STRUCTURAL STUDIES ON NEW TERNARY OXIDES BA8TA4TI3O24 AND BA10TA7.04TI1.2O30

The ternary oxides Ba8Ta4Ti3O24 and Ba10Ta7.04Ti1.2O30 were synthesize d and their crystal structures and defects were studied by means of X- ray powder diffraction, electron diffraction, and high resolution elec tron microscopy. The crystal structure of Ba8Ta4Ti3O24 is based on the 8H (cchc)(2) close-packed stacking (a 10.0314 Angstrom, c = 18.869 An gstrom, SG P6(3)/mcm, Z = 3) and that of Ba10Ta7.04Ti1.2O30 and on the 10H (cchcc)(2) close-packed stacking (a = 5.7981 Angstrom, c = 23.755 Angstrom, SG P6(3)/mmc, Z = 1) of BaO3 layers. The structural refinem ents gave the following values for the R factors for Ba8Ta4Ti3O24 (Ba1 0Ta7.04Ti1.2O30) R(I) = 0.041 (0.039), R(P) = 0.108 (0.118), and R(wP) = 0.094 (0.099). The main feature of both structures is the presence of two types of face-sharing octahedra (FSO) with different occupancie s by Ta atoms, Ti atoms, and vacancies, which results in the formation of a superstructure. It was shown that in the Ba8Ta4Ti3O24 structure these pairs of FSO occur in an ordered fashion and in the Ba10Ta7.04Ti 1.2O30 structure in a disordered fashion. The existence of the wide ra nge of solid solutions was shown to be also a consequence of the prese nce of one of the two types of face-sharing octahedra. (C) 1995 Academ ic Press, Inc,