We propose a modified classification scheme for the crystal structures of the CuO2 layer based high temperature superconductors (Cu-HTSs) and related materials. Six layers of different atomic arrangements are derived from the perovskite, rock-salt, and fluorite structures. Stacking sequences of these six building layers are used to describe the crystal structures of all Cu-HTSs and related materials. The copper-oxygen configuration, the approximate unit cell size, and general symmetry information are easily obtained from the layer stacking sequence. A preference for certain elements due to charge and size considerations in different building layers is found. This elemental preference provides additional information in phase identification and in the design of new Cu-HTSs.
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