Abstract
Density functional theory calculations were carried out for three entropic rocksalt oxides, (Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5, termed J14, and J14 + Li and J14 + Sc, to understand the role of charge neutrality and electronic states on their properties, and to probe whether simple expressions may exist that predict stability. The calculations predict that the average lattice constants of the ternary structures provide good approximations to that of the random structures. For J14, Bader charges are transferable between the binary, ternary, and random structures. For J14 + Sc and J14 + Li, average Bader charges in the entropic structures can be estimated from the ternary compositions. Addition of Sc to J14 reduces the majority of Cu, which show large displacements from ideal lattice sites, along with reduction of a few Co and Ni cations. Addition of Li to J14 reduces the lattice constant, consistent with experiment, and oxidizes some of Co as well as some of Ni and Cu. The Bader charges and spin-resolved density of states (DOS) for Co+3 in J14 + Li are very different from Co+2, while for Cu and Ni the Bader charges form continuous distributions and the two DOS are similar for the two oxidation states. Experimental detection of different oxidation states may therefore be challenging for Cu and Ni compared to Co. Based on these results, empirical stability parameters for these entropic oxides may be more complicated than those for non-oxide entropic solids.
Highlights
AND MOTIVATIONMulti-component high entropy alloys (MHEAs), defined as four or more components in roughly equi-atomic concentrations randomly arranged on a single phase crystalline lattice, are of current interest due to both their potential for unique thermodynamic phase stability1–3 and for their potential applications.4–10 Up until recently, this class of high entropy material was restricted to metals and refractory ceramics11–14 with relatively simple crystal structures within which all of the lattice sites are part of the entropic phase
We have carried out an extensive set of Density Functional Theory (DFT) calculations on three entropic oxides, J14, J14 þ Li, and J14 þ Sc, to explore their properties, to understand the role of charge distribution and electronic states on their structure, and to probe whether simple empirical expressions may exist that could predict the stability of different compositions
Our calculations predict that the lattice constants for the random structures can be predicted reasonably well as the average of those for the ternary structures
Summary
Multi-component high entropy alloys (MHEAs), defined as four or more components in roughly equi-atomic concentrations randomly arranged on a single phase crystalline lattice, are of current interest due to both their potential for unique thermodynamic phase stability and for their potential applications. Up until recently, this class of high entropy material was restricted to metals and refractory ceramics with relatively simple crystal structures within which all of the lattice sites are part of the entropic phase. The current dearth of data related to the formation of entropic oxides hinders the empirical development of reliable parameters for stability With these and similar issues in mind, we have carried out Density Functional Theory (DFT) calculations on the J14, J14 þ Sc, and J14 þ Li compositions in a rocksalt structure for both binary and equi-atomic ternary compositions, as well as for relatively large systems that represent supercells of the equi-atomic high-entropy rocksalt structures. The radii associated with the altered oxidation states do not necessarily fall within the range of effective ionic radii defined by the ions in the J14 composition, potentially complicating expressions that rely on similar radii to predict stability of the entropic rocksalt structure. As discussed in detail below, these changes in charge state can add disorder to the system in terms of both Bader charges associated with the atoms as well as in displacements of atoms from their ideal lattice sites
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