SYNTHESIS, STRUCTURE, AND REACTIVITY OF NOVEL LANTHANUM TUNGSTATES

Two new compounds, La2W3O12 and LaFeW3O12, were prepared from La2O3 an d WO3 or from Fe2O3, La2O3, and WO3. La2W3O12 has a structure which ca n be considered as a distorted superstructure of the scheelite structu re CaWO4 in which 1/3 of the Ca sites are vacant. It is monoclinic, wi th space group C2/c and a = 7.873(2) angstrom, b = 11.841(2) angstrom, c = 11.654(2) angstrom, and beta = 109.25(3)degrees. LaFeW3O12 is tri clinic with a = 7.569(3) angstrom, b = 7.537(2) angstrom, c = 32.41(2) angstrom, alpha = 90.13(4)degrees, beta = 94.75(3)degrees, and gamma = 98.90(3)degrees, and the structure consists of two distinct layers, one of which is a La tungstate which consists of a sheet of La in eigh t coordination and a sheet of WO4 tetrahedra and another which consist s of a sheet of edge-sharing FeO6 octahedra sandwiched between two she ets of edge-connected WO6 octahedra. The reactivity of these compounds with H-2 was investigated by TGA. La2W3O12 is reduced at 1100-degrees -C to yield a mixture of La2O3 and W metal. The products of the reduct ion of LaFeW3O12 were found to depend on the composition of the atmosp here. In N2 with 5% H-2 the compound yields a complex mixture of La(x) (WO3)y compounds with W and a W-Fe alloy, while in 100% H-2, La2O3, W, and the alloys Fe6W7 or Fe2W are formed. (C) 1994 Academic Press, Inc .