Anomalous Metal–Insulator Transitions in Reduced Molybdenum Oxides,A4Mo18O32(A=Ca, Y, Gd–Yb) with Mon(n=2, 4, 6) Cluster Chains

Abstract

Single crystals ofA4Mo18O32(A=Ca, Y, Gd–Yb) large enough for transport measurements were grown by high-temperature reactions or molten-salt electrolysis and their electrical and magnetic properties were investigated. The crystal structure of these phases indicates the presence of three distinctly different Mo metal atom clusters (Mo2, Mo4, and Mo6), which extend in complex chains along the monoclinicbaxis. The temperature-dependent resitivity displays a sharp metal–insulator transition in the range 70–120 K for the Y and all the rare earth members. The metal–insulator transition temperature (TIM) was observed to increase with decreasing size of the rare earth ions. No anomalies were observed in their magnetic susceptibilities at temperatures in the vicinity of metal–insulator transition. The corresponding Ca phase, however, is semiconducting. An attempt has been made to correlate the observed physical properties with the structural arrangement of the Moncluster chains.