Improved Synthesis of Alkali Metal Vanadates Using a Hydrothermal Method

High product yield of highly crystalline layered MxV3O8 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) Å, c = 11.297(3) Å, V = 787.2(3) Å3, 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]n2n- layer with distorted ZnO4 tetrahedra