A3Sc(PO4)2 (A = Li, Na): rare earth cations effectively regulate phosphate structures for enhanced birefringence

Abstract

Phosphates possessing a rich variety of structural types and intriguing properties are widely used in fields such as ionic conductors, phosphors, and nonlinear optical materials. In this paper, two novel rare earth phosphates, A₃Sc(PO₄)₂ (A = Li, Na), were successfully predicted using the artificial bee colony (ABC) algorithm along with ab initio total-energy calculations. The results of electronic structure and optical property calculations indicate that A₃Sc(PO₄)₂ (A = Li, Na) not only retains the advantage of the large bandgap (E_g) of phosphates but also ameliorates the defects of the smaller birefringence (Δn) of phosphates (Li₃Sc(PO₄)₂: E_g = 6.077 eV and Δn = 0.046@1064 nm; Na₃Sc(PO₄)₂: E_g = 5.085 eV and Δn = 0.058@1064 nm). The real-space atom-cutting (RSAC) results show that the birefringence of A₃Sc(PO₄)₂ (A = Li, Na) is mainly derived from the ScO₆ group. This study enriches the variety of rare earth phosphates and confirms that the introduction of rare earth cations is an effective strategy for obtaining phosphates with a balance of a large bandgap and suitable birefringence.