Disorder and symmetry : structural and compositional interplay of high entropy oxides

High entropy oxides (HEOs) are an extension of solid solutions that possess high enough configurational entropy (Sconf) to aid in the stabilization of a single crystal lattice. This is synthetically achieved by combining a large number of principal elements, usually five, and annealing at high temperatures. This new advent of combinatorially complex materials has attracted wide research efforts for their promise of unique properties enabled by the interplay of configurational disorder and multifaceted chemical composition. This work contributes to the vein of exploratory synthesis of HEOs and broadly encompasses two research directions. The first is an exploration of the compatibility of high entropy design principles in complex, lower-symmetry crystal structures. Grounding the inquiry from the unusually flexible accommodation of high entropy solid solutions in the perovskite lattice, a natural extension of the perovskite structure is the Ruddlesden-Popper (RP) phases. In this work, 10 novel high entropy compositions were targeted for various orders of RP phases and structurally examined by powder x-ray diffraction. While the majority of these synthetic attempts did not yield phase-pure samples, one composition that has the stoichiometry of a 214 tetragonal phase was characterized as having a 113 cubic structure. The composition Sr₂(Ti,Zr,Sn,Hf)O₄ is instead found to be Sr(Sr0.33Ti0.16Zr0.16Sn0.16Hf0.16)O3–δ and this transformation yields a large system-wide Sconf increase. The second direction concerns the exploration of novel high entropy systems in the spinel structure. Normal spinels have the general formula AB₂O₄ where A and B have tetrahedral and octahedral coordination geometry with oxygen ligands. Spinels, however, are prone to site mixing, a process in which some B cations will occupy tetrahedral sites and displace those A cations to the octahedral. While this selectivity is governed by several factors including thermodynamics, a prominent contributor to this selectivity are crystal field stabilization effects (CFSE). In this contribution, the design principle is to synthesize high entropy spinels which minimize CFSE to study their site mixing. Of the three targeted compositions, only one could be synthesized phase-pure. The novel spinel (Mg,Zn,Al,Fe,Ga,In)₃O₄ minimizes CFSE contributions and has magnetic characteristics suggestive of spin glass behaviour.