The APSe6 (A = K, K(1–x)Cs x , Cs) family of one-dimensional (1D) materials was studied to examine the effects of the cation size on the nonlinear optical (NLO) response. The family has high-performing NLO properties with the noncentrosymmetric parent material, KPSe6, having infinite 1D chains of 1/∞[PSe6 –]. This structure has been successfully substituted with cesium up to K0.6Cs0.4PSe6 while retaining the polar character. All compounds crystallize in the space group Pca21 and have band gaps of 2.1 eV. In situ powder X-ray diffraction experiments using synchrotron radiation were used to determine the specifics of the amorphous to crystalline behavior and the crystallization and melting kinetics of the APSe6 system. These measurements revealed a new phase, β-CsPSe6, which is metastable and crystallizes in the noncentrosymmetric tetragonal space group P 4̅21 c with a = 12.526(2) Å, c = 12.781(3) Å, V = 2005.3(6) Å3, and Z = 8. The structure is composed of 2 sets of mutually perpendicular 1/∞[PSe6 –] chains charge-balanced by Cs cations, and the band gap of β-CsPSe6 is 1.9 eV. Second harmonic generation (SHG) measurements demonstrate that substitution of Cs into KPSe6 maintains the strong NLO signal with a very high SHG coefficient (χ(2)) of ∼150 pm/V for K(1‑x)Cs x PSe6 and 30 pm/V for β-CsPSe6. Laser-induced damage threshold analysis reveals APSe6 exhibits two-photon absorption (2PA) at 1064 nm with input laser intensity greater than 1 GW/cm2 and optical damage from 2PA at ∼2 GW/cm2. The materials also exhibit strong third harmonic generation (THG) with THG coefficients (χ(3) × 105) for KPSe6, K0.6Cs0.4PSe6, α-CsPSe6, and β-CsPSe6 to be 2.6, 3.1, 1.8, and 1.1 pm2/V2, respectively.

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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.

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The cation size effect on the NLO response of the APSe6 (A = K, K(1‐x)Csx, Cs) family of one‐dimensional nonlinear optical (NLO) materials is studied.