Subsolidus Phase Relations in the Systems M₂ ⁺O–M²⁺O–V₂O₅(M⁺ = Li, Na, K, Rb, Cs; M²⁺ = Mg, Ca)

Abstract

Subsolidus phase relations in the M₂O(M₂CO₃)–MgO–V₂O₅ and M₂O(M₂CO₃)–CaO–V₂O₅ (M = Li, Na, K, Rb, Cs) systems are studied. Twenty mixed vanadates are obtained, of which Rb₂CaV₂O₇ , Cs₂CaV₂O₇ , LiMg₄(VO₄)₃ , RbCaVO₄ , and CsCaVO₄ are identified for the first time. Structural data are summarized for all of the mixed vanadates: the space group and lattice parameters are indicated for 14 compounds (for 6 compounds, such data are obtained for the first time), and I and d data are presented for 8 compounds. Partial series of Ca₃(VO₄)₂-based solid solutions with the general formula Ca_{3 – x} M_2x (VO₄)₂ (M = Na, K, Rb, Cs) are identified in the range 0 < x ≤ 0.14. Six phase diagrams (M⁺ = Li, Rb, Cs; M²⁺ = Mg, Ca) are investigated and are compared with the phase diagrams of the other ternary systems in question. The key features of the ternary phase diagrams and, hence, the reactivity of the constituent oxides are shown to vary systematically in going from Li₂O to Cs₂O and from MgO to SrO, which is interpreted in terms of the variation in the ionic radius of the alkali and alkaline-earth metals.