  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 XRD and X-ray total scattering measurements on putative Cs2AgBiI6
nanocrystals made via anion exchange from parent Cs2AgBiBr6 nanocrystals.
Rietveld refinement of XRD and PDF data shows that these nanocrystals indeed
exhibit a tetragonal (I4-m) 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.
