Pressure-Induced Suppression of Static Disorder and Lattice Distortion in Pyrochlore Bi2Ru2O7
Bi2Ru2O7 is a weak metal pyrochlore with static disorder from lone-pair electrons. The suppression of static disorder can be observed by in situ Raman spectroscopy, and the abnormal turning point in resistivity can be found at high pressure. X-ray photoelectron spectroscopy (XPS) of bismuth and ruthenium, low-temperature magnetic susceptibility, and magnetization curve were studied, indicating that there were different valence states between bismuth and ruthenium. The in situ high-pressure structure evolution and conductivity in Bi2Ru2O7 were investigated up to 30.4 and 32.7 GPa, respectively. The electronic structure evolution was studied by first-principles calculations up to 30.0 GPa. Some of the Raman peaks related to static disorder were suppressed at around 2.5 and 18.0 GPa, respectively. The [RuO6] octahedron was compressed and distorted with pressure, and the triangular distortion increased in the range of 18.0 and 24.4 GPa. The high-pressure resistivity was measured by the four probe method. It slowly decreased at first and then increased slowly after 2.5 GPa due to the effects of static disorder and the applying pressure. After 18.0 GPa, the resistivity increased rapidly because of the pressure-induced lattice distortion, which led to the weakening of the conductivity of the sample under high pressure. All observations showed that the lattice distortion was influenced by the combined effects of static disorder and pressure before 18.0 GPa, which caused the resistivity to decrease first and then increase. As static disorder is suppressed, Bi2Ru2O7 is close to standard pyrochlore and the conductivity after 18.0 GPa, which is consistent with the theoretical calculation results of band gap.
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