Abstract
Interfacial structures of c-axis-oriented YBa2Cu3O7–y (Y123) and Nd1+xBa2–xCu3O7–y (Nd123) films were investigated by high-resolution transmission electron microscopy (HRTEM) in conjunction with geometrical lattice matching and molecular orbital calculations. These films were formed on MgO(001) substrates by liquid-phase epitaxy. Despite the similarity in lattice constants between Y123 and Nd123, the in-plane orientation relationship (OR) to the substrates is different: [100]film//[100]substrate(I) for Y123 and [110]film//[100]substrate(II) for Nd123. From the results of HRTEM observations and image simulations, it was found that the Y123 and Nd123 films are terminated by BaO and CuO-chain layers at the interfaces, respectively. For both the Y123/MgO and Nd123/MgO systems, the OR(I) is assessed to be the most favorable in point of geometrical matching and the OR(II) is the second among the rotational misorientations on the [001]film and [001]MgO. The molecular orbital calculations reveal that the interface with the OR(II) and the CuO-chain layer termination is preferable in terms of covalent bonding for both the systems. Consequently, we suggest that the preferential interfacial structures are delicately determined by a balance of the geometrical and chemical factors at the interfaces, resulting in making the lowest interfacial free energies.
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