First-Principles Thermodynamics of CsSnI₃.

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Cesium tin iodide (CsSnI3) is an eco-friendly material for energy harvesting.
• CsSnI3 exhibits polymorphism, existing as a black perovskite or a yellow 1D double-chain structure.
• The yellow phase degrades irreversibly in air, limiting its application.

Purpose of the Study:

• To determine the relative thermodynamic stability of CsSnI3 polymorphs.
• To investigate the role of quantum and anharmonic ionic fluctuations in phase stability.
• To validate a first-principles computational methodology for metal halide systems.

Main Methods:

• First-principles calculations were used to sample the CsSnI3 finite-temperature phase diagram.
• A comprehensive treatment of anharmonicity was incorporated into the simulations.
• Calculated results were compared with experimental data for transition temperatures and thermal expansion.

Main Results:

• The perovskite polymorphs of CsSnI3 are thermodynamically stable above 270 K.
• Anomalously large quantum and anharmonic ionic fluctuations drive the phase stability.
• Simulations accurately predicted experimental transition temperatures and thermal expansion coefficients.
• An abnormal decrease in heat capacity was observed in the cubic black perovskite upon heating.

Conclusions:

• The study elucidates the driving forces behind CsSnI3 phase stability.
• The developed computational methodology accurately reproduces experimental data.
• This approach is applicable to the systematic study of other metal halides for energy applications.