First-Principles Thermodynamics of CsSnI3

Investigating the role of dispersion corrections and anharmonic effects on the phase transition in SrZrS3: A systematic analysis from AIMD free energy calculations

A thermally driven needle-like (NL) to distorted perovskite (DP) phase transition in SrZrS3 was investigated by means of ab initio free energy calculations accelerated by machine learning. As a first step, a systematic screening of the methods to include long-range interactions in semilocal density functional theory Perdew-Burke-Ernzerhof calculations was performed. Out of the ten correction schemes tested, the Tkatchenko-Scheffler method with iterative Hirshfeld partitioning method was found to yield the best match between calculated and experimental lattice geometries, while predicting the correct order of stability of NL and DP phases at zero temperature. This method was then used in free energy calculations, performed using several approaches, so as to determine the effect of various anharmonicity contributions, such as the anisotropic thermal lattice expansion or the thermally induced internal structure changes, on the phase transition temperature (TNP→DP). Accounting for the full anharmonicity by combining the NPT molecular dynamics data with thermodynamic integration with harmonic reference provided our best estimate of TNL→DP = 867 K. Although this result is ∼150 K lower than the experimental value, it still provides an improvement by nearly 300 K compared to the previous theoretical report by Koocher et al. [Inorg. Chem. 62, 11134-11141 (2023)].

Cation Dynamics as Structure Explorer in Hybrid Perovskites-The Case of MAPbI3

Hybrid organic-inorganic perovskites exhibit remarkable potential as cost-effective and high-efficiency materials for photovoltaic applications. Their exceptional chemical tunability opens further routes for optimizing their optical and electronic properties through structural engineering. Nevertheless, the extraordinary softness of the lattice, stemming from its interconnected organic-inorganic composition, unveils formidable challenges in structural characterization. Here, by focusing on the quintessential methylammonium lead triiodide, MAPbI3, we combine first-principles modeling with high-resolution neutron scattering data to identify the key stationary points on its shallow potential energy landscape. This combined experimental and computational approach enables us to benchmark the performance of a collection of semilocal exchange-correlation functionals and to track the local distortions of the perovskite framework, hallmarked by the inelastic neutron scattering response of the organic cation. By conducting a thorough examination of structural distortions, we introduce the IKUR-PVP-1 structural data set. This data set contains nine mechanically stable structural models, each manifesting a distinct vibrational response. IKUR-PVP-1 constitutes a valuable resource for assessing thermal behavior in the low-temperature perovskite phase. In addition, it paves the way for the development of accurate force fields, enabling a comprehensive understanding of the interplay between the structure and dynamics in MAPbI3 and related hybrid perovskites.