Intrinsic Instabilities and Mechanical Anisotropy in Halide Perovskite Monolayers

TL;DR

This study uses first-principles simulations to analyze the structural, electronic, and mechanical properties of halide perovskite monolayers, revealing their anisotropic stiffness, stability issues, and electronic behavior variations with composition.

Contribution

It provides a comprehensive first-principles analysis of various halide perovskite monolayers, highlighting their stability, anisotropic mechanical properties, and electronic characteristics, which were not previously detailed.

Findings

ABX4 stoichiometry is thermodynamically and mechanically unstable.

Covalent Pb--X bonds induce anisotropic mechanical properties.

Band gap increases with halogen electronegativity and octahedral tilting.

Abstract

Halide perovskites have been extensively studied owing to their excellent optoelectronic properties and their unique lattice characteristics, that are very soft and anharmonic. Recent studies indicate the importance of a deep understanding of their surfaces and, in the limit, the properties of low-dimensional structures based on these materials. To investigate the structural and electronic properties of halide perovskite monolayers (i.e., perovskenes), this work uses first-principles simulations. We have studied three different stoichiometries (ABX3, ABX4, and A2BX4) and structural phases for iodide, bromide, and chloride perovskite monolayers. Their thermodynamic behavior was evaluated through the construction of phase diagrams, highlighting the instability of the ABX4 stoichiometry, which was further supported by its mechanical instability. Structurally, the covalent characteristics of the Pb--X bond, in contrast to the Cs--X bonds, induce a strong anisotropy in the Young's modulus and Poisson's ratio along different crystallographic directions, and also account for the lower stiffness observed in the phases where the octahedra are not aligned. The electronic properties are somewhat similar to those of their 3D counterparts, but with a slightly larger band gap; in the monolayers, the band gap increases with halogen electronegativity (I, Br, Cl) and octahedral tilting. Moreover, the non-symmetric ABX3 stoichiometry exhibited a spin splitting due to the internal dipole moment in these layers. Overall, our work lays the groundwork for a deeper understanding of low-dimensional structures based on halide perovskites.