Symmetry over Chemistry: Harmonic Generation of Low-Dimensional Alkali Chalcopnictates RbMP₂S₆ (M = Sb, Bi)

We have discovered RbSbP₂S₆ and β-RbBiP₂S₆, two alkali chalcopnictates that crystallize as one-dimensional chains. These near-isostructural analogs differ primarily in the coordination environment of the central metal: Sb has a coordination number (CN) of 4, while Bi exhibits a CN of 5. Notably, β-RbBiP₂S₆ is another form of a Rb/Bi/P/S compound with a 1126 stoichiometry. The β-phase, detailed in this work, is synthesized via direct combination, whereas the previously reported α-phase forms through a reactive Rb/Bi salt flux. The α- and β-phases also differ primarily by the Bi coordination sphere: α-phase Bi CN = 7; β-phase Bi CN = 5. RbSbP₂S₆ and β-RbBiP₂S₆ are both semiconductors. RbSbP₂S₆ has a bandgap of 2.68 eV while β-RbBiP₂S₆ has a bandgap of 2.06 eV. Neither material is congruently melting, but both have successfully been grown into large single crystals via slow cooling of the melt. Nonlinear optical (NLO) results in powder samples show that the symmetry breaking, which leads to lower dimensionality, also leads to significantly lower second harmonic generation (SHG) intensity (∼0.1x AgGaSe₂ for β-RbBiP₂S₆ vs ∼12x AgGaS₂ for α-RbBiP₂S₆). Third Harmonic Generation (THG) in powder samples show similar intensities (∼0.3x AgGaSe₂ for β-RbBiP₂S₆ and ∼0.1x AgGaSe₂ for RbSbP₂S₆). These are corroborated by single crystal SHG results for β-RbBiP₂S₆ (∼0.5x AgGaSe₂). The nonzero NLO coefficients d₁₄, d₂₅, and d₃₆ at 1550 nm fundamental wavelength are measured to be 9.5 ± 2.4, 17.2 ± 3.2, and 3.6 ± 2.3 pm/V, respectively. β-RbBiP₂S₆ has a comparable laser-induced damage threshold (LIDT) to AgGaSe₂ while RbSbP₂S₆ has a LIDT approximately 3x that of β-RbBiP₂S₆ attributed to its higher bandgap.