TANTALUM OXIDES BY GAS-PHASE HYDROLYSIS, PRESSURE DIGESTION, AND CHEMICAL-TRANSPORT OF 2H-TAS2 - SYNTHESIS OF TT-TA2O5, T-TA2O5 AND X-RAY STRUCTURE DETERMINATION OF T-TA2O5

TT-Ta2O5 is obtained by pressure digestion in water using 2H-TaS2 as starting material at a temperature of 200-degrees-C followed by a dehydratization procedure of initially formed amorphous HTaO3. The X-ray powder diagram of TT-Ta2O5 revealed a hexagonal unit cell with space group P6/mmm and lattice constants a = 3.639, c = 3.901 angstrom. This material undergoes a phase transition during heating from 20 to 900-degrees-C. The resulting product is crystallizing orthorhombic with space group Pmm2 and lattice constants a = 43.997(8), b = 3.894(1), c = 6.209(3) angstrom and Z = 12. Single crystals of this phase (T-Ta2O5) have been prepared by chemical transport reaction using TT-Ta2O5 as starting material and NH4Cl as transport agent. The structure of T-Ta2O5 consists of pentagonal-bipyramidal coordinated tantalum atoms connected with distorted TaO6 octahedra. The Ta-O distances vary between 1.84 and 3.09 angstrom and the mean Ta-O distance is 1.99 angstrom.