Order in the Disordered State

Structural Entities in Complex Oxides While materials at low temperatures are often characterized in terms of crystal chemical concepts, descriptions of their high-temperature properties are usually based on defect chemistry. Both approaches are obviously of great value but also have clear limitations. While the defect chemistry is often weakly linked to the crystal structure of the material and basically provides an ideal solution model where the solutes are either simple defects or defect clusters, crystal chemistry most often gives a static view of the material and tries to reduce the real complex structure to one in which the local structure does not vary throughout the crystal. Thus descriptions based on partial occupancy and random distributions of different species are frequently encountered. We approach the local and average structure of grossly disordered materials by configurational averaging using energies of individual configurations optimized by density functional theory or force-field models. We interpret the average structure observed experimentally at any temperature as a time and spatial average of the different local environments (corresponding to separate local minima on the potential energy surface) which are energetically accessible at that temperature. These environments are regarded as representing snapshots of small regions of the disordered material and thus to show local structural environments and structural correlations. Our alternative description bridges the two usual approaches for describing the structure of disordered materials and provides a unified picture of the main structural features of disordered oxides both at low and high temperatures.