High-Entropy Oxides: Epitaxial Growth, Kinetic Dependencies, and Linear Optical Properties
Restricted (Penn State Only)
- Author:
- Kotsonis, George
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 15, 2022
- Committee Members:
- Susan Trolier-McKinstry, Major Field Member
Venkatraman Gopalan, Major Field Member
Jon-Paul Maria, Chair & Dissertation Advisor
Brian Foley, Outside Unit & Field Member
Vincent Crespi, Outside Field Member
John Mauro, Program Head/Chair - Keywords:
- Oxides
Entropy
Epitaxy - Abstract:
- High-entropy oxides (HEOs) are a class of chemically complex solid solutions typically consisting of around five constituent oxides in roughly equimolar proportions. The high-entropy paradigm violates historical materials design norms by relying on entropy maximization, rather than enthalpy minimization, to synthesize new materials. HEOs possess two inherent attributes that motivate this work: profound chemical flexibility, and kinetic metastability that leads to local crystalline disorder and unique physical properties. The chemical degrees of freedom afforded by mixing five constituents leads to physical property tunability, while local crystalline disorder disrupts structural and electromagnetic order parameters to generate potentially useful macroscopic properties. Applications of interest include low-thermal conductivity coatings, bespoke magnetic materials, robust electrochemical materials, and non-linear dielectric materials. The present work leverages the non-equilibrium kinetics of physical vapor deposition (PVD) techniques to grow high fidelity epitaxial crystals for structural and optical characterization. PVD rapidly condenses precursors from a high-entropy initial state, allowing kinetic stabilization of a broader palette of compositions than bulk synthesis under near-equilibrium conditions. Additionally, the flexibility of growth conditions afforded by pulsed laser deposition allows for studies on kinetic reconfiguration processes experienced by HEO crystals. As-deposited films can nucleate initially in a high-entropy state, but slow growth rates or elevated growth temperatures facilitate a controllable degree of phase decomposition. Crystal growth and phase reconfiguration studies are presented for the prototype HEO compositions Mg1/5Co1/5Ni1/5Cu1/5Zn1/5O and Sc1/6Mg1/6Co1/6Ni1/6Cu1/6Zn1/6O. Both initially nucleate in a chemically homogeneous rocksalt structure, but subsequently decompose into heterogeneous microstructures if a kinetic quench is not imposed to halt atomic reconfiguration towards equilibrium. Mg1/5Co1/5Ni1/5Cu1/5Zn1/5O experiences the nucleation of nano-scale secondary phases that are crystallographically coherent to the rocksalt matrix, while Sc1/6Mg1/6Co1/6Ni1/6Cu1/6Zn1/6O appears to reconfigure via spinodal decomposition. Refractive index models and growth temperature dependencies are discussed for single phase Mg1/5Co1/5Ni1/5Cu1/5Zn1/5O, Sc1/6Mg1/6Co1/6Ni1/6Cu1/6Zn1/6O, and four-component derivative compositions, which can all be grown epitaxially and kinetically quenched under suitable growth conditions. Compositions containing Co exhibit a structural and optical dependence on the growth temperature. At high substrate temperatures (greater than 400 °C), all cations appear to take their expected 2+ valence states. However, lower growth temperatures facilitate an appreciable amount of Co3+ in the rocksalt lattice that results in a smaller unit cell volume and increased optical absorption at visible light frequencies. Optical models suggest an increased band gap and more pronounced absorption tail for films grown at lower temperatures, owing to the increased Co3+ concentration. The near-infrared refractive index appears to depend on cation selection, but the optical dependence on Co valence appears to more strongly influence the infrared refractive index and electronic polarizability. The prototype HEO Y1/5La1/5Ce1/5Pr1/5Sm1/5O2-d, which takes a fluorite-derived crystal structure, was investigated to study influence of Pr multivalency and significant oxygen sublattice entropy on crystal symmetry, optical properties, and electrical transport properties. The symmetry of as-deposited single phase films appears to be that of the fluorite structure, but reactively sintered bulk ceramics exhibit lower symmetry characteristic of bixbyite-type crystals, indicating that processing conditions can influence the degree of chemical ordering in Y1/5La1/5Ce1/5Pr1/5Sm1/5O2-d specimens. Thin film capacitor devices were measured to see if the combination of oxygen sublattice entropy and Pr multivalency could lead to resistive switching behavior characteristic of memristive materials. Consecutive switching cycles are presented that demonstrate memristivity, illustrating promise for fluorite-derived HEOs as memristors and mixed electronic-ionic conductors. Lastly, structural and optical characterization of novel ABO3-type ternary HEOs are presented. Ba(Zr1/5Mg1/15Nb2/15Sc1/5Ta1/5Ti1/5)O3 exhibits a cubic perovskite structure and is notable for incorporating cations with four different valence states on the same sublattice. This observation expands the palette of accessible perovskite oxides to include those with significant multivalency, provided that charges are mixed in a way that maintains net charge neutrality. The (Mg1/5Mn1/5Co1/5Ni1/5Zn1/5)TiO3 system takes a single phase ilmenite structure, a chemically ordered derivative of the prototype corundum structure. The degree of chemical ordering is shown to depend on thin film growth conditions. Lower growth temperatures and lower oxygen pressures during growth enable the recovery of chemically disordered crystals with an observed symmetry similar to corundum, while higher growth temperatures and oxygen pressures favor well defined ilmenite-type ordering. This tunable amount of chemical ordering in (Mg1/5Mn1/5Co1/5Ni1/5Zn1/5)TiO3 films may result in correspondingly tunable electromagnetic order parameters and macroscopic properties. Overall, this work explores the relationship between processing kinetics, phase structure, and crystal symmetry in HEO systems. Non-equilibrium synthesis techniques facilitate HEO phase formation, but the nucleated high entropy phase is always subject to thermodynamic relaxation via atomic reconfiguration or phase decomposition. HEO systems were named for their inherently large configurational entropy (i.e. number of possible atomic configurations). This large number of possible configurations complicates atomic reconfiguration and thermodynamic relaxation, but also allows one to recover unique nanoscale microstructures. Exerting tight control over kinetic parameters appears to allow regulation of subtle local atomic reconfiguration, with potential ramifications on physical properties.