The twinning associated with the tetragonal to orthorhombic I (T/OI) phase transition in YBCO, between 500 and 600C was considered. In situ neutron diffraction measurements revealed the coexistence of the T and split OI peaks in the vicinity of the transition. However, by direct numerical simulation, it was shown that this was to be expected from the twinned OI structure when the separation of adjacent twinning planes and the orthorhombicity both approach zero. It was demonstrated that the observed hkl-dependent peak broadening was consistent with this interpretation. A comparison of the measured and simulated peak shapes permitted an approximate estimate to be made of the actual twin widths for a given orthorhombicity.

Observations of Twinning in YBa2Cu3O6+x, 0 < x < 1, at High Temperatures. B.Khoshnevisan, D.K.Ross, D.P.Broom, M.Babaeipour: Journal of Physics - Condensed Matter, 2002, 14[41], 9763-78