A2(TeO)P2O7 (A = K, Rb, Cs): Three new tellurite-pyrophosphates with large birefringence

Highlights: • Three new tellurite-pyrophosphates were successfully synthesized A₂(TeO)P₂O₇ (A = K, Rb, Cs). • Three new tellurite-pyrophosphates exhibit two-dimensional layer structure. • Large birefringence: Δn = 0.070, 0.090 and 0.060 for A₂(TeO)P₂O₇ (A = K, Rb, Cs), respectively. • The SHG responses for K₂(TeO)P₂O₇ and Rb₂(TeO)P₂O₇ are about 0.1 × KDP. • A₂(TeO)P₂O₇ (A = K, Rb, Cs) show noticeable thermal stabilities up to 602 °C, 609 °C and 568 °C, respectively. -- Abstract: Three new tellurite-pyrophosphates K₂(TeO)P₂O₇, Rb₂(TeO)P₂O₇, and Cs₂(TeO)P₂O₇ have been synthesized through high-temperature solution method and their structures have been determined on single crystals. The three title compounds exhibit two-dimensional layered structures and composed of two different [TeO₅] square pyramids and [P₂O₇] units. The compounds show quite large birefringences of Δn = 0.070, 0.090 and 0.060 for K₂(TeO)P₂O₇, Rb₂(TeO)P₂O₇ and Cs₂(TeO)P₂O₇, respectively, as indicated by first-principles theoretical calculations based on density functional theory (DFT). In addition, K₂(TeO)P₂O₇ and Rb₂(TeO)P₂O₇ show a weak second-harmonic generation (SHG) effects, with strengths about 0.1 × KDP. The deformation charge density indicate that the stereo-active lone-pair electrons on the Te(IV) atoms play an important role for the optical properties. Furthermore, the title compounds show noticeable thermal stabilities up to 602 °C, 609 °C and 568 °C, respectively.