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.