Long-period superstructures in Cu_3±xPd

The one- and two-dimensional long-period superstructures in Cu₃Pd are analysed by means of high-resolution electron microscopy and electron diffraction. Minor, but nevertheless significant, modifications of the phase diagram are suggested from detailed microstructural observations. The half-period M varies not only with composition but also with temperature. A systematic decrease in M with increasing temperature has been established. At low Pd concentrations the M values are incommensurate; this is confirmed by structural imaging. At high Pd concentrations, M decreases and locks in at simple commensurate values such as 4·5, 3·5, or 3. A comparison of high-resolution images of the periodic antiphase boundaries of the one-dimensional long-period superstructure with computer simulations seems to indicate that the diffuse character of the boundary images is due to compositional disorder along the boundaries.