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

Abstract

The cubic pyrochlores Cd 2 Nb 2 O 7 , Cd 2 Ta 2 O 7 , Sn 2 Nb 2 O 7 , Sn 2 Ta 2 O 7 . Cd 2 Sb 7 O 7 , Ca 2 Sb 2 O 7 , Mn 2 Sb 2 O 7 , Pb 2 Sb 2 O 7 , and related compounds were prepared and investigated by a number of methods. On heating above 700°. Ca 2 Sb 2 O 7 (pyrochlore) transformed to Ca 2 Sb 2 O 7 (weberite), while Pb 2 Sb 2 O 7 (pyrochlore) changed to a rhombohedrally distorted Pb 2 Sb 2 O 7 pyrochlore. Refinement of the crystal structures of Cd 2 Nb 2 O 7 and Cd 2 Ta 2 O 7 from powder diffractometer intensities yielded 0.434(2) and 0.434(3) respectively as the best estimates of x(O 2 ). Specimens of natural bindheimite and stibiconite were also examined. Stibiconite from San Luis Potosi (Mexico) was shown, on the evidence of its Mössbauer 121 Sb spectrum, to contain Sb(V) and Sb(III) in the approximate ratio of 0.2.BaCd 2 Cl 6 •5H 2 O and BaCd 2 Cl 6 •2H 2 O both give powder diffraction patterns of the pyrochlore type. The chlorine could be partially replaced by Br to give mixed crystals BaCd 2 Cl 6−z Br z •5H 2 O 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.