Stability and Dynamics of Sn-based Halide Perovskites: Insights from MACE-MP-0 and Molecular Dynamics Simulations

TL;DR

This work tackles the stability and finite-temperature phase behavior of lead-free Sn-based halide perovskites, focusing on CsSnBr3 and Cs2SnBr6. It employs the foundational MACE-MP-0 interatomic potential to perform MD in the $NpT$ ensemble over $100$–$500$ K, analyzing observables such as enthalpy, specific heat, RDFs, translational order $\tau$, Br–Sn–Br angle distributions, and vibrational spectra via VACF-derived $g(\omega)$ and $I(\omega)$. The results show CsSnBr3 undergoes a low-temperature orthorhombic-to-cubic transition without capturing the intermediate tetragonal phase, while Cs2SnBr6 remains cubic with a particularly rigid octahedral framework; both maintain short-range order up to $500$ K, with Cs2SnBr6 displaying higher structural coherence and stiffer lattice dynamics. Overall, MACE-MP-0 reproduces qualitative thermal and structural trends, validating its use as a first-step tool, though system-specific DFT fine-tuning may be needed to resolve subtle phase behavior. The work also provides the code and data access through a public repository for reproducibility.

Abstract

Tin-based halide perovskites have emerged as promising lead-free alternatives for optoelectronic applications, yet their structural stability and phase behavior at finite temperatures remain challenging to predict. Here, we assess the predictive capabilities of the foundational machine learning model MACE-MP-0 - trained on a broad chemical space and applied without system-specific fine-tuning - for the temperature-dependent behavior of CsSnBr3 and Cs2SnBr6. Molecular Dynamics simulations in the NpT ensemble were performed from 100 K to 500 K, and thermodynamic and structural descriptors including enthalpy, specific heat, radial distribution functions, translational order, bond angle distributions, and vibrational spectra were analyzed. Our results show that CsSnBr3 undergoes a low-temperature orthorhombic-to-cubic phase transition, evidenced by both the evolution of lattice parameters and subtle anomalies in enthalpy and specific heat, although the experimentally observed intermediate tetragonal phase is not captured. In contrast, Cs2SnBr6 remains cubic and maintains a more rigid octahedral framework across the entire temperature range. Overall, MACE-MP-0 qualitatively reproduces key thermal and structural features of these materials, highlighting its usefulness as a first step for studying new materials. For cases where capturing more subtle phase behavior is required, system-specific fine-tuning with Density Functional Theory data should be considered.

Figures (8)

• Figure 1: Crystal structures of the simulated supercells: (a) cubic CsSnBr3 and (b) cubic Cs2SnBr6, both with $4 \times 4 \times 4$ replication.
• Figure 2: Average values obtained during the production stage ($T = 100$–$500$ K): (a) cell angles $\alpha$, $\beta$, and $\gamma$ for CsSnBr3; (b) lattice parameters $a$, $b$, and $c$ normalized per octahedral unit for CsSnBr3; (c) cell angles $\alpha$, $\beta$, and $\gamma$ for Cs2SnBr6; and (d) lattice parameters $a$, $b$, and $c$ normalized per octahedral unit for Cs2SnBr6.
• Figure 3: (a) CsSnBr3 enthalpy $H$ as function of the system's temperature $T$, (b) Cs2SnBr6 enthalpy $H$ as function of the system's temperature $T$ and (c) the specific heat at constant pressure $c_p$ for both systems as function of the temperature $T$. The lines are for guiding the eyes.
• Figure 4: Radial distribution function $g(r)$ for (a) CsSnBr3 and (b) Cs2SnBr6 computed along the isobar from 100 K to 500 K. The curves highlight how thermal fluctuations progressively broaden and lower the peaks, especially beyond the first coordination shell. Vertical dashed lines indicate characteristic interatomic distances at 0 K.
• Figure 5: Translational order parameter $\tau$ as a function of temperature for CsSnBr3 (black) and Cs2SnBr6 (red). Both systems exhibit a monotonic decrease in $\tau$, indicating loss of positional order. Cs2SnBr6 consistently shows higher values of $\tau$, suggesting greater structural coherence.