On the question of dynamic domains and critical scattering in cubic methylammonium lead triiodide

TL;DR

This study uses neutron spectroscopy to examine the presence of dynamic tetragonal domains in cubic methylammonium lead iodide, finding evidence inconsistent with dynamic domains and supporting a first-order phase transition with static domains.

Contribution

It provides experimental evidence against dynamic domain models in cubic methylammonium lead iodide and suggests static domains and a first-order transition.

Findings

R-point scattering is resolution-limited, inconsistent with dynamic domains.

The transition is confirmed to be first-order.

Scattering likely originates from static tetragonal domains near defects.

Abstract

We investigate the hypothesis of dynamic tetragonal domains occuring in cubic CH$_3$NH$_3$PbI$_3$ using high-resolution neutron spectroscopy to study a fully deuterated single crystal. The $\textit{R}$-point scattering above the 327.5 K tetragonal-cubic phase transition is always resolution-limited in energy and therefore inconsistent with dynamic domain predictions. This behavior is instead consistent with the central peak phenomenon observed in other perovskites. The scattering may originate from small, static, tetragonal-phase domains nucleating about crystal defects, and the temperature dependence demonstrates the transition is first-order.