Tellurium–oxygen group enhanced birefringence in tellurium phosphates: a first-principles investigation

Phosphates possess a relatively large UV/DUV cutoff edge, but these compounds usually have very small birefringence. Recently the Te(2)P(2)O(9) crystal was synthesized and its birefringence was reported to be as large as 0.106 at 1013.98 nm. Herein, we investigated the electronic structure and optical properties of Te(2)P(2)O(9) using the first-principles method. The obtained results are in good agreement with the experimental values. The Born effective charges and SHG density of Te(2)P(2)O(9) show that the contribution to the birefringence and SHG response mainly originates from the TeO(5) group. The electronic structures and optical response of Ba(2)TeO(PO(4))(2) and Te(3)O(3)(PO(4))(2) were also investigated for comparison. The results show that these two tellurium phosphates also possess a large birefringence similar to Te(2)P(2)O(9). Also, the birefringence originates from the TeO(x) polyhedrons, which was confirmed by the real-space atom-cutting results and distortion indices.