Odd-parity multipole order in the spin-orbit coupled metallic pyrochlore Pb2Re2O7-δ

The pyrochlore oxide Pb2Re2O7-delta (PRO) is a candidate spin-orbit coupled metal (SOCM) that exhibits a structural phase transition with inversion symmetry breaking. In this paper, we report on the results of detailed x-ray diffraction (XRD) measurements on single crystals of PRO to clarify the crystal structure below the phase transition temperature at Ts = 300 K. In the XRD patterns, a clear peak splitting is observed below Ts, indicating a cubic-to-tetragonal transition. Based on the group-subgroup relationship and the observed reflection conditions, the space group of the low-temperature phase is proposed to be I4122, which agrees with optical second harmonic generation measurements. This space group is the same as that of the lowest-temperature structure of the analogous SOCM Cd2Re2O7 (CRO), which is realized by the emergence of odd-parity multipole order. The comparison between PRO and CRO allows for advancing our understanding of the symmetry-lowering complex order exhibited by SOCMs.