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

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 (T_IM) 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 Mo_ncluster chains.