The nature of the twinning associated with the tetragonal-to-orthorhombic phase transition at 500 to 600C was studied. In situ neutron diffraction measurements revealed the coexistence of T and split OI peaks in the vicinity of the transition. However, by using 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 approached 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.

Neutron Diffraction Line Broadening of Twins in YBa2Cu3O6+x (0 < x < 1) at High Temperatures. B.Khoshnevisan, D.K.Ross: Physica Status Solidi C, 2006, 3[9], 3166-9