New Advances in Early Transition Metal High-Entropy Oxide Electroceramics

High-entropy oxides unlock new potential for application-driven materials design. High configurational entropy bestows thermodynamic stability to structures with distinct local environments, which in turn give rise to diverse and tailorable properties. The <i>A</i>₆<i>B</i>₂O₁₇ (<i>A </i>= Zr, Hf; <i>B</i> = Nb, Ta) high-entropy oxides adopt a complex, disordered cation sublattice which induces a unique macroscopic dielectric repsonse, making them important candidates for electroceramics applications. Here, we investigate the interplay between strcuture and dielectric properties of disordered <i>A</i>₆<i>B</i>₂O₁₇ phases across length scales, from bulk ceramics to thin films.<br/><br/>Tandem <i>in situ</i> and <i>ex situ</i> X-ray diffraction identify <i>A</i>₆<i>B</i>₂O₁₇ stabilization temperatures which are consistent with calculations for a strongly cation-disordered sublattice. Reactive sintering at high temperatures (≥ 1300 °C) yields dense (&gt; 95% <i>ρ_Theoretical</i>), phase-homogenous <i>A</i>₆<i>B</i>₂O₁₇ ceramics which are used for bulk dielectric measurements and as targets for thin film sputter deposition. Room temperature Hakki-Coleman measurements on bulk ceramics indicate a relative permittivity of ~60 with loss &lt; 0.01. Modified temperature resonant post measurements indicate an atypical temperature-dependent dielectric response: we observe a positive permittivity temperature coefficient, suggesting contributions from a disorder-derived polarization mechanism. Dielectric measurements from sputter-deposited films corroborate bulk data. We observe a strong connection between oxygen partial and total pressure deposition conditions on film microstructure and morphology as well as electrical performance. X-ray diffraction, X-ray reflectometry, atomic force microscopy, and X-ray photoelectron spectroscopy highlight structural differences between films grown under different deposition conditions; this structural data is used to improve understanding of dielectric property trends. We explore extended solubility in <i>A</i>₆<i>B</i>₂O₁₇ phases and impacts from different solute cations on structure and electronic properties, including high field responses. We employ computational tools, including density functional theory and machine learning, to corroborate and elucidate experimental trends in both structure and property behavior.