HYDROTHERMAL SYNTHESIS OF CALCIUM-NIOBIUM AND TANTALUM OXIDES WITH THE PYROCHLORE STRUCTURE

The mixed metal oxides Ca2B2O7 (B = Nb, Ta) have been synthesized with the pyrochlore structure, The oxides were prepared by hydrolysis of Nb and Ta alkoxides and subsequent crystallization in the presence of the divalent cation in alkaline solution at temperatures up to 100-degrees-C. The reactions yield crystalline phases directly, though with small particle size and high surface area. The pyrochlore oxide Ca2B2O7 was also found to undergo ion-exchange in acid under mild conditions to form the hydrated oxide HNbO3.xH2O, also with small particle size and hi-h surface area. Confirmation of the structures of Ca2Nb2O7 and HNbO3.xH2O was obtained by Rietveld analysis of the powder X-ray diffraction data.