Anharmonic phonons and phase transitions in the vacancy-ordered double perovskite Cs2SnI6 from first-principles predictions

Abstract

Tin-based vacancy-ordered double perovskite ${\mathrm{Cs}}_{2}{\mathrm{SnI}}_{6}$ has emerged as a potential candidate for nontoxic, nonhygroscopic, and high efficient light absorber in the field of photovoltaics. By using lattice-dynamics calculations, we demonstrate that a series of phase transitions in ${\mathrm{Cs}}_{2}{\mathrm{SnI}}_{6}$ from cubic ($Fm\overline{3}m$) to tetragonal ($P4/mnc$) and to monoclinic ($P{2}_{1}/n$) is driven by anharmonic phonon modes, associated with octahedral tilting. For all the anharmonic phonon soft modes, double-well potentials are found with different well depths, which are used to estimate the hopping rate of a structural transition between two symmetry-breaking local minima. Phonon calculations of the lower-symmetry structures reveal that the anharmonic phonon modes are partly or fully eliminated. Furthermore, we calculate Helmholtz free energy difference to predict the phase transition temperatures, which are 44 K for monoclinic-tetragonal and 137 K for tetragonal-cubic transitions.