Cation Substitution Studies in Some Vanadate-Phosphate Systems

Abstract

<p>Cation substitution studies in several vanadate-phosphate systems have revealed substantial substitution between pentavalent V and P in tetrahedral sites in both ring and chain structures. The phase Zn₃V₀․₅P₁․₅O₈ has been prepared and its structure has been determined and found to be similar to that of α-Zn₃(PO₄)₂. It crystalizes in the space group C2/c with rings of Zn(1)O₄ and (V,P)O₄ tetrahedra sharing corners to form chains, which are linked to neighbouring chains by Zn(2)O₄ tetrahedra. The edge-sharing between Zn tetrahedra which is found in α-Zn₃(PO₄)₂ is absent in Zn₃V₀․₅P₁․₅O₈. There is no evidence of long-range ordering of V⁵⁺ and P⁵⁺ cations in this structure.</p> <p>Three pyroxene-like phases, related to NaVO₃ and characterized by tetrahedral chains, have been prepared. The NaV⅔P⅓O₃ structure closely resembles that of NaVO₃, and shows no long-range ordering of V⁵⁺ and P⁵⁺. The mixed phases Na․₈₇₅K․₁₂₅VO₃ and Na₀․₅K₀․₅VO₃ were prepared in order to study the effects of M⁺ substitution on the NaVO₃ structure. The structure of Na․₈₇₅K․₁₂₅VO₃ resembles that of NaVO₃ and NaV⅔P⅓O₃, while Na₀․₅K₀․₅VO₃ has the true diopside structure, with Na and K ordered in two nonequivalent sites. All three structures were refined in the space group C2/c and the effects of cation substitution on the configuration of the tetrahedral chains are discussed.</p>