Dynamic nanoscale spatial heterogeneity in a perovskite to brownmillerite topotactic phase transformation

TL;DR

This study uses in-situ X-ray photon correlation spectroscopy to reveal nanoscale spatial and dynamical heterogeneity during a phase transformation in a perovskite material, showing continuous evolution of domain dynamics over hours.

Contribution

It demonstrates the capability of Bragg XPCS to probe nanoscale dynamics in phase transformations and uncovers persistent, evolving heterogeneity in domain behavior under constant conditions.

Findings

Stable domain growth timescale with specific wall speed

Accelerating dynamics following an aging power law

Nanoscale dynamics evolve over hours, affecting device performance

Abstract

Phase transitions are omnipresent in modern condensed matter physics and its applications. In solids, phase transformations typically occur by nucleation and growth under non-equilibrium conditions. Under constant external conditions, $\textit{e.g.}$, constant heating temperature and pressure, the nucleation and growth dynamics are often thought of as spatially and temporally independent. Here, $\textit{in-situ}$ Bragg X-ray photon correlation spectroscopy (XPCS) reveals nanoscale spatial and dynamical heterogeneity in the perovskite to brownmillerite topotactic phase transformation in La$_{0.7}$Sr$_{0.3}$CoO$_3$ (LSCO) thin films under constant reducing conditions over a time-span of multiple hours. Specifically, a timescale associated with domain growth remains stable, with a corresponding domain wall speed of $v_d = 6 \pm 0.5 \times10^{-4}$ nm/s ($2 \pm 0.2$ nm/h), while a slower timescale, associated with temperature driven de-pinning of domains, leads to accelerating dynamics with timescales following an aging power law with exponent $-2.2 \pm 0.5$. The experiment demonstrates that Bragg XPCS is a powerful tool to study nanoscale dynamics in phase transformations. The results are relevant for phase engineering of phase-change devices, as they show that nanoscale dynamics, linked to domain and domain-wall motion, can continuously evolve and speed up with time, even hours after the initiation of the phase transformation, with potential repercussions on electrical performance.