Dynamic Nanoscale Spatial Heterogeneity in a Perovskite-to-Brownmillerite Topotactic Phase Transformation

Area of Science:

• Condensed matter physics
• Materials science
• Nanoscale science

Background:

• First-order phase transformations in solids typically involve nucleation and growth under nonequilibrium conditions.
• These dynamics are often assumed to be spatially and temporally independent under constant external conditions.

Purpose of the Study:

• To investigate nanoscale spatial and dynamical heterogeneity during the perovskite-to-brownmillerite topotactic phase transformation.
• To utilize in situ Bragg X-ray photon correlation spectroscopy (XPCS) for studying these dynamics.

Main Methods:

• In situ Bragg X-ray photon correlation spectroscopy (XPCS) was employed.
• La_0.7Sr_0.3CoO₃ thin films were annealed under constant reducing conditions for several hours.
• Nanoscale domain motion and domain wall dynamics were analyzed.

Main Results:

• Spatial and dynamical heterogeneity was revealed during the phase transformation.
• A stable time scale for domain growth was observed with a domain wall speed of 6 ± 0.5 × 10^-4 nm/s.
• Accelerating dynamics, following an aging power law with exponent -2.2 ± 0.5, were linked to temperature-driven depinning of domains.

Conclusions:

• Bragg XPCS is a powerful tool for quantitative in situ study of nanoscale dynamics in structural phase transformations.
• Nanoscale dynamics, including domain and domain-wall motion, can evolve and accelerate over extended periods.
• These findings have implications for phase engineering in phase-change devices, particularly concerning electrical performance.