Configurational Entropy and Phase Stability in Lead-Free Mixed-Halide CsSn(Br_xI_1-x)₃.

Area of Science:

• Materials Science
• Solid State Physics
• Computational Chemistry

Background:

• Metal halide perovskites offer tunable properties but face challenges with phase segregation and instability, especially in lead-free tin-based systems.
• Halide mixing in these materials is crucial for tuning optoelectronic properties but often leads to undesirable phase separation.

Purpose of the Study:

• To investigate the thermodynamic stability of Br/I alloying in lead-free CsSn(BrₓI₁₋ₓ)₃ perovskites.
• To understand the influence of different crystallographic phases (cubic, tetragonal, orthorhombic) on the miscibility of bromide and iodide ions.

Main Methods:

• Employed density functional theory (DFT) calculations to determine the thermodynamics of alloying.
• Utilized partition functions to analyze all symmetry-inequivalent configurations across different phases.
• Calculated free-energy differences to assess phase stability and miscibility boundaries.

Main Results:

• The orthorhombic phase shows the lowest mixing free-energy curve, indicating higher stability for Br/I alloying.
• The cubic phase is least favorable for Br/I mixing, with a significant positive free-energy difference (ΔF_cub-orth) across all compositions at 300 K.
• Enhanced stability in the low-symmetry orthorhombic phase is attributed to more effective local structural relaxation and octahedral distortions.

Conclusions:

• The orthorhombic phase of CsSn(BrₓI₁₋ₓ)₃ exhibits a robust thermodynamic preference over the cubic phase, crucial for stable mixed-halide compositions.
• Local structural relaxation and octahedral distortions are key factors influencing the thermodynamic stability of lead-free tin perovskites.
• These findings provide theoretical guidance for designing stable lead-free tin-based mixed-halide perovskites by controlling compositional and structural parameters.