Structure and electronic properties of the heavy transition metal pyrochlores and weberitet A2B2O7, A=Ca, Cd, Hg, Tl, Pb; B=Os, Ru, Re

Pyrochlores, A(2)B(2)O(7), containing late transition metals have gained considerable recent interest with the discovery of superconducting behaviour in Cd2Re2O7 and the characterisation of a pure Slater-like metal-semiconductor transition in Cd2Os2O7. In this presentation we will report the synthesis and full structural characterisation of a number of new and known phases of this type and in particular the solid solution Ca2-xCdxOs2O7. The synthesis of a range of 2(nd) and 3(rd) row transition metal pyrochlores has been achieved by direct reaction of component metal oxides in sealed tubes. Products, which include Ca2Os2O7, Hg2Os2O7, Cd2-xCdxOs2O7, Cd2Re2O7, Cd2Ru2O7, Hg2Os2O7, Tl2Os2O7-y, and Pb2Os2O7-x have been characterised by, powder neutron and X-ray diffraction over the temperature range 2K-900K; non-absorbing cadmium isotopes have been used where appropriate. Ca2Os2O7 adopts the weberite structure while the other A(2)B(2)O(7) type compounds crystallise with the pyrochlore structure. Calcium may be doped into Cd2Os2O7, as in Ca2-xCdxOs2O7, up to x =0.5 with retention of the pyrochlore structure. The electronic properties of these materials as a: function of structure and temperature are reported; all materials show metallic or poor metallic behaviour near room temperature.