Observation of a soft-phonon mode and a pre-martensitic phase in the intermetallic compound Ti50Ni47Fe3studied by inelastic neutron scattering

Phonon dispersion in the equiatomic intermetallic compound Ti 50 Ni 47 Fe 3 has been studied by inelastic neutron scattering in view of the pre-martensitic phase transition which transforms the high-temperature CsCl structure into an intermediate phase. The TA 2 transverse acoustical phonon branch along (110), measured at room temperature (RT), 83, 150, 250 and 350 degrees C, shows a minimum around q=( 1 / 3 , 1 / 3 , 0) 2 pi /a which decreases dramatically as the temperature is decreased from 350 degrees C to RT. A new superlattice peak, close to ( 5 / 3 , 4 / 3 , 0), appears at 20 degrees C. The general features resemble low-dimensional systems where Fermi-surface instabilities and the formation of charge density waves have been observed. The new superstructure of the intermediate phase, which initially appears to be incommensurate, becomes commensurable at lower temperatures.