Phase-change behavior and nonlinear optical second and third harmonic generation of the one-dimensional K(1- x)CsxPSe6 and metastable β-CsPSe6

The APSe6 (A = K, K(1–x)Csx, 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 421c with a = 12.526(2) A, c = 12.781(3) A, V = 2005.3(6) A3, and Z = 8. The structure is composed of 2 sets of mutually perpendicular 1/∞[PSe6–] chains cha...