Cation Short-Range Ordering of MgAl₂O₄and NiAl₂O₄Spinel Oxides at High Temperatures via in Situ Neutron Total Scattering

Complex oxides that adopt the isometric spinel structure (AB2O4) are important for numerous technological applications and are relevant for certain geological processes, which involve exposure to extreme environments such as high pressures and temperatures. Recent studies have shown that the changes to the spinel structure caused by these environments are complex and depend on the material length scale under consideration. In this study, we have expanded this approach to the behavior of spinels under high temperatures. In situ neutron total scattering experiments, coupled with pair distribution function analysis, performed on two spinel compositions with various levels of pre-existing disorder (MgAl2O4 and NiAl2O4) revealed that both compositions trend to a state of maximum disorder where the A and B cations are randomly distributed among the two available sites. Temperature-induced cation inversion, conventionally understood as an exchange of cations on the A and B sites, is locally expressed as an atomic rearrangement to a tetragonal symmetry, a correlation that is retained up to the maximum temperature studied (1000 °C). A complex thermal expansion behavior is revealed wherein the oxide materials expand heterogeneously at the level of coordination polyhedra with an apparent dependence on bond strength.