Phase-Change Behavior and Nonlinear Optical Second and Third Harmonic Generation of the One-Dimensional K_(1–x)Cs_xPSe₆ and Metastable β-CsPSe₆

The A PSe 6 ( 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, KPSe 6, having infinite 1D chains of 1 / ∞ [PSe 6 – ]. This structure has been successfully substituted with cesium up to K 0.6 Cs 0.4 PSe 6 while retaining the polar character. All compounds crystallize in the space group Pca 2 1 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 A PSe 6 system. These measurements revealed a new phase, β-CsPSe 6, which is metastable and crystallizes in the noncentrosymmetric tetragonal space group P 4̅2 1 c with a = 12.526(2) Å, c = 12.781(3) Å, V = 2005.3(6) Å 3, and Z = 8. The structure is composed of 2 sets of mutually perpendicular 1 / ∞ [PSe 6 – ] chains charge-balanced by Cs cations, and the band gap of β-CsPSe 6 is 1.9 eV. Second harmonic generation (SHG) measurements demonstrate that substitution of Cs into KPSe 6 maintains the strong NLO signal with a very high SHG coefficient (χ (2) ) of ∼150 pm/V for K (1- x ) Cs x PSe 6 and 30 pm/V for β-CsPSe 6 . Laser-induced damage threshold analysis reveals A PSe 6 exhibits two-photon absorption (2PA) at 1064 nm with input laser intensity greater than 1 GW/cm 2 and optical damage from 2PA at ∼2 GW/cm 2 . The materials also exhibit strong third harmonic generation (THG) with THG coefficients (χ (3) × 10 5 ) for KPSe 6, K 0.6 Cs 0.4 PSe 6, α-CsPSe 6, and β-CsPSe 6 to be 2.6, 3.1, 1.8, and 1.1 pm 2 /V 2, respectively.