Synthesis, structure, and reactivity of novel lanthanum tungstates

Two new compounds, La₂W₃O₁₂ and LaFeW₃O₁₂, were prepared from La₂O₃ and WO₃ or from Fe₂O₃, La₂O₃, and WO₃. La₂W₃O₁₂ has a structure which can be considered as a distorted superstructure of the scheelite structure CaWO₄ in which 1/3 of the Ca sites are vacant. It is monoclinic, with space group C2/c and a = 7.873(2) Å, b = 11.841(2) Å, c = 11.654(2) Å, and β = 109.25(3)°. LaFeW₃O₁₂ is triclinic with a = 7.569(3) Å, b = 7.537(2) Å, c = 32.41(2) Å, α = 90.13(4)°, β = 94.75(3)°;, and γ = 98.90(3)°, and the structure consists of two distinct layers, one of which is a La tungstate which consists of a sheet of La in eight coordination and a sheet of WO₄ tetrahedra and another which consists of a sheet of edge-sharing FeO₆ octahedra sandwiched between two sheets of edge-connected WO₆ octahedra. The reactivity of these compounds with H₂ was investigated by TGA. La₂W₃O₁₂ is reduced at 1100°C to yield a mixture of La₂O₃ and W metal. The products of the reduction of LaFeW₃O₁₂ were found to depend on the composition of the atmosphere. In N₂ with 5% H₂ the compound yields a complex mixture of La_x(WO₃)_y compounds with W and a W-Fe alloy, while in 100% H₂, La₂O₃, W, and the alloys Fe₆W₇ or Fe₂W are formed.