Verwey-type transition in Pb$_{3}$Rh$_{7-x}$Mn$_{x}$O$_{15}$ Solid Solution

Mixed-valence compound Pb$_{3}$Rh$_{7}$O$_{15}$ shows an isotropic room temperature resistivity ($\sim $1 x 10$^{-3}$ ohm cm) that slowly increases with decreasing temperature until 185K where the resistivity increases more rapidly as temperature is further decreased. Magnetic susceptibility and thermopower measurements on Pb$_{3}$Rh$_{7}$O$_{15}$ 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 and it is likely that this transition at 185 K is related to a charge ordering. Pb$_{3}$Mn$_{7}$O$_{15}$ also crystallizes in a similar hexagonal structure. A complete solid solution of Pb$_{3}$Rh$_{7-x}$Mn$_{x}$O$_{15}$ has been prepared and crystals with varying Mn content have been grown in a PbO flux. The phase transition at 185 K fades away as the Mn substitutes for Rh. Structural, electrical and magnetic studies are discussed.