[110]TA1 Phonon Branch and Anomalous 2/3[110] Elastic Peak in Heusler and B2 Phases of a AuCuZn2 Alloy

The present study on the [110]TA 1 phonon dispersion relation in a AuCuZn 2 alloy reveals that the lattice instability towards the 18R martensite is characteristic of only the Heusler phase existing stably below about 670 K, which is the disordering temperature to the B2-phase. Anomalous peak appearing near ζ=2/3[110] in the Heusler-phase is also examined by neutron elastic scattering experiments in a wide range of temperature. It is evident that such a peak has no relation to the 2/3-dip on the [110]TA 1 phonon branch observed in the Heusler-phase but it is originated from vacancies existing inherently and regularly in the B2-lattice.