Fluorite M1 − x R xF2 + x phases (M = Ca, Sr, Ba; R = rare earth elements) as nanostructured materials

Abstract

The structural defects (M 2+ and R 3+ in the noncubic environment of F−, interstitial F−, and anion vacancies) in nonstoichiometric M 1 − x RxF2 + x crystals with the CaF2 structure form {M 8[R 6F68-69]} superclusters of nanometer linear dimensions. This fact allows one to classify the M 1 − x R xF2 + x phases as nanostructured materials (NSM). The superclusters concentrate rare-earth ions (R 3+ = RE). In a M 1 − x R xF2 + x crystal with the fluorite cation motif, two chemically different parts can be separated: the R 3+-depleted matrix and the R 3+-enriched clusters. The M 1 − x R xF2 + x phases are the first NSM among fluorides; they constitute a new type of these materials in which different chemical compositions of the matrix and nanoinclusions are combined with their isostructurality and coherent conjugation of the crystal lattices. Superclusters can also form associates with linear dimensions of tens or hundreds of angstroms. A model is suggested which describes the main characteristic of such NSMs. These materials behave as single crystals in X-ray, neutron, and electron diffraction experiments. The influence of microheterogeneity on some physical properties of the M 1 − x R xF2 + x phases is also considered.