Phonon dispersion anomaly in the premartensitic β1-phase Au-Zn alloys

Transverse acoustic phonon dispersion relations along the [110] axis (e//[110]) in binary Au-Zn alloys were measured at 293,150 and 110 K by means of triple-axis neutron spectrometer installed in JRR-3M. Each dispersion relation curve was observed to have the anomalous dip. Phonon energy decreased with decreasing temperature. The effective interplaner force constants between the origin and nth neighbor atomic planes Φn's were determined through the least-squares fitting analysis on the phonon dispersion curves (N = 1, 2 and 3). The Φ2's were negative, while Φ3's were positive. As temperature decreased, Φ2's became smaller, but Φ3's became larger. This means that the (110) planes of the second nearest neighbors are not coupled with each other, and the third nearest ones are strongly coupled. Thus the stacking structure with a periodic unit of three layers is considered to form. The dips appearing on the [110]TA1 phonon dispersion curves mainly originated from a characteristic combination of Φ2 and Φ3 in the β1 phases. It was concluded that lattice dynamical precursor in the premartensitic β1 state causes the change of Φ2 and Φ3. (author)