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 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 K_0.6Cs_0.4PSe₆ 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 4 2₁c with a = 12.526(2) Å, c = 12.781(3) Å, V = 2005.3(6) ų, 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_xPSe₆ 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, K_0.6Cs_0.4PSe₆, α-CsPSe₆, and β-CsPSe₆ to be 2.6, 3.1, 1.8, and 1.1 pm²/V², respectively.