Epitaxial entropy-stabilized oxides: growth of chemically diverse phases via kinetic bombardment
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esearch Letter
Epitaxial entropy-stabilized oxides: growth of chemically diverse phases via kinetic bombardment George N. Kotsonis, Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, 27606, USA; Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Christina M. Rost, Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia 22904, USA David T. Harris, Materials Science and Engineering, University of Wisconsin – Madison, Madison, Wisconsin 53706, USA Jon-Paul Maria, Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, 27606, USA; Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA Address all correspondence to George N. Kotsonis at [email protected] (Received 25 May 2018; accepted 15 August 2018)
Abstract This paper explores thin films of the entropy-stabilized oxide (ESO) composition MgxNixCoxCuxZnxScxO (x ∼ 0.167) grown by laser ablation in incremental gas pressures and O2/Ar ratios to modulate particle kinetic energy and plume reactivity. Low pressures supporting high kinetic energy adatoms favor the kinetic stabilization of a single rocksalt phase, while high pressures (low kinetic energy adatoms) result in phase separation. The pressure threshold for phase separation is a function of O2/Ar ratio. These findings suggest large kinetic energies facilitate the assembly and quench of metastable ESO phases that may require immoderate physical or chemical conditions to synthesize using near-equilibrium techniques.
Introduction In 2015, Rost et al. demonstrated that configurational entropy can be exploited to stabilize many-component oxide solid solutions.[1] They performed a thermodynamic study of the entropy-stabilized oxide (ESO) composition: MgxNixCoxCuxZnxO (x = 0.2), henceforth denoted J14; which undergoes an entropydriven phase transformation at ∼875 °C to a randomly distributed rocksalt (RS) solid solution that is metastable at 25 °C.[1] This J14 composition represents a model ESO system, because it has an easily accessible transition temperature, at which the configurational entropy gained by solid solution formation overcomes the enthalpy penalty for incorporating Cu and Zn into a RS structure.[1] The most common synthesis method for RS ESOs remains solid-state sintering,[1–6] but J14 has also been synthesized via nebulized spray pyrolysis, flame spray pyrolysis, reverse co-precipitation,[7] and pulsed-laser deposition (PLD).[1,8] Researchers reported the successful equimolar incorporation of Li+ into J14;[2] as well as the co-incorporation of Ga3+ with Li+,[2,4] Na+, or K+[3] via solid-state reactions of binary oxide powders. However, it remains difficult to incorporate aliovalent 3+ or 4+ cations without charge compensation from co-additions such as Li+.[2,5,6] Increasing interest in ESO discovery and characterization motivates an evaluation of synthesis methods that may expand the palette of
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