The universal defect cluster architecture of fluorite-type nanostructured crystals

Abstract

A universal cluster model (UCM) of a defect (cluster) structure was developed on examples of nanostructured nonstoichiometric crystals (NNCs) M_1−xR_xF_2+x (M = Ca, Sr, Ba; R = La–Lu, Y) of the fluorite type (CaF₂). This family of NNCs consists of 48 phases of different elemental (M, R) compositions. Their RF₃ content reaches 50 ± 3 mol%. Crystal matrices MF₂ and RF₃ control the cluster structure and properties of the NNCs. They are components of MF₂–RF₃ systems in which these NNCs are formed. The UCM was created on the basis of the studied structures of the ordered phases M₄R₃F₁₇ (M = Sr, Ba) and M₂RF₇ (M = Ca) and the mineral tveitite Ca₁₄Y₅F₄₃ with a structure derived from the CaF₂ type. Clusters [M₈R₆F_68–70] are the structural units in them. The M₄R₃F₁₇ architecture was first presented as an assembly of blocks of MF₂ matrix structure and blocks of both types of structures of the second system components: LaF₃ [R₆F₄₄] and β-YF₃ [R₆F₄₀]. Rare-earth clusters in MF₂ matrices are treated as nanoscale blocks (R-enriched “nanophases”). The model is based on the octa-cubic cluster [M_14−xR_xF_64+x] with the cuboctahedral anionic core {F₁₂}. The new model describes the structure of M_1−xR_xF_2+x with all M and R and any x. The UCM is of fundamental interest for the most common nonstoichiometry in inorganic fluorides of the CaF₂ structural type. The properties of fluorite-type NNCs are controlled by a defect structure, which allows them to be used in quantum electronics, optics, solid state ionics, photonics, in vivo deep-tissue bioimaging NaRF₄:(R' + R′′) and other fields.