Preparation, thermal expansion, high pressure and high temperature behavior of Al2(WO4)3

The titled compound Al-2(WO4)(3) was synthesized by a conventional solid state reaction and characterized by powder XRD. It crystallizes in an orthorhombic (Pbcn, No. 60) lattice, with unit cell parameters as 12.582(2), 9.051(1), 9.128(2) Angstrom, and V = 1039.5(3) (Angstrom)(3). The compound was found to show negative thermal expansion (NTE) behavior in the temperature range of 25 to 850degreesC. The average linear NTE coefficient (alpha(1)), in this temperature range, was -1.5 x 10(-6) K-1. The effect of pressure at ambient temperature, was studied by a Bridgman Anvil (BA) apparatus, to reveal that there is no irreversible phase transition up to 8 GPa. The effect of high pressure and high temperature on this compound was studied by a Toroid Anvil (TA) apparatus. This compound has a limited stability under high pressure and temperature, as it undergoes a decomposition to AlWO4 and WO3-x with a partial oxygen loss. As an off-shoot of this work, certain new modifications of WO3-x under pressure and temperature were observed, viz., monoclinic, tetragonal and an orthorhombic modifications at 5 GPa/1400degreesC, 3 GPa/900degreesC and 1.8 GPa/1030degreesC, respectively. The detailed XRD studies of the products are presented here. (C) 2002 Kluwer Academic Publishers.