Quantifying the coupling between strain and cation valence in high entropy oxide thin films using electron microscopy

TL;DR

This study uses advanced electron microscopy techniques to analyze how growth conditions influence strain and cobalt valence in high entropy oxide thin films, revealing the potential to tailor their properties.

Contribution

It introduces a detailed nanoscale analysis linking strain, chemical, and electronic structure variations to growth temperature in high entropy oxides.

Findings

Nanoscale strain variations correlate with Co valence changes.

Identical compositions can have different strain and defect states.

Growth conditions can be used to manipulate strain and valence.

Abstract

High entropy oxides (HEOs) are a class of materials with vast compositional space and tunable properties, making them attractive for applications in thermoelectrics, magnetism, ionic conduction, and beyond. However, their metastable nature makes the local structure, and consequently their properties, highly sensitive to growth conditions. It is therefore essential to probe the local modulations in atomic, chemical, and electronic structure as a function of growth conditions. Here, advanced S/TEM techniques, including 4D-STEM combined with electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy are used to investigate the effect of substrate temperature on structure and strain at the nanoscale regime in HEO thin films. We quantify how nanoscale strain variations correlate with Co valence and subtle chemical differences in the films with the same nominal composition but different growth temperatures. Our results demonstrate that identical HEO compositions can accommodate distinct strain and defect states in thin film form and highlight how synthesis conditions can be leveraged to manipulate strain and Co valence. These findings establish a framework to tailor functional properties via strain and valence control in high entropy oxide thin films.