Bi_2–<i>x</i>Ca_<i>x</i>Ir₂O_6+<i>y</i> Pyrochlore Phases: Structure and Properties with Varied Ir Oxidation State from 3.9+ to 4.3+

Pyrochlore phases Bi_2–<i>x</i>Ca_<i>x</i>Ir₂O₆O_<i>y</i>^′ with <i>x</i> 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 <i>x</i>, above 4+ for high <i>x</i>, and is very close to 4+ at <i>x</i> = 0.5. All samples show metallic properties with an electrical resistivity of ∼10^–3 Ω·cm at room temperature. For low <i>x</i>, the sign of the Seebeck coefficient is negative at low temperature but becomes positive at high temperature. For high <i>x</i>, 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 <i>x</i>. All Bi is in its ideal position for all values of <i>x</i>, 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.