Localisation of vibrational modes in high-entropy oxides.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• High-entropy oxides (HEOs) present a novel class of materials with both crystalline order and significant atomic disorder.
• Disordered solids like glasses and alloys are known to exhibit localized vibrational excitations.
• Understanding phonon behavior in HEOs is crucial for their technological applications.

Purpose of the Study:

• To investigate the phenomenon of disorder-induced phonon localization in the prototypical rock-salt structured HEO, Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$O.
• To model and analyze the phononic excitation spectrum of this HEO.
• To explore strategies for enhancing phonon localization in HEOs.

Main Methods:

• Developed a model for cation-oxygen interatomic potentials by fitting to parent binary oxide properties.
• Validated the model against experimental crystal structure and optical conductivity data.
• Analyzed the phonon spectrum, including participation ratio and correlation amplitude, to identify localized modes.

Main Results:

• The phonon spectrum of the HEO exhibits wave-like propagating modes at low energies and localized modes at high energies.
• Signatures of localization, such as participation ratio and correlation amplitude, were identified.
• A hypothetical high-entropy telluride-oxide demonstrated enhanced localization with additional modes in the mid-spectrum.

Conclusions:

• Disorder in HEOs can induce localized vibrational excitations, distinct from traditional disordered materials.
• Increasing mass disorder offers a route to enhance phonon localization in HEOs.
• HEOs serve as a promising experimental platform for studying Anderson localization of phonons.