Kinetic control of order in the layered perovskite La₂Ba₂Cu₂Ti₂O₁₁

Abstract

Definitive evidence on the variable degrees of order/disorder in the layered perovskite La₂Ba₂Cu₂Ti₂O₁₁ is presented. This oxide was prepared by the ceramic method with varying reaction times and cooling rates; the average and local structures of the resulting solids were studied by x-ray diffraction, electron diffraction, and transmission electron microscopy (low and high resolution). The data clearly show the presence of an a_p × a_p × 4a_p layered perovskite superstructure with a high concentration of defects in samples prepared over 48 h but with increasing degree of order for samples with longer reaction times. For samples reacted over 17 days, the order is complete and identical to that found for other related derivatives such as Ln₂Ba₂Cu₂Ti₂O₁₁ (Ln = Nd, Eu, Tb). This behavior clearly indicates the existence of a kinetic control of order in this phase.