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

Pyrochlores, ABO, containing late transition metals have gained considerable recent interest with the discovery of superconducting behaviour in CdReO and the characterisation of a pure Slater-like metal-semiconductor transition in CdOsO. 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 CaCdOsO. The synthesis of a range of 2 and 3 row transition metal pyrochlores has been achieved by direct reaction of component metal oxides in sealed tubes. Products, which include CaOsO, HgOsO, CaCdOsO, CdReO, CdRuO, HgOsO, TlOsO, and PbOsO 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. CaOsO adopts the weberite structure while the other ABO type compounds crystallise with the pyrochlore structure. Calcium may be doped into CdOsO, as in CaCdOsO, 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.