Order-Disorder Transition at High Temperature and Microdomain Formation in Oxidized Ferrites
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ORDER-DISORDER TRANSITION AT HIGH TEMPERATURE AND MICRODOMAIN OXIDIZED FERRITES
FORMATION IN
J.C. Grenier*, M. Pouchard*, P. Hagenmuller*, M.J.R. Henche**, M. Vallet**, J.M.G. Calbet**, and M. A. Alario-Franco** *Laboratoire de Chimie du Solide du CNRS, 351, cours de la Lib~ration, 33405 Talence Cedex, France ;* Departamento de Qufmica Inorganica, de Ciencias Qufmicas, Universidad Complutense, Madrid-3, Spain.
Facultad
ABSTRACT At high temperature, nonstoichiometric perovskite-type related ferrites can be oxidized in air. At first sight, this phenomenon appears rather surprising since it corresponds to a partial formation of iron +IV. Moreover a structural transition occurs from a vacancy ordering state to a pseudodisordering one characterized by a cubic perovskite-type X-ray diffraction pattern. A TEM study has shown in fact a more complex situation attributed to a microdomain texture. This is explained from a crystallographic point of view and correlated to the existence of mixed valencies of iron.
41 nA/supposes 3
p(n co)
aT bB
A M 02.50
M 02.5 (n = 2)
kap~
(=
In previous works, it has been shown that nonstoichiometric oxides AMO3_y (0 4 y < 0.50) belonging to series (y=l/n ; n>2) the AnMnO3n-j can accomodate composition variations by means of a vacancy ordering. This that the cation is able to fit various types of coordination; a typical example is trivalent iron which can be 6 or 4-fold coordinated [1]. This was more particularly studied in the La 1 _2 yA2 yFeO3_y(A-CaSr) and CaTil_2yFe2yO3_y systems where it was found that a long range vacancy ordering occurs if y • 0.25(n.6 4). In this way, for 0.25 < y < 0.50, we observed perpendicularly to the b axis intergrowths more or less ordered of the perovskite (y=0,AMO3 ), the brownmillerite (y = 0.50,AMO2 50 and the Grenier phase (G phase) (y = 0.33, AM02. 6 7 ) (Fig. 1). This was mainly described in previous papers [2,33.
lap'if
FIG. 1. Idealized structures of AMO or AnMnO3n-1 phases for y=0, 0,50 and 0,33 (n = co,2 and 3).
For y X 0.25, M6ssbauer resonance studies and calculations of the vacancy distribution have shown the existence of a short range vacancy ordering [4,5]. These materials which were
low temperature
generally synthesized at relatively (t = 1000*C) and under controlled partial oxygen pressure
Mat.
Proc. Vol. 21 (1984) CElsevier Science Publishing Co.,
Res.
Soc. Symp.
Inc.
388
(10-25 6 po 2
10-2 atm), contain only trivalent iron.
After firing at high temperature (t > 1400'C) in air, a quite different situation arises. Two phenomena are then observed - a structural transition - an oxygen gain corresponding to a partial formation of tetravalent iron. The stabilization of iron +IV at high temperature appears to be rather surprising and needs to be elucidated. Based on HRTEM observations, we propose in this work some explanatiorsof such a phenomenon. EXPERIMENTAL LaixCaxFeO3 _ were prepared starting from nitrates which Samples of were decomposed at 800*C and fires at 1300'C. After a last annealing at 1400'C in air, the X-ray
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