Structure and Stability of the Iodide Elpasolite, Cs2AgBiI6

TL;DR

This study confirms the first structural characterization of iodide elpasolite Cs2AgBiI6 nanocrystals, revealing surface free energy's role in stabilizing this otherwise predicted unstable phase.

Contribution

It provides the first definitive structural analysis of iodide elpasolite Cs2AgBiI6 nanocrystals and investigates stabilization mechanisms.

Findings

Cs2AgBiI6 nanocrystals have a tetragonal elpasolite structure

Surface free energy stabilizes the iodide elpasolite phase

Nanocrystal synthesis via anion exchange from bromide precursors

Abstract

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.