Bi2–xCaxIr2O6+y Pyrochlore Phases: Structure and Properties with Varied Ir Oxidation State from 3.9+ to 4.3+

Pyrochlore phases Bi2–xCaxIr2O6Oy′ 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.

Pyrochlore 相 Bi2-xCaxIr2O6Oy′（其中 x 的取值范围为 0.0 至 1.0）的评估基于对中子衍射数据的 Rietveld 分析，以及 3 至 756 K 范围内的电导率和热电势数据，以及 3 至 298 K 范围内的磁化率数据。在低 x 值下，平均 Ir 氧化态小于 4+，在高 x 值下超过 4+，而在 x = 0.5 时，氧化态与 4+ 非常接近。所有样品均表现出金属特性，在室温下电导率约为 ∼10–3 Ω·cm。对于低 x 值，塞贝克系数在低温下为负，但在高温下变为正。对于高 x 值，塞贝克系数在 3 至 756 K 范围内均为正。磁性测量表明，对于所有 x 值，在 3 K 以下均未观察到磁序。在所有 x 值下，Bi 均处于其理想位置，但大部分 Ca 离子从理想的 A 阳离子位置发生了强烈偏移。这种 Ca 离子的偏移显然仅在 O′ 位点上存在相邻空位时才会发生。