SYNTHESIS AND CHARACTERIZATION OF CUBIC PYROCHLORE-TYPE RUBIDIUM AND CESIUM TUNGSTATES WITH A LACUNAR HOST LATTICE

Cubic pyrochlore-type tungstates of general composition A1.1W1.65O5.5 (where A = Rb, Cs) were synthesized by reacting hexagonal WO3 and alkali carbonates in the solid state at 300-degrees-C. The preparation of these new phases can be related to the structural analogy between the WO6 octahedra arrangements in hexagonal WO3 and in cubic pyrochlore lattices. DSC and DTA studies showed an exothermic peak on heating curves, which indicates an irreversible transition to known tetragonal phases. Annealing of these tetragonal phases produced two cubic pyrochlore-type tungstates structurally similar to the starting cubic phases. The analytical results (chemical and thermal analyses, IR spectroscopy, density measurements) and structural studies of the cubic phases led to a lacunar pyrochlore lattice with about 20% of W vacancies. Compounds of the AB2O6 pyrochlore-type possess a lacunar lattice without any substituent for the B cation.