Evolution of the Cluster Structure of Fluorite Nonstoichiometric Crystals of the Homological Series CaRF (R = Sc, Y, La–Lu) in the Composition–Temperature System

The unified model of the cluster structure of fluorite nanostructured crystals proposed by the authors in the temperature–composition (T–x) system was used to clarify the evolution of the cluster structure of the crystals of the homologous series CaRF (R = La–Lu, Y). Defective structures of CaNdF, CaTbF and CaYF were studied by X-ray diffraction analysis at 293 and 85 K. They are based on octa-cubic clusters with cuboctahedron cores (F). The nuclei are formed by interstitial anions at position 32f (f-type clusters in CaNdF and CaTbF) and position 48i (i-type clusters in CaYF). Obtaining diffraction data with a resolution of 0.29 Å made it possible to identify cation displacements at positions 24e and 32f in CaYF and clarify the occupancies of these positions. Lowering the temperature from 293 to 85 K does not change the cluster structure of CaNdF, CaTbF and CaYF. An electron diffraction study of the CaNdF crystal revealed, for the first time, diffuse scattering from structural defects with dimensions on the order of one unit cell in crystals of the CaRF homologous series.