE06, PWGGA and PWGGA+13%HF. It was found that for all three functionals, the minimum of total energy corresponds to P6‾ space group. Secondly, in order to study the effects associated with the Ce3+ impurity and the F center (radiation defect), the P6‾ β-NaYF4 structure with the F center and Ce3+ or with both was carefully modelled. Taking into account that fluorine atoms have different nearest neighbours, several types of fluorine vacancies were simulated and an appropriate formation energies were determined. Finally, the effects of Ce3+ ion substitution of Y ions in different positions as well as formation of Ce3+, the F center defect pairs were also studied and an appropriate incorporation energies were calculated.