Abstract
Metal oxides that have particularly strong electrical responses very frequently have the perovskite structure, or incorporate elements of that structure into more complex arrangements. These include ferroelectrics, piezoelectrics, and superconducting oxides. These structures often involve alternating layers in coherent, intergrowth patterns. It will be shown how a wide variety of active metal oxides with extremely complex compositions can be organized into systematic families. The perovskite structure is so favorable in these materials that it persists even when the composition is far from the ideal ABO 3 .