We study halide perovskites materials, which have lately emerged as potential materials for low-cost, high-efficiency solar cells due to their ease of synthesis and relatively abundant constituent elements, as well as their intriguing photo-physical features. Increasing evidences has showed that the drivers of their remarkable properties is related to a temperature-activated symmetry-lowering distortion effect, which was originally named emphanisis in the context of compounds like rock-salt group IV chalcogenides. Here we study a class of ABX₃ compounds [A⁺ = Cs or CH₃NH₃ (MA) or CH(NH₂)₂ (FA); B²⁺ = Sn or Pb; X^- = Br or I] to understand the driving forces behind emphanisis or dynamical off-centering instability. We show that electron-phonon interaction is the driving force with the cov...[ Read more ]

We study halide perovskites materials, which have lately emerged as potential materials for low-cost, high-efficiency solar cells due to their ease of synthesis and relatively abundant constituent elements, as well as their intriguing photo-physical features. Increasing evidences has showed that the drivers of their remarkable properties is related to a temperature-activated symmetry-lowering distortion effect, which was originally named emphanisis in the context of compounds like rock-salt group IV chalcogenides. Here we study a class of ABX₃ compounds [A⁺ = Cs or CH₃NH₃ (MA) or CH(NH₂)₂ (FA); B²⁺ = Sn or Pb; X^- = Br or I] to understand the driving forces behind emphanisis or dynamical off-centering instability. We show that electron-phonon interaction is the driving force with the covalency of BX₃ framework and the dipole fluctuations of the A-site cation plays an essential role in determining the off-center temperature. We propose a general formula that capture the observed off-centering temperature dependence on aforementioned factors. Our theory suggests that A-site driven polaron-like model might be beneficial to understand the off-centering phenomenon in halide perovskites.