Nonstoichiometry in inorganic fluorides: I. Nonstoichiometry in MF m -RF n (m < n ≤ 4) systems

The manifestation of gross nonstoichiometry in MF _m -RF _n systems (m < n ?? 4) has been studied. Fluorides of 34 elements, in the systems of which phases of practical interest are formed, are chosen. To search for new phases of complex composition, a program for studying the phase diagrams of the condensed state (??200 systems) has been carried out at the Institute of Crystallography, Russian Academy of Sciences. The main products of high-temperature interactions of the fluorides of elements with different valences (m ?? n) are grossly nonstoichiometric phases of two structural types: fluorite (CaF₂) and tysonite (LaF₃). Systems of fluorides of 27 elements (M ¹⁺ = Na, K; M ²⁺ = Ca, Sr, Ba, Cd, Pb; R ³⁺ = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; R ⁴⁺ = Zr, Hf, Th, U) are selected; nonstoichiometric M _{1 ? x} R _x F _{m(1 ? x) + nx} phases, which are of greatest practical interest, are formed in these systems. The gross nonstoichiometry in inorganic fluorides is most pronounced in 80 MF₂ ? RF₃ systems (M = Ca, Sr, Ba, Cd, Pb; R are rare earth elements). The problems related to the growth of single crystals of nonstoichiometric phases and basic fields of their application as new fluoride multicomponent materials, the properties of which are controlled by the defect structure, are considered.