Application of KR₃F₁₀ (R = Gd–Lu and Y) Polymorphism for Modeling the Defect Structure of Nonstoichiometric Multicomponent Fluorite-Type Crystals in T-x Systems

Nonstoichiometric nanostructured crystals (NSC) α-M0.5−xR0.5+xF2+2x (M = Na and K; R = Sc, Y, and La–Lu) and M1−xRxF2+x (M = Ca, Sr, Ba, Cd, and Pb) with a fluorite-type (F) structure are widely used in quantum electronics, optics, solid-state ionics (superionic conductors), and bioimaging. However, the temperature dependence of their defect crystal structure, which is fundamental for understanding the numerous properties of F-NSC, remains unexplored. In this study, temperature changes in the F-NSC defect structure in T-x (temperature-composition) systems were simulated. Data on the KR3F10 (R = Gd–Lu and Y) polymorphism were used for modeling. A fragment representing an octa-cubic cluster (OCC) [M14−nRnF64+n] with a hybrid-type (i-f-) anionic core was isolated in the structure of low-temperature β-KR3F10 (R = Er and Yb). It includes all types of interstitial anions involved in the formation of i- and f-type clusters in the unified cluster model (UCM) of the F-NSC defect structure. There is a common tendency for hybrid (i + f) and f-type OCCs to form at low T and i-type OCCs at high T. The UCM was applied to verify T changes in the Ca0.94Y0.06F2.06 defect structure within the 293 ÷ 1423 K T range.