Observations of twinning in YBa2Cu3O6+x, 0 < x < 1, at high temperatures

We report here on the nature of the twinning associated with the tetragonal to orthorhombic I (T/OI) phase transition in YBCO, for temperatures between 500 and 600ºC. In situ neutron diffraction measurements show the coexistence of the T and split OI peaks in the vicinity of the transition. However, by direct numerical simulation, we show that this is to be expected from the twinned OI structure when the separation of adjacent twinning planes and the orthorhombicity both approach zero. It is demonstrated that the observed hkl-dependent peak broadening is consistent with this interpretation. A comparison of the measured and simulated peak shapes enables us to make an approximate estimate of the actual twin widths for a given orthorhombicity