Softened phonons in Cu-Zn-Al: a study using calorimetry, susceptibility and inelastic neutron scattering

Abstract

Experiments on specific heat (1.3-40K), susceptibility (4-300K) and neutron inelastic scattering (6 and 215K) have been performed on Cu-Zn-Al samples for concentrations close to the onset of the martensitic transformation at 0K. The electronic specific heat term is small and insensitive to the proximity of the transition: the electron-phonon enhancement of the electronic specific heat must be unimportant. For nearly martensitic samples, the phonon specific heat is anomalous: in the range 1.3-4K, the T3 term reaches 0.089 mJ K-4 mol-1; at higher temperatures, the results are well described by a Debye spectrum with, essentially, an Einstein peak added, containing about 25% of the modes, and with a characteristic temperature of 78K. The latter figure is in agreement with the frequency corresponding to the Brillouin zone boundary for the softened TA2 branch as measured by neutrons on one of the authors samples. However, the high mode content of the Einstein specific heat term shows that the softening of this branch extends well outside the direction explored. The neutron results also show that the TA2 branch is not altered appreciably when the temperature varies from 6 to 215K. This tends to show that the transition is first order. The same indication can be drawn from the strong hysteresis shown by the magnetic susceptibility of the samples exhibiting a transformation, and from the fact that anharmonic processes can be accounted for by mode renormalisation.