Influence of Alkaline-Earth-Metal Substitutions on the Bismuth Ruthenate Structure: Bi2-xA′xRu2O6O′1-y (A′ = Mg, Ca, Sr)

Solid solutions with the formula of Bi(2-x)A'xRu2O7-y (A' = Mg, Ca, Sr; 0 <= x <= 0.2 for Mg, 0 <= x <= 1 for Ca, and 0 <= x <= 0.5 for Sr) have been synthesized and characterized. The crystal structures for these phases are found to be in the pyrochlore family, crystallizing in the cubic space group Fd (3) over barm with complex A/A' cation coordination environments. The Bi cation is found to be off-center from the ideal position because of a lone-pair distortion, while the positions of the substituted A' cations vary based on the size and ionicity. The neutron structure refinements reveal a similar propensity to off-center regarding Ca and Sr, while Mg features the largest static displacement of up to 0.48 angstrom. Interestingly, this is one of only two known pyrochlores with Mg2+ located in an 8-coordinated site. The average Ru oxidation state for each substitution is found to increase, and charge compensates for the lower divalent A' substitution. The solid solutions show temperature-independent resistance across the series with small changes in magnitude that scale with the amount of substitution, while displaying Pauli paramagnetic behavior throughout.