A neutron diffraction study of AuCu3near the order-disorder temperature

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Abstract
Neutron diffraction has been used to study the thermal parameters of AuCu3 from 658 to 688K, i.e. from Tc-10K to Tc+20K, where Tc 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 Tc-5K due to anharmonic contributions, becomes very large close to Tc. 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 Tc. It increases by approximately 20% at Tc and remains at a large value for increasing T. This behaviour of beta above Tc is in disagreement with earlier experimental results.

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