Abstract
Thea−b microtwinning in the 90 K superconductors of the Y(1)Ba(2)Cu(3)O(7−δ) family of perovscites is studied by transmission electron microscopy betwen 300 and 85 K. Thermally stable twin spacings of order 300 A are observed in dense polycrystalline material. In grains with free surfaces of sufficiently regular shape, the twin pattern is observed to refine at low temperature, either spontaneously or by low temperature thermal cycling, from a spacing of about 1000 A to about 300 A. In grains with free surfaces of less regular shape, the twins can be brought to disappear at low temperature after a sufficient electron dosis, and to reappear reversibly. A correlation between transition temperature and twin spacing is discussed.
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