Demixion of the K2NiF4 type aluminate LaCaAlO4: precursor role of the local ordering of lanthanum and calcium

The modeling of the modulation of the X-ray diffuse scattering background i s applied to get evidence of local ordering phenomena of La3+ and Ca2+ in t he K2NiF4 type aluminate LaCaAlO4. Such short range ordering phenomena play a precursor role in the demixion of the intergrowth structure of LaCaAlO4 in LaAlO3 and CaO, the perovskite and rock-salt type parent oxides, respect ively. As the first stage of formation of twinned LaAlO3 precipitates with the matrix, a La3+ enrichment occurs in the {113} planes of the tetragonal cell of LaCaAlO4. The tendency to get simultaneously a short range ordering of the first nine neighbours and a clustering of the 8 second neighbours o f Ca2+ accounts well for the formation of 'LaO3' compact planes, ensuring t he progressive coming out of the perovskite structure. In comparison with L aCaAlO4, no local ordering is detected in the stable aluminate NdCaAlO4, as in YCaAlO4 a slight tendency to get a short range ordering of both the fir st and second neighbours results in the formation of mixed 'Y,CaO3' compact planes. The local ordering phenomena prove not to be contradictory to the overall randomness of the cationic distribution which is the usual crystal- chemical property of the A(3+)A'2+AlO4 aluminates. (C) 1999 Elsevier Scienc e S.A. All rights reserved.