Phase Stability and Transformations in CsSnI₃: Is Anharmonicity Negligible?

Abstract

Metal halide perovskites (MHPs) have soft lattices with strong anharmonicity and will undergo entropy-driven solid−solid phase transitions upon heating. Here, we investigate the polymorph stabilities and phase transitions in one of the lead-free MHPs, CsSnI 3 , by several molecular simulation techniques. Three different phase transitions (γ ↔ β, β ↔ α, and yellow → black) in CsSnI 3 have been successfully reproduced by molecular dynamics (MD) simulations with a newly developed empirical force field. The heating and annealing MD simulations and free-energy calculations with the non-equilibrium thermodynamic integration (NETI) method predict the transition temperatures of 275, 385, and 280 K for the γ ↔ β, β ↔ α, and yellow → black transitions, respectively. Lattice dynamics (LD) simulations within the harmonic approximation fail to predict the correct phase stability in CsSnI 3 at high temperatures. The quasiharmonic approximation (QHA) calculations that include the volume dependence of the phonon frequencies and lattice energies correctly predict all phase transitions in CsSnI 3 . However, the transition temperatures of the γ ↔ β and β ↔ α transitions predicted by the QHA calculations significantly deviate from those by MD simulations. By comparing the Gibbs free energies calculated by the LD simulations within the QHA and MD-based NETI method, we find the differences of 3−30 meV for different polymorphs. Although calculations based on the harmonic model can provide valuable information, the anharmonic terms need to be included for accurate predictions of transition temperatures of phase transitions in CsSnI 3 and other MHPs.

Structure of β-CuI: Stacking of 2D Bilayers

“…However, the QHA-predicted transition points may deviate from the actual values. 47 Additionally, our DFT-QHA calculations cannot predict the β → α solid-state phase transition occurring at a higher temperature since the α phase is a superionic phase and its Gibbs free energy cannot be accurately calculated within the QHA framework. To confirm that the CuI with a bilayered stacking structure near the predicted transition point does not reach a superionic state but can still be described within the QHA framework, we performed AIMD simulations on CuI with the ZB and CS2 structures.…”

“…Third, the neglect of anharmonic effects in the QHA framework can lead to a deviation of the predicted phase transition temperature from the accurate value . Although the partial neglect of anharmonic contributions in QHA does not affect the prediction of solid–solid phase transitions, , possibly because the anharmonic contributions to the free energy from two compared solid phases largely cancel each other out. However, the QHA-predicted transition points may deviate from the actual values .…”

“…Although the partial neglect of anharmonic contributions in QHA does not affect the prediction of solid–solid phase transitions, , possibly because the anharmonic contributions to the free energy from two compared solid phases largely cancel each other out. However, the QHA-predicted transition points may deviate from the actual values . Additionally, our DFT-QHA calculations cannot predict the β → α solid-state phase transition occurring at a higher temperature since the α phase is a superionic phase and its Gibbs free energy cannot be accurately calculated within the QHA framework.…”

Critical Evaluation of Five Classical Force Fields for CsPbI3: Limitations in Modeling Phase Stability and Transformations

“…Despite the fact that LJ-Coul has the simplest functional form and fewest parameters among five classical force fields considered in this work, it shows the best performance that can correctly predict all phase stability, solid–solid transitions, and melting of CsPbI 3 . This result is not surprising in the sense that the functional form of LJ-Coul is able to capture the ionic feature of CsPbI 3 crystals and other partially ionic crystals ,− and the parametrization process of this force field targeted accurate description of the CsPbI 3 polymorph stability. However, the simplicity of its functional forms and the fewer force field parameters also imply limitations in accuracy and transferability compared to those of more complex models.…”

Benchmark thermodynamic analysis of methylammonium lead iodide decomposition from first principles

“…One option is to apply ab-initio molecular dynamics and thermodynamic integration of anharmonic terms to the problem of MAPbI 3 thermodynamics and decomposition [28,41,45], but this method is computationally demanding, with either significant computational cost (if carried out by a direct first-principles approach) or a nontrivial workflow by machine-learning an interpolated potential energy surface with lower computational cost and sufficient precision [4,27]. For simpler solids, a widely employed method to capture a first approximation of temperature/entropy components is the quasiharmonic method (QHM) [15,28,45,46], which uses the locally stable equilibrium geometry and takes lattice thermal expansion into account, but otherwise remains within the bounds of the harmonic approximation. While the quasiharmonic approximation does not account for all consequences of anharmonicity [47], some of its limitations can potentially be mitigated.…”