A neutron diffraction study of AuCu3near the order-disorder temperature

Abstract

Neutron diffraction has been used to study the thermal parameters of AuCu₃ from 658 to 688K, i.e. from T_c-10K to T_c+20K, where T_c is the temperature of the order-disorder transformation. In a previous study the present authors, together with J Faber, (see ibid., vol.15, p.6699, 1982) showed that large fourth-order anharmonic effects are present at lower temperatures in ordered AuCu₃. The present study shows that the slight anisotropy at the Cu atom site, which is present up to T_c-5K due to anharmonic contributions, becomes very large close to T_c. The effect can be satisfactorily modelled with second-order thermal parameters. This suggests that phonon anomalies may be responsible for the first-order transition. They have also measured the average thermal vibration B (or beta ) through T_c. It increases by approximately 20% at T_c and remains at a large value for increasing T. This behaviour of beta above T_c is in disagreement with earlier experimental results.