Inorganic syntheses and properties of novel hexagonal layered compounds

The structures of a new family of hexagonal layered molybdenum, tungsten, and vanadium oxide compounds are described. Hydrothermal techniques were used to survey the syntheses of several molybdenum and tungsten selenites of the formula {dollar}rm Asb2(MOsb3)sb3SeOsb3{dollar} (where A = Cs, Rb, Tl or NH{dollar}sb4{dollar} and M = Mo and W), and the vanadium selenites with the formula {dollar}rm A(VOsb2)sb3(SeOsb3)sb2{dollar} (where A = K, NH{dollar}sb4{dollar}, Tl and Rb). These compounds possess structures which contain hexagonal-WO{dollar}sb3{dollar} (HTO) type layers composed of corner sharing MO{dollar}sb6{dollar} octahedra. In the vanadium series three adjacent octahedra are capped by Se atoms on both faces of the hexagonal layer. However, in the molybdenum and tungsten series the Se atom caps only one side of the hexagonal layer. The monovalent cations occupy inter-layer sites to compensate for the charge of the anionic layers.; In light of the unusual layers structures formed, an alternative capping anion was chosen, {dollar}rm (CHsb3POsb3)sp{lcub}2-{rcub}{dollar}. This resulted in the syntheses of {dollar}rm A(VOsb2)sb3(POsb3CHsb3)sb2{dollar} (where A = K, NH{dollar}sb4{dollar}, Tl and Rb), and {dollar}rm Asb2(MOsb3)sb3POsb3CHsb3{dollar} (where A = Cs, Rb, Tl and NH{dollar}sb4{dollar}, and M = W and Mo). For the vanadium phases double capping also occurs, while with molybdenum and tungsten compounds the {dollar}rm CHsb3POsb3sp{lcub}2-{rcub}{dollar}anion also caps only one side of the hexagonal layer.; All of these phases have non-centrosymmetric space groups and contain MO{dollar}sb6{dollar} octahedra that are distorted. In the vanadium compounds the V(V) atom is displaced toward an octahedral edge, whereas in the molybdenum compounds the Mo(VI) atom is displaced toward an octahedral face resulting in three short and three long Mo-O bond distances. Due to the stacking of the layers the compounds with a SeO{dollar}sb3{dollar} group are hexagonal (P6{dollar}sb3{dollar}) while the compounds with a {dollar}rm CHsb3POsb3{dollar} group are rhombohedral (R32 and R3). In the vanadium, molybdenum and tungsten selenites the layers form a two layer repeating sequence, whereas in the vanadium, molybdenum and tungsten methylphosphonates the layers form a three layer repeating sequence. In all these structure types the layers are displaced in the hexagonal ab plane with respect to the adjacent layers, resulting in no infinite 6-ring channels like those observed in HTO.; The thermal decomposition of the alkali metal selenite phases result in the sublimation of SeO{dollar}sb2{dollar} and the formation of alkali metal vanadates, molybdates and tungstates. In contrast, the decomposition of {dollar}rm (NHsb4)sb2(WOsb3)sb3SeOsb3{dollar} results in the formation of hexagonal WO{dollar}sb3.{dollar} Decomposition of {dollar}rm (NHsb4)sb2(MoOsb3)sb3SeOsb3{dollar} form yet another hexagonal phase unlike HTO. The solid solutions with the composition {dollar}rm (NHsb4)sb2(Wsb{lcub}1-x{rcub}Mosb{lcub}x{rcub}Osb3)sb3SeOsb3{dollar} were synthesized and characterized. The decomposition products for a complete series of solid solutions were studied. The hexagonal WO{dollar}sb3{dollar} phase is stable over a similar composition range to the previously reported solid solutions {dollar}rm Wsb{lcub}1-x{rcub}Mosb{lcub}x{rcub}Osb3cdot1/3Hsb2O{dollar} reported by Figlarz (1989).; The reaction conditions surveyed also led to the synthesis of a number of other new compounds, for example the molybdenum selenite chain structures with the formula {dollar}rm Asb2(MoOsb3)sb6SeOsb3cdot xHsb2O{dollar} (A = Rb, K, Na). A new chain structure without a Se or P atom incorporated into the structure was also formed, tris(piperazinium) octamolybdate, ({dollar}rm Nsb2Csb4Hsb{lcub}12{rcub}rbracksb3Mosb8Osb{lcub}27{rcub}.{dollar} Other compounds were cluster anions such as the salt of the Keg...