Structural Determination of Epitaxial Formations in Layered Cuprates

Abstract

From the discovery of high-Τ superconductors the layered cuprates were carefully analyzed in order to reveal the correlation between the structure and physical properties. Basing on the experimental observations, the temperature of the transition to superconducting state has been correlated with the number of Cu-O planes and with particular interatomic distances, hence emphasizing the role of crystalline structure. Due to the growth problems of thin films and heteroepitaxial layers, a further attention was paid to the study of commonalities and differences between superconducting compounds of various compositions and cocrystallizing phases. As a result, various classification schemes have been developed in order to reveal the general regularities in the structure for the whole group of cuprates. The aim of the present paper was to analyze the genetic relationship between the structures of various layered cuprates, to which the major part of high-Τ superconductors is attributed. It will be shown that from the viewpoint of the structural regularities, the formation of epitaxial intergrowths is not only typical but a predetermined feature for the family of these compounds. Previous classification schemes of cuprate structures [1-3] distinguished the following main features: (i) the layered character of structure; (ii) the presence of copper-oxygen fragments, containing single Cu-O plane or doubled Cu-O planes with cation layer between them; (iii) the existence of blocking layers of rock-saltand fluorite-type which are located between the Cu-O fragments; (iv) a similarity of cuprate structure to the perovskite.