TWIN BOUNDARY ENERGIES IN YBA2CU3O7-X AND LA2CUO4

Colonies of parallel twins are observed in YBa2Cu3O7-x and La2CuO4, as well as in many other materials which undergo a tetragonal to orthorhombic or equivalent transformation. The spacing of such twins is found to vary with the colony size. It is shown that this variation can be used to calculate the twin boundary energy, gamma. The total energy of a twin colony is the sum of the twin boundary energy associated with the presence of the twin boundaries and the strain energy of the coherent lenticular ends. Minimization of the total energy of the system shows that the twin thickness (t), should be proportional to (gamma-L/epsilon-2)1/2 where L is the colony size and epsilon is the transformation strain. Plots of t against square-root L yield straight lines for both YBa2Cu3O7-x and La2CuO4 and gamma can be calculated from the slopes giving values of 2.8 and 5.3 mJm-2 respectively.