Oxygen-vacancy organization in compositionally complex fluorites beyond the host reference

In conventional doped fluorites, oxygen-vacancy organization is interpreted relative to a defined host lattice and a limited set of dopant-vacancy interactions. When several chemically distinct cations share the fluorite sublattice, this reference becomes ambiguous: vacancies experience locally variable cation environments, and the energetics governing their behavior are better described as distributions rather than single values. In the limited conductivity data reported so far, multi-host fluorites designed around vacancy concentration and configurational entropy have not shown clear gains over optimized single-dopant fluorites. This observation motivates a shift in emphasis: from asking whether these materials are more disordered to determining whether their local vacancy preferences are compatible or competing. This Perspective develops a diagnostic classification based on four limiting regimes: random vacancy disorder, dominant defect association, short-range defect order, and frustrated defect landscapes. No existing dataset yet assigns these regimes unambiguously; defining the evidence standard is a central open problem for the field. This Perspective develops a diagnostic classification for compositionally complex fluorites based on random vacancy disorder, dominant defect association, short-range defect order, and frustrated defect landscapes.