Spontaneous Octahedral Tilting in the Cubic Inorganic Caesium Halide Perovskites CsSnX$_3$ and CsPbX$_3$ (X = F, Cl, Br, I)

TL;DR

This study reveals that cubic inorganic caesium halide perovskites inherently exhibit octahedral tilting due to vibrational instabilities, affecting their structural stability and optoelectronic properties relevant for solar cells and LEDs.

Contribution

The paper provides the first detailed quantum-chemical analysis of vibrational instabilities and anharmonic potentials causing octahedral tilting in CsSnX3 and CsPbX3 perovskites.

Findings

All studied perovskites show vibrational instabilities linked to octahedral tilting.

Anharmonic double-well potentials are present with barriers from 108 to 512 meV.

Structural distortions cause a spectral blueshift impacting optoelectronic performance.

Abstract

The local crystal structures of many perovskite-structured materials deviate from the average space group symmetry. We demonstrate, from lattice-dynamics calculations based on quantum chemical force constants, that all the caesium-lead and caesium-tin halide perovskites exhibit vibrational instabilities associated with octahedral titling in their high-temperature cubic phase. Anharmonic double-well potentials are found for zone-boundary phonon modes in all compounds with barriers ranging from 108 to 512 meV. The well depth is correlated with the tolerance factor and the chemistry of the composition, but is not proportional to the imaginary harmonic phonon frequency. We provide quantitative insights into the thermodynamic driving forces and distinguish between dynamic and static disorder based on the potential-energy landscape. A positive band gap deformation (spectral blueshift) accompanies the structural distortion, with implications for understanding the performance of these materials in applications areas including solar cells and light-emitting diodes.