RE3Mo14O30 and RE2Mo9O19, Two Reduced Rare-Earth Molybdates with Honeycomb-Related Structures (RE = La–Pr)

The previously unreported RE 3 Mo 14 O 30 and RE 2 Mo 9 O 19 phases were synthesized in vacuo from rare-earth oxides, molybdenum oxide, and molybdenum metal using halide fluxes at 875-1000 °C. Both phases adopt structures in the triclinic P1̅ space group albeit with several notable differences. The structures display an ordering of layers along the a direction of the unit cell, forming distinct honeycomb-related lattice arrangements composed of MoO 6 octahedra and vacancies. Mo-Mo bonding and clusters are present; the RE 3 Mo 14 O 30 structure contains Mo dimers and rhomboid tetramers, while the RE 2 Mo 9 O 19 structure contains rhomboid tetramers and an unusual arrangement of planar tetramers, pentamers, and hexamers. The magnetic measurements found the RE 2 Mo 9 O 19 phases to be simple paramagnets, while La 3 Mo 14 O 30 was observed to order antiferromagnetically at 18 K. Electrical resistivity measurements confirmed all of the samples to behave as nondegenerate semiconductors.