Defect crystal structure of α-Na0.5-xR0.5+xF2+2x (R = Dy-Lu, Y) on X-Ray and electron diffraction data. I. MethoD of Defect structure modelling on the α-Na0.35Dy0.65F2.30 example

For the first time, the crystal α-Na0.35Dy0.65F2.30 was studied using X-ray diffraction at 293 K and 85 K and electron diffraction at 293 K. A unified cluster model of the defective structure of nanostructured crystals with a fluorite-type structure, based on the polymorphism of ordered phases KR3F10 (R = Er, Yb), was expanded with a matrix part model based on the structure of the KYF4 compound. The unified cluster model was applied to construct the defective structure of α-Na0.35Dy0.65F2.30. It was found that the matrix part of the crystal contains Na+ и Dy3+ cations in a 1:1 ratio. Some of the anions in the matrix are displaced to the 32f positions (space group Fm3m). The excess Dy3+ forms octahedral-cubic clusters with Na+ [Na14–nDynF64+n] with cores in the form of distorted and regular cuboctahedrons {F12}. These are composed of interstitial anions in two 32f positions and one 48i position. The cluster component of the α-Na0.35Dy0.65F2.30 crystal contains octahedral-cubic clusters of f-, f–i- and i-types. Electron diffraction showed that α-Na0.35Dy0.65F2.30 is a nanostructured crystal. Its cluster component is in the form of plate-like inclusions about 5 nm thick with superstructural ordering and individual octahedral-cubic clusters. A model of their structure was proposed. Lowering the temperature to 85 K increases the number of interstitial F(32f)1 anions in the matrix component of the crystal.