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# Bi_{2–x}Ca_{x}Ir_{2}O_{6+y} Pyrochlore
Phases: Structure and Properties with Varied Ir Oxidation State from 3.9+ to 4.3+

journal contribution

posted on 2017-04-05, 13:54 authored by Gabriella Giampaoli, Jun Li, Arthur P. Ramirez, Arthur W. Sleight, M. A. SubramanianPyrochlore phases Bi

_{2–x}Ca_{x}Ir_{2}O_{6}O_{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.