Cubic-to-hexagonal structural phase transition in metal halide compounds: a DFT study

Abstract

Transitions to photo-inactive structural phases have impacted the use of metal halide perovskites as photovoltaic materials. However, the chemical composition allows us to mitigate this problem. This study investigated the cubic-to-hexagonal phase transition by means of density functional theory in nine metal halide compounds: APbX₃, with A = HC(NH₂)₂⁺, CH₃NH₃⁺, or Cs⁺, and X = I⁻, Br⁻, or Cl⁻. Fourty-three different structures were used to explore this transition. Variations in unit cell volumes were observed for all compounds. The energy barrier can be modulated by the chemical composition: it increases when the effective radii of the monovalent cation in the A-site decreases, and it increases when the mass of the halogen in the X-site decreases. Moreover, we also examined changes in the metal halide bond lengths during the phase transition, which were influenced by internal octahedral distortions, halogen electronegativity, and the presence of hydrogen bonds. Furthermore, the band gaps of all compounds increased once their octahedra started to distort, and, in most cases, only indirect wide band gaps were obtained once the phase transition took place. Our findings confirmed the possibility of preventing the cubic-to-hexagonal phase transition by employing smaller monovalent cations (MA or Cs) and lighter halogens (Br or Cl).