Synthesis and characterization of novel Group VI metal (Mo, W) nitride and oxide compounds
Binary Group VI metal nitrides are characterized by thermodynamic instability towards dissociation to N2 and the N-saturated elements at high temperature. Our investigations into the preparation of tungsten nitrides have involved the synthesis of molecular precursors, and their conversion to tungsten nitrides at relatively low temperatures. Two interesting molecular precursors, (WNCl3·NCCH3] 4 and WN(N3)3· xNCCH3, have been prepared and characterized. The molecular structure of (WNCl3·NCCH3] 4 consists of a W4N4 tetrameric core with multiple and single W-N bonds arranged in an alternating fashion. Three new solid state phases, amorphous W3N5, cubic WN, and W2N2(C2N2), have been discovered by solid state and chemical vapor transport reactions. Cubic WN has a rock salt structure. W2N2(C2N2) has a novel three-dimensional network structure consisting of W2 dimers, hydrazido ligands N24-, and 1,4-diazabutenido ligands represented by three resonance structures, (N-C=C-N) 6-, (N=C-C-N) 6- and (N-C-C=N) 6-;Ternary reduced molybdenum oxides have caused great interest, since the discovery of NaMo4O6 containing trans-edge-shared Mo6 octahedra. Recent investigations have yielded a new family of LnMO8O14 compounds containing cis- and trans-Mo8 bicapped octahedra. A systematic study in the Ln2O3-MoO3-MO (Ln = La, Ce, Pr, Nd, Sm) system has been explored to better understand LnMO8O14. The study has shown that the sizes of the rare-earth cations affect the formation of these phases. Larger cations (La, Ce, and Pr) aid in the formation of trans-Mo8 bicapped octahedra, and the smaller cations (Nd, Sm) only stabilize the cis-Mo8 bicapped octahedra. Magnetic susceptibility measurements have indicated that no effective moment contribution arises from the Mo8 metal clusters, even though the cis-Mo8 cluster in LnMo8O14, containing all cis-Mo8 octahedra, apparently contains an odd number of electrons (23). Electrical resistivity measurements and electronic structure calculations have shown that the LnMo8O14 compounds containing cis-Mo8 clusters are metallic, and the LnMo8O14 compounds containing a 1:1 ratio of cis- to trans-Mo8 clusters are semiconducting.