Growth, Structure and Optical Characterization of Rb₃Ti₃P₅O₂₀ Single Crystal

Phosphate crystals attract much attention on account of their rich crystal structures and excellent physical and chemical properties. Herein, Rb₃Ti₃P₅O₂₀ single crystals were grown by the high temperature solution method using Rb₂CO₃ and NH₄H₂PO₄ as the fluxes. This crystal, with non-centrosymmetric Pca2₁ space group, presents a three-dimensional framework structure composed of [TiO₆] octahedron, [PO₄] tetrahedra, and [P₂O₇] dimers. The electronic structure was measured via X-ray photoelectron spectroscopy. The measurements found that Rb₃Ti₃P₅O₂₀ has stronger Ti-O ionic bonding properties and weaker P-O covalent bonding properties compared to RbTiOPO₄. Optical measurements indicated that Rb₃Ti₃P₅O₂₀ has a 3.54 eV band gap and a wide transmission range (0.33-4.5 μm). Theoretical calculations showed that Rb₃Ti₃P₅O₂₀ 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 TiO₆ octahedron played a significant role for the optical properties.