Structure and Stability of the Iodide Elpasolite, Cs₂AgBiI₆

Iodide elpasolites (or double perovskites, A₂B′B″I₆, B′ = M⁺, B″ = M³⁺) are predicted to be promising alternatives to lead-based perovskite semiconductors for photovoltaic and optoelectronic applications, but no iodide elpasolite has ever been definitively prepared or structurally characterized. Iodide elpasolites are widely predicted to be unstable due to favorable decomposition to the competing A₃B₂I₉ (B = M³⁺) phase. Here, we report the results of synchrotron X-ray diffraction (XRD) and X-ray total scattering measurements on putative Cs₂AgBiI₆ nanocrystals made via anion exchange from parent Cs₂AgBiBr₆ nanocrystals. Rietveld refinement of XRD and pair distribution functions (PDF) data shows that these nanocrystals indeed exhibit a tetragonal (I4̅m) elpasolite structure, making them the first example of a structurally characterized iodide elpasolite. A series of experiments probing structural relaxation and the effects of surface ligation or grain size all point to the critical role of surface free energy in stabilizing the iodide elpasolite phase in these nanocrystals.