Pressure effect and electron diffraction on the anomalous transition in ternary superconductor Bi ₂ Rh ₃ Se ₂

The effect of external hydrostatic pressure up to 22.23 kbar on the temperature-dependent transport properties of the ternary compound Bi ₂Rh₃Se₂ is investigated. Interestingly, the resistive anomaly at T_s~250 K, previously proposed as a charge-density-wave (CDW) transition, is shifted to higher temperature with increasing pressure, in distinct contrast to an established knowledge for CDW. Using temperature-dependent electron-diffraction characterizations, we have unraveled that this transition is, in effect, of a structural phase-transformation nature, experiencing the symmetry reduction from a high-symmetry C-centered monoclinic lattice to a low-symmetry primitive one below T_s. A more elaborately determined room-temperature C-centered lattice was also proposed.