Improved Synthesis of Alkali Metal Vanadates Using a Hydrothermal Method

Abstract

High product yield of highly crystalline layered M_xV₃O₈ compounds was obtained under hydrothermal conditions from the V₂O₅-Zn(OAc)₂.2H₂O-alkali metal salt-H₂O system at significantly lower reaction temperature than previously reported in neutral media for x = 1 and M = K⁺, Rb⁺, and Cs⁺, and in basic media for x = 2 and M = K⁺. Unreacted Zn(OAc)₂ may act as a catalyst and/or HOAc/OAc^- acts as a buffer in obtaining monopotassium salts while enMe used to adjust pH can also act as a reducing agent in producing the dipotassium salt. A new layered dipotassium zinc pyrovanadate compound, K₂ZnV₂O₇, was formed when twice the stoichiometric ratio of the zinc salt was used. Single crystal X-ray diffraction shows the zinc compound to crystallize in the tetragonal space group P4₂/mnm (No. 136) with a = 8.3478(16) Ã, c = 11.297(3) Ã, V = 787.2(3) ó, and thus to be isomorphous with K₂MgV₂O₇. The pentagonal-motif layer structure of K₂ZnV₂O₇ is similar to that of K₂(VO)V₂O₇, but the unit cell parameters differ with approximate doubling of the c axial length when replacing the square-pyramidal VO₅ units of the [(V⁴⁺O)V⁵⁺₂O₇]_n^2n- layer with distorted ZnO₄ tetrahedra.