Phase Transition, Conformational Exchange, and Nonlinear Optical Third Harmonic Generation of ACsP2Se8 (A = K, Rb, Cs)

The soluble molecular selenophosphate salts ACsP2Se8 (A = K, Rb, Cs) crystallize in the orthorhombic space group Ccce with a = 14.982(3) Å, b = 24.579(5) Å, and c = 13.065(3) Å for the Cs salt and a = 14.782(3) Å, b = 23.954(5) Å, and c = 13.044(3) Å for the K analogue. ACsP2Se8 is composed of the molecular 6-membered ring, [P2Se8]2−, in the twist conformation charge balanced by alkali metals. The band gaps of these compounds are 2.44 ± 0.2 eV for Cs2P2Se8, 2.41 ± 0.2 eV for RbCsP2Se8, and 2.36 ± 0.2 eV for KCsP2Se8. The amorphous versions of these materials can be made by water quenching the melt and have band gaps for all ACsP2Se8 of 2.12 ± 0.2 eV. Raman spectroscopic studies exhibit active modes of PSe4 and Se−Se in the compound. Solution 31P NMR studies shed light into the interesting conformational fluxionality of the [P2Se8]2− anion, including a conformation that has not been previously observed. Thermal analysis reveals ACsP2Se8 exhibits a phase transition, which we investigate by in situ synchrotron powder X-ray diffraction. Third harmonic generation (THG) nonlinear optical measurements determined the THG coefficient, χ(3), for amorphous and crystalline Cs2P2Se8 of 1.8 ± 0.2 × 105 pm2/V2 and 2.4 ± 0.1 × 105 pm2/V2, respectively.