Missed ferroelectricity in methylammonium lead iodide

TL;DR

This study uses first-principles calculations to show that methylammonium lead iodide, despite its potential, is a paraelectric material with a missed ferroelectric state due to structural distortions.

Contribution

The paper reveals that methylammonium lead iodide is inherently a missed ferroelectric, clarifying its structural and electronic properties through first-principles analysis.

Findings

Disordered tetragonal phase is paraelectric.

Iodine octahedra rotations lead to an antipolar orthorhombic ground state.

Material has potential to be ferroelectric but is hindered by structural distortions.

Abstract

Methylammonium lead iodide, as related organometal halide perovskites, emerged recently as a particularly attractive material for photovoltaic applications. The origin of its appealing properties is sometimes assigned to its potential ferroelectric character, which remains however a topic of intense debate. Here, we rationalize from first-principles calculations how the spatial arrangement of methylammonium polar molecules is progressively constrained by the subtle interplay between their tendency to bond with the inorganic framework and the appearance of iodine octahedra rotations inherent to the perovskite structure. The disordered tetragonal phase observed at room temperature is paraelectric. We show that it should a priori become ferroelectric but that iodine octahedra rotations drive the system toward an antipolar orthorhombic ground state, making it a missed ferroelectric.