Unraveling the Mechanism of Reversible Phase Transformation in Sb(III)-Doped Metal Halide Semiconductors

We investigate the reversible phase transformation of two Sb-doped metal halide semiconductors that respond in reverse to environmental changes and propose that this transformation follows Le Chatelier’s principle. Sb:Cs2InCl5·H2O and Sb:Cs2KInCl6 undergo reversible transformations triggered by external heat and moisture. Both experimental and density functional theory calculations are combined to provide mechanistic insights into this reversible transformation. The key finding is that phase transformations are governed by equilibrium shifts that counteract external perturbations, such as temperature or humidity. At high temperatures, Sb:Cs2InCl5·(H2O) absorbs heat from the environment and transforms into Sb:Cs2KInCl6, an endothermic process favored by the higher formation energy of Sb:Cs2KInCl6. Under high humidity, crystals absorb H2O molecules from the environment and reduce the ambient humidity, stabilizing the Sb:Cs2InCl5·(H2O) phase. Based on these results, we propose that the reversible phase transformation follows Le Chatelier’s principle, wherein Sb:Cs2KInCl6 and Sb:Cs2InCl5·(H2O) exist in a dynamic equilibrium that shifts in a direction that counteracts external environmental changes.