A crystal of alpha -lead phosphate having planar domain walls in at least two different directions is heated to establish a temperature gradient such that the isotherms are essentially planar and the Curie temperature isotherm crosses the crystal so that a region of the crystal containing only a single set of domain walls remains below the Curie temperature, the domain walls being at a large angle to the isotherm. On cooling so that the Curie point isotherm moves laterally through the plate, the domain walls extend behind the isotherm to cover the plate. Crystals so poled can be utilized in optical switches and line scanners wherein the crystal plate is switched from one strain state to another by lateral motion of the domain walls.
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