Many energy-related materials rely on the uptake and release of large quantities of ions, for example, Li⁺in batteries, H⁺in hydrogen storage materials, and O²⁻in solid-oxide fuel cell and related materials. These compositional changes often result in large volumetric dilation of the material, commonly referred to as chemical expansion. This article reviews the current knowledge of chemical expansion and aspires to facilitate and promote future research in this field by providing a taxonomy for its sources, along with recent atomistic insights of its origin, aided by recent computational modeling and an overview of factors impacting chemical expansion. We discuss the implications of chemical expansion for mechanical stability and functionality in the energy applications above, as well as in other oxide-based systems. The use of chemical expansion as a new means to probe other materials properties, as well as its contribution to recently investigated electromechanical coupling, is also highlighted.

In this work, chemical expansion in perovskite oxides was characterized in detail, motivated, inter alia, by a desire to understand the lower chemical expansion coefficients observed for perovskites in comparison to fluorite-structured oxides. Changes in lattice parameter and in local atomic arrangements taking place during compositional changes of perovskites, i.e., stoichiometric expansion, were investigated by developing an empirical model and through molecular dynamics and density functional theory atomistic simulations. An accurate empirical expression for predicting lattice constants of perovskites was developed, using a similar approach to previous reports. From this equation, analytical expressions relating chemical expansion coefficients to separate contributions from the cation and anion sublattices, assuming Shannon ionic radii, were developed and used to isolate the effective radius of an oxygen vacancy, r_V. Using both experimental and simulated chemical expansion coefficient data, r_V for a variety of perovskite compositions was estimated, and trends in r_V were studied. In most cases, r_V was slightly smaller than or similar to the radius of an oxide ion, but larger than in the fluorite structured materials. This result was in good agreement with the atomistic simulations, showing contractive relaxations of the closest oxide ions towards the oxygen vacancy. The results indicate that the smaller chemical expansion coefficients of perovskites vs. fluorites are largely due to the smaller change in cation radii in perovskites, given that the contraction around the oxygen vacancy appears to be less in this structure. Limitations of applicability for the model are discussed.

Holes can be readily doped into small-gap semiconductors such as Si or GaAs, but corresponding $p$-type doping in wide-gap insulators, while maintaining transparency, has proven difficult. Here, by utilizing design principles distilled from theory with systematic measurements in the prototype $A_{2}B$O${}_4$ spinel Co${}_2$ZnO${}_4$, we formulate and test practical design rules for effective hole doping. Using these, we demonstrate a 20-fold increase in the hole density in Co${}_2$ZnO${}_4$ due to extrinsic (Mg) doping and, ultimately, a factor of 10${}^4$ increase for the inverse spinel Co${}_2$NiO${}_4$, the $x$ $=$ 1 end point of Ni-doped Co${}_2$Zn${}_{1-x}$Ni${}_x$O${}_4$.