NaTePO5, SrTeP2O8 and Ba2TeP2O9: Three tellurite-phosphates with large birefringence

Two new tellurite-phosphates NaTePO5, SrTeP2O8 and the previously reported Ba2TeP2O9 were synthesized successfully via high-temperature solid-state reactions and their structures were determined by single-crystal X-ray diffraction. The structures of NaTePO5 and SrTeP2O8 feature two-dimensional (2D) [TePO5]- and [TeP2O8]2- anionic layers interleaved with Na+ and Sr2+ cations, respectively. The 31P NMR spectra show that electron density around phosphorus atoms in SrTeP2O8 is slightly larger than that in Ba2TeP2O9, owing to the connection modes of [TeO5] and [PO4] units and counter-ion effects. The calculated electric field strength (Z/a2) based on the 31P chemical shift indicate as the chemical shift increases the Z/a2 also increases, as evidenced by the binding energy of P 2p determined by the X-ray photoelectron spectroscopy. Further, first-principle calculations show that the NaTePO5, SrTeP2O8 and Ba2TeP2O9 exhibit large birefringence of 0.212, 0.133 and 0.126 at 1064 nm, respectively, compared to other phosphates.