RESEARCH ON THE BABIO3-DELTA SYSTEM -THAN-OR-EQUAL-TO-DELTA-LESS-THAN-OR-EQUAL-TO-0.5)

The reduction isobar T vs delta (pO(2) = 0.21 atm) of BaBiO3-delta bet ween 750 and 1050 degrees C shows a trend with three near linear secti ons in the ranges 750-850 degrees C, 850-1000 degrees C, and 1000-1045 degrees C, respectively. The quenching in liquid N-2 of first-section solids (phase I) does not prevent the transformations Fm(3) over bar m --> R (3) over bar --> I2/m typical of the stoichiometric compound a nd their XRD patterns at room temperature are not different from that of the stoichiometric BaBiO3. The quenching of the cubic structure Pm( 3) over bar m of solids in the second section (phase II) still origina tes a monoclinic structure, but one strongly perturbed by the high con centration of oxygen vacancies. As already pointed out by other author s, by neutron diffraction at high temperatures, the phase II to phase III transition (third section of the isobar) occurs, unusually, in a g radual way and it is nearly complete about the melting temperature (10 45 degrees C). The close resemblance of the XRD patterns of the terms of the Aurivillius series with those of BaBiO2.56 (phase III) leads us to think that this last phase is the term limit of the above series. Its structure would therefore be characterized by a random distributio n of Ba and Bi ions in the perovskite A and B sites. The reduction of BaBiO3 in high purity N-2 (pO(2) approximate to 10(-6) atm) yields the polymorphous solid BaBiO2.50. At 1000 degrees C it still shows a stru cture with the tetragonal symmetry of phase III. Between 1000 and 800 degrees C this bright red solid shows a pseudocubic perovskite-type st ructure, while for lower temperatures it assumes a bright yellow color and a structure with monoclinic no-longer-perovskite-type symmetry. T he equilibria between the solids of the examined system were also stud ied through annealing and quenching in water of solids with delta = 0. 2, 0.3, and 0.44, sealed in silica glass tubes. The results agree with the diagram already published by Beyerlein et al. (R. A. Beyerlein, A . J. Jacobson, and L. N. Yacullo, Mater. Res. Bull. 20, 877 (1985)) on ly for T values higher than 700 degrees C. The eutectoid decomposition of phase II (delta = 0.2) to phase I and phase III (at T = 668 +/- 2 degrees C) and the subsequent decomposition of phase III to phase I pl us the yellow phase BaBiO2.5 (at 663 +/- 2 degrees C) disagrees, on th e contrary, with the results of the above authors. The results of the simultaneous DTA, TG, and DTG analyses on the stoichiometric BaBiO3 at pO(2) = 0.21 and 1 atm leads us to think that monophasic region II (< Pm(3)over bar >m) for adequate pO(2) (pO(2) > 1 atm) extends as far as the stoichiometric solid itself (delta = 0). (C) 1995 Academic Press, Inc.