Lone-Pair-Induced Structural Ordering in the Mixed-Valent 0D Metal-Halides Rb23BiIII x SbIII 7-x SbV 2Cl54 (0 ≤ x ≤ 7).

ABSTRACT

Mixed-valent metal-halides containing ns2 lone pairs may exhibit intense visible absorption, while zero-dimensional (0D) ns2-based metal-chlorides are generally colorless but have demonstrated promising optoelectronic properties suitable for thermometry and radiation detection. Here, we report solvothermally synthesized mixed-valent 0D metal-halides Rb23BiIII x SbIII 7-x SbV 2Cl54 (0 ≤ x ≤ 7). Rb23SbIII 7SbV 2Cl54 crystallizes in an orthorhombic space group (Cmcm) with a unique, layered 0D structure driven by the arrangement of the 5s2 lone pairs of the SbIIICl6 octahedra. This red material is likely the true structure of a previously reported monoclinic "Rb2.67SbCl6" phase, the structure of which was not determined. Partially or fully substituting SbIII with isoelectronic BiIII yields the series Rb23BiIII x SbIII 7-x SbV 2Cl54 (0 < x ≤ 7), which exhibits a similar layered 0D structure but with additional disorder that yields a trigonal crystal system with an enantiomorphic space group (R32). Second harmonic generation of 532 nm light from a 1064 nm laser using Rb23BiIII 7SbV 2Cl54 powder confirms the noncentrosymmetry of this space group. As with the prototypical mixed-valent pnictogen halides, the visible absorption bands of the Rb23BiIII x SbIII 7-x SbV 2Cl54 family are the result of intervalent SbIII-SbV and mixed-valent BiIII-SbV charge transfer bands (CTB), with a blueshift of the absorption edge as BiIII substitution increases. No PL is observed from this family of semiconductors, but a crystal of Rb23BiIII 7SbV 2Cl54 exhibits a high resistivity of 1.0 × 1010 Ω·cm and X-ray photoconductivity with a promising µτ product of 8.0 × 10-5 cm2 s-1 V-1. The unique 0D layered structures of the Rb23BiIII x SbIII 7-x SbV 2Cl54 family highlight the versatility of the ns2 lone pair in semiconducting metal-halides, pointing the way toward new functional 0D metal-halide compounds.