Synthesis, Crystal Structure, and Electrical and Magnetic Characterizations of the Monoclinic Compounds Ln 3 Mo 4+ x Si 1- x O 14 (Ln = La, Ce, Pr, and Nd; x = 0 and 0.35) Containing Mo 3 Clusters and Mo 2 Dimers.

Powder samples of the new monoclinic compounds Ln 3 Mo 4 SiO 14 (Ln = La, Ce, Pr, and Nd) and single crystals of Pr 3 Mo 4.35 Si 0.65 O 14 were obtained by the solid-state reaction. The crystal structure of Pr 3 Mo 4.35 Si 0.65 O 14 was determined by single-crystal X-ray diffraction. Pr 3 Mo 4.35 Si 0.65 O 14 crystallizes in the monoclinic space group P 2 1 / 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 F 2 using 7544 independent reflections for 203 refinable parameters results in R 1 = 0.0359 and w R 2 = 0.0831. The structure contains chains of Mo 3 O 13 clusters and chains of edge-sharing MoO 6 octahedra with alternately short (2.508 Å) and long (3.161 Å) Mo-Mo distances running parallel to the a axis that are separated by 8- or 10-coordinate Pr-O polyhedra. Magnetic susceptibility measurements on the Ln 3 Mo 4 SiO 14 (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 confirms the presence of one unpaired electron per Mo 3 as expected. Resistivity measurements on a single crystal show that Pr 3 Mo 4.35 Si 0.65 O 14 is a small band gap semi-conductor.