Atomistic insights into the degradation of inorganic halide perovskite CsPbI3: A reactive force field molecular dynamics study
Summary¶
A ReaxFF parameter set for inorganic CsPbI\(_3\) is fitted to DFT (PBE + DFT-D3(BJ)) reference data computed in VASP, including equations of state, charges, formation energies, and defect energetics/migration for perovskite phases and precursors (CsI, PbI\(_2\)). Monte Carlo force-field optimization in AMS 2020 starts from literature Cs/I electrolyte-water parameters and Pb parameters adapted from Pt (Fantauzzi et al.). The letter then applies ReaxFF MD to phase stability versus temperature and to defective cells, showing anharmonic lattice fluctuations drive phase behavior and that iodine vacancies seed Frenkel defects and Pb–I cluster formation leading to PbI\(_2\)-like decomposition.
Opening motivation ties metal-halide perovskite deployment to unresolved environmental stability: moisture, heat, and illumination can trigger phase segregation and lead-halide precipitation, so atomistic reactive MD on CsPbI\(_3\) with explicit vacancies links iodine transport to decomposition microstructures at elevated temperature.
Methods¶
Force-field training (ReaxFF for Cs–Pb–I). Reference data are from DFT with PBE + DFT-D3(BJ) in VASP (see SI Supporting Notes 1–3 for convergence and supercells). The training set covers perovskite-related phases and precursors (including CsI and PbI\(_2\)) with equations of state, atomic charges, formation energies, and defect formation and migration barriers. Optimization uses the Monte Carlo-based ReaxFF optimizer in AMS 2020; the starting point combines Cs/I ReaxFF parameters from Fedkin et al. (electrolyte–water set) and Pb-like parameters adapted from Pt (Fantauzzi et al.), then adjusted for Pb. Optimized parameters are in the Supporting Information.
MD application (ReaxFF molecular dynamics in AMS 2020). All ReaxFF molecular dynamics in the study is run in AMS 2020 with the fitted potential (SI gives full integration and cell settings). The letter uses these runs for: (i) bulk CsPbI\(_3\) phase stability vs T on a ~100–700 K grid; (ii) NPT @ 1 atm self-diffusion of I vacancies and I interstitials at 450–700 K (Supporting Note 2); and (iii) 600 K dynamics of iodine-vacancy-enriched supercells (doping order ~3×10\(^{19}\) cm\(^{-3}\) in the letter). 3D PBC perovskite supercells; system size, time step (fs), equilibration vs production (ps–ns), thermostat/barostat tuning, and Ewald/QEq details are in the letter+SI (not re-copied here). External E-field, shear, metadynamics — N/A in these runs. Barostat — NPT for the 1 atm defect-diffusion block; other segments as tabulated in the SI.
Findings¶
Outcomes and mechanisms. Phase behavior: the letter attributes phase instability in part to anharmonic lattice motion and Cs displacement from preferred sites at low T, favoring conversion toward a non-perovskite (yellow) phase, while at higher T the dynamics are more isotropic. Defects: both I vacancies and I interstitials are mobile; the authors report Arrhenius-like diffusion with activation energies of order ~0.19 eV (vacancy) and ~0.28 eV (interstitial) in the temperature ranges quoted. Vacancy-assisted decomposition: I vacancies are argued to be especially detrimental—they promote local restructuring (edge-sharing octahedral motifs, Pb\(_x\)I\(_y\) clusters) that can evolve toward PbI\(_2\), whereas other vacancy types may remain stable to higher temperatures before decomposing.
Comparisons (model vs DFT). The fitted ReaxFF matches bulk EOS, formation energetics, and key defect/migration quantities vs the PBE+D3 training data; a highlighted check is the octahedral tilting barrier (~0.14 eV ReaxFF vs 0.17 eV DFT in the main text).
Sensitivity. Phase metrics and diffusivities are tracked as functions of temperature and defect loading; readers should use the main text + SI for exact run conditions when transferring numbers.
Limitations¶
ReaxFF accuracy is bounded by the DFT rung and training set; explicit device interfaces (contacts, moisture) are outside this parameterization.
Relevance to group¶
First ReaxFF line for all-inorganic halide perovskite stability with van Duin co-authorship.
Citations and evidence anchors¶
- J. Phys. Chem. Lett. 12, 5519–5525 (2021); SI for parameters and diffusion analysis.