Pyrochlores. VIII. Studies of some 2–5 Pyrochlores and Related Compounds and Minerals

Abstract

The cubic pyrochlores Cd2Nb2O7, Cd2Ta2O7, Sn2Nb2O7, Sn2Ta2O7. Cd2Sb7O7, Ca2Sb2O7, Mn2Sb2O7, Pb2Sb2O7, and related compounds were prepared and investigated by a number of methods. On heating above 700°. Ca2Sb2O7(pyrochlore) transformed to Ca2Sb2O7(weberite), while Pb2Sb2O7(pyrochlore) changed to a rhombohedrally distorted Pb2Sb2O7 pyrochlore. Refinement of the crystal structures of Cd2Nb2O7 and Cd2Ta2O7 from powder diffractometer intensities yielded 0.434(2) and 0.434(3) respectively as the best estimates of x(O2). Specimens of natural bindheimite and stibiconite were also examined. Stibiconite from San Luis Potosi (Mexico) was shown, on the evidence of its Mössbauer 121Sb spectrum, to contain Sb(V) and Sb(III) in the approximate ratio of 0.2.BaCd2Cl6•5H2O and BaCd2Cl6•2H2O both give powder diffraction patterns of the pyrochlore type. The chlorine could be partially replaced by Br to give mixed crystals BaCd2Cl6−zBrz•5H2O almost up to z = 2.The crystal chemistry of 2–5 oxide pyrochlores and the relationship of the weberite to the pyrochlore phases are discussed.