Configurational Entropy in Complex Multi-Component Oxides

Configurational entropy embodies the composition complexity that materials can exhibit. Ab-initio methods such as the cluster expansion (CE) technique can be used to develop coarse-grained Hamiltonians that parameterize the energy dependence on configurational state but have rarely been used beyond simple binary or ternary metallic alloys. In this presentation, I will demonstrate the application of cluster expansion techniques to complex oxides with a high number of cation and anion components. The extension of the cluster expansion is non-trivial due to the combinatorial scaling of basis functions, the inability to sample all characteristic cation configurations leading to unavoidable degeneracy in the ab-initio to CE mapping, and the charge neutrality constrain in systems with charged ions. I will show how modern compressive sensing techniques, combined with physical constraints, can be used to build accurate and stable cluster expansions which can be used with Monte Carlo simulations to determine the state of order in complex oxides. These complex expansions have been used to develop DRX cathode materials, which are an exciting earth abundant replacement for current Ni and Co-based cathodes, but often have a high number of cations present, making their thermodynamic assessment difficult.