Synthesis, crystal structure, and electrical and magnetic characterizations of the monoclinic compounds Ln3Mo4+xSi1-xO14 (Ln = La, Ce, Pr, and Nd; X = 0 and 0.35) containing Mo3 clusters and Mo2 dimers

Powder samples of the new monoclinic compounds Ln3Mo4SiO14 (Ln = La, Ce, Pr and Nd) and single crystals of Pr3Mo4.35Si0.65O14 were obtained by solid state reaction. The crystal structure of Pr3Mo4.35Si0.65O14 was determined by single-crystal X-ray diffraction. Pr3Mo4.35Si0.65O14 crystallizes in the monoclinic space group P21/n with unit-cell parameters a = 5.6361 (2) Å, b = 17.5814 (8) Å, c = 10.9883 (4) Å and Z = 4. Full-matrix least-squares refinement on F2 using 7544 independent reflections for 203 refinable parameters results in R1 = 0.0359 and wR2 = 0.0831. The structure contains chains of Mo3O13 clusters and chains of edge-sharing MoO6 octahedra with alternately short (2.508 Å) and long (3.162 Å) Mo-Mo distances running parallel to the a axis that are separated by eight- or ten-coordinate Pr-O polyhedra. Magnetic susceptibility measurements on the Ln3Mo4SiO14 (Ln = La, Ce, Pr and Nd) are in agreement with a trivalent state of the rare earths for the Ce, Pr and Nd compounds, while that on the lanthanum compound confirm the presence of one unpaired electron per Mo3 as expected. Resistivity measurements on a single crystal show that Pr3Mo4.35Si0.65O14 is a small band gap semi-conductor.