Synthesis, Crystal Structure, and Electrical and Magnetic Characterizations of the Monoclinic Compounds Ln₃Mo₄₊Si_1-O₁₄ (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 Pr₃Mo_4.35Si_0.65O₁₄ were obtained by the solid-state reaction. The crystal structure of Pr₃Mo_4.35Si_0.65O₁₄ was determined by single-crystal X-ray diffraction. Pr₃Mo_4.35Si_0.65O₁₄ 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 (3.161 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 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 confirms the presence of one unpaired electron per Mo₃ as expected. Resistivity measurements on a single crystal show that Pr₃Mo_4.35Si_0.65O₁₄ is a small band gap semi-conductor.