FORMATION OF PEROVSKITE TYPE LACRO3 BY PYROLYSIS OF LA(III)-CR(VI) PRECURSOR

Perovskite type lanthanum chormium oxide, LaCrO3, was prepared by pyrolysis of La. (CH3COO)(CrO4).n H2O (n = 2) precursor which was obtained as residue of the evaporation of an equimolar mixture of lanthanum acetate and chromium trioxide. The pyrolysis process was investigated by TG-DTA, XRD, IR, EPR, chemical analysis, and elemental analysis. It was found that pyrolysis reaction of the precursor is affected by partial pressure of oxygen, P(O2), in the atmosphere. An increase in P(O2) caused a change in mass loss-temperature curves from a one-step to a two-steps type (Figs.3, 8). For P(O2) greater-than-or-equal-to 0.10 atm, the reaction clearly proceeded in two steps via the formation of monoclinic LaCrO4 as an intermediate (Figs.4, 5). The first step was corresponded to the combustion of acetate moiety in the precursor and crystallization to form LaCrO4, and the second step was an elimination of the oxygen of LaCrO4 and the phase transition to orthorhombic LaCrO3. For P(O2) less-than-or-equal-to 0.028 atm, LaCrO3 was formed by the one-step reaction, and the transition temperature to LaCrO3 was lower than at higher P(O2) (Fig.8). The intermediate compound, LaCrO4, was obtained as a single phase at 575-625-degrees-C in air, and it was stable at 600-degrees-C for at least 12 h (Fig. 10). Specific surface area of LaCrO3 formed by 3h pyrolysis at 650-degrees-C in N2 was 19+/-2 m2.g-. A-site substituted perovskite type oxide could also be prepared by a similar method La0.8Ca0.2Cro3 was prepared by pyrolysis at 800-degrees-C for 5 h in N2 from the precursor, which was obtained from a mixture of lanthanum acetate, calcium carbonate, and chromium trioxide (Fig. 14).