Bi_2-xCa_xIr₂O_6+y Pyrochlore Phases: Structure and Properties with Varied Ir Oxidation State from 3.9+ to 4.3

Pyrochlore phases Bi_2-xCa_xIr₂O₆O_y^' with x from 0.0 to 1.0 have been evaluated based on Rietveld analysis of neutron diffraction data, electrical resistivity and thermopower data from 3 to 756 K, and magnetic susceptibility data from 3 to 298 K. The average Ir oxidation state is less than 4+ at low x, above 4+ for high x, and is very close to 4+ at x = 0.5. All samples show metallic properties with an electrical resistivity of ∼10^-3 Ω·cm at room temperature. For low x, the sign of the Seebeck coefficient is negative at low temperature but becomes positive at high temperature. For high x, the sign of the Seebeck coefficient is positive from 3 to 756 K. Magnetic measurements indicate no magnetic ordering down to 3 K for all values of x. All Bi is in its ideal position for all values of x, but much of the Ca is strongly displaced from the ideal A cation site. This displacement of Ca apparently only occurs when there is an adjacent vacancy at the O' site.