Growth, Structure and Optical Characterization of Rb 3 Ti 3 P 5 O 20 Single Crystal.

Phosphate crystals attract much attention on account of their rich crystal structures and excellent physical and chemical properties. Herein, Rb 3 Ti 3 P 5 O 20 single crystals were grown by the high temperature solution method using Rb 2 CO 3 and NH 4 H 2 PO 4 as the fluxes. This crystal, with non-centrosymmetric Pca2 1 space group, presents a three-dimensional framework structure composed of [TiO 6 ] octahedron, [PO 4 ] tetrahedra, and [P 2 O 7 ] dimers. The electronic structure was measured via X-ray photoelectron spectroscopy. The measurements found that Rb 3 Ti 3 P 5 O 20 has stronger Ti-O ionic bonding properties and weaker P-O covalent bonding properties compared to RbTiOPO 4 . Optical measurements indicated that Rb 3 Ti 3 P 5 O 20 has a 3.54 eV band gap and a wide transmission range (0.33-4.5 μm). Theoretical calculations showed that Rb 3 Ti 3 P 5 O 20 crystals have a moderate birefringence of 0.079 at 1064 nm. In addition, the relationship of the structure-property was studied using first-principles method. The results demonstrated that TiO 6 octahedron played a significant role for the optical properties.