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

초록

We have discovered RbSbP2S6 and beta-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, beta-RbBiP2S6 is another form of a Rb/Bi/P/S compound with a 1126 stoichiometry. The beta-phase, detailed in this work, is synthesized via direct combination, whereas the previously reported alpha-phase forms through a reactive Rb/Bi salt flux. The alpha- and beta-phases also differ primarily by the Bi coordination sphere: alpha-phase Bi CN = 7; beta-phase Bi CN = 5. RbSbP2S6 and beta-RbBiP2S6 are both semiconductors. RbSbP2S6 has a bandgap of 2.68 eV while beta-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 (similar to 0.1x AgGaSe2 for beta-RbBiP2S6 vs similar to 12x AgGaS2 for alpha-RbBiP2S6). Third Harmonic Generation (THG) in powder samples show similar intensities (similar to 0.3x AgGaSe2 for beta-RbBiP2S6 and similar to 0.1x AgGaSe2 for RbSbP2S6). These are corroborated by single crystal SHG results for beta-RbBiP2S6 (similar to 0.5x AgGaSe2). The nonzero NLO coefficients d 14, d 25, and d 36 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. beta-RbBiP2S6 has a comparable laser-induced damage threshold (LIDT) to AgGaSe2 while RbSbP2S6 has a LIDT approximately 3x that of beta-RbBiP2S6 attributed to its higher bandgap.

키워드