Possible Verwey-Type Transition in Pb3Rh7O15

Crystals of the mixed-valence compound Pb3Rh43+Rh34+O15 have been grown in a PbO flux. These hexagonal crystals show an isotropic electrical resistivity of about 1 × 10−3 ohm cm at room temperature. This resistivity slowly increases with decreasing temperature until 185 K, where the resistivity begins increasing more rapidly as temperature is further decreased. Specific heat measurements on polycrystalline Pb3Rh7O15 show a sharp, lambda-shaped peak at 185 K. Magnetic susceptibility and thermopower measurements on Pb3Rh7O15 also show a discontinuity at about 185 K. Structural analyses of X-ray diffraction data obtained above and below 185 K indicate that a change in space group has occurred at 185 K. It is likely that the transition at 185 K is related to a change in charge modulation. The phase transition at 185 K fades away as Bi is substituted for Pb in Pb3Rh7O15.