Metal–insulator transitions in reduced molybdenum oxides Sm4Mo18O32 and Nd4Mo18O32

Synthesis, structure and electronic properties are reported on single crystals of Sm4Mo18O32 and Nd4Mo18O32. The triclinic crystal structure has three distinctly different Mo metal atom clusters (Mo2, Mo4 and Mo6), which extend in complex chains. The temperature dependence of the electrical resistivity displays a broad minimum around 150–200 K with a pronounced increase in the 30–50 K range. However, unlike the related monoclinic reduced Mo oxides, A4Mo18O32 (A = Y, Gd–Yb), there is no true metal–insulator transition in the Sm and Nd analogs. We discuss these observations in terms of correlations among the Mo clusters.