SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF REDUCED TERNARY TANTALUMOXIDES IN THE TETRAGONAL TUNGSTEN BRONZE STRUCTURE

Single crystals of Ba3Ta5O15 were prepared by heating a pressed pellet of Ba5Ta4O15, Ta2O5, and Ta powder with 10 wt % BaCl2 in a sealed tan talum tube at 1640 degrees C for 12 h. Ba3Ta5O15 crystallizes in the t etragonal tungsten bronze structure (space group P4/mbm) with a = 12.5 90(1) Angstrom and c = 3.967(2) Angstrom. Ba cations occupy the two la rgest types of cation sites in the structure. An understanding of the thermal parameters of the Ba atom in the large tricapped trigonal pris matic site requires the inclusion of four additional more distant oxyg en atoms in the coordination sphere. Ba stoichiometry is observed to r ange from Ba3Ta5O15 to the fully oxidized Ba2.5Ta5O15 without any chan ge in structure type. A negative deviation from Vegard's law suggests some degree of cation ordering among available sites occurs. Propertie s of Ba3Ta5O15 clearly indicate it should be classified as a metal oxi de bronze. Two additional isostructural compounds, Sr3Ta5O15 (a = 12.3 79(2), c = 3.8796(8) Angstrom) and Eu3Ta5O15 (a = 12.386(3), c = 3.875 (1) Angstrom), have also been prepared.