Irreversible expansion and distortion relief of bismuth ruthenate under high temperature and high pressure

Bi2Ru2O7 is a weak metal pyrochlore with good conductivity. Due to the static disordered displacement caused by the Bi 6s lone pair, which distorts the [RuO6] octahedron, the t2g orbitals split into e'g and a1g states. This results in the opening of a bandgap at the Fermi level, leading to reduced electrical conductivity in the sample compared to its standard structure. The electrical resistivity, temperature difference, and Seebeck coefficient of the sample were synchronously observed using high-temperature and high-pressure in situ testing assembly. The Seebeck coefficient and temperature discontinuity of the sample occurred at 4.0 GPa at 878.9 K, and the final value of resistivity showed a significant decrease after returning to ambient temperature at 4.0 GPa, but remained within the range of 10−4 Ω m. The x-ray diffraction (XRD) analysis after high-temperature and high-pressure treatment confirmed that the sample underwent irreversible expansion at 4.0 GPa at 878.9 K. In situ high-temperature XRD indicates that irreversible expansion of the sample cannot be achieved by high-temperature treatment at ambient pressure. High-temperature and high-pressure treatment effectively expands the atomic spacing, which can alleviate the displacement of shared oxygen in [RuO6] octahedra caused by the Bi 6s lone pair, reduce octahedral distortion, and thus improve the semi-metallic bandgap caused by static disordered displacement, which provides a new idea for improving the performance of bismuth ruthenate and its composite electrode materials. In addition, expansion is beneficial for the replacement process of the sample as a matrix material, indicating its potential application value in the field of nuclear waste solidification and as a doped matrix material.