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

Three new tellurite-pyrophosphates K-2(TeO)P2O7, Rb-2(TeO)P2O7, and Cs-2(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 [P2O2] units. The compounds show quite large birefringences of Delta n = 0.070, 0.090 and 0.060 for K-2(TeO)P2O7, Rb-2(TeO)P2O7 and Cs-2(TeO)P2O7, respectively, as indicated by first-principles theoretical calculations based on density functional theory (DFT). In addition, K-2(TeO)P2O7 and Rb-2(TeO)P2O7 show a weak second-harmonic generation (SHG) effects, with strengths about 0.1 x 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 degrees C, 609 degrees C and 568 degrees C, respectively. (C) 2021 Elsevier B.V. All rights reserved.