Studies of Transverse Phonon Modes in Premartensitic Indium-Thallium Alloys§

The measured phonon dispersion relations along the high symmetry directions for a face-centered In-Tl alloy have been reasonably well fitted using a general Born-von Karman force constant model. However, for the (zeta zeta 0)(zeta anti zeta 0) branch at low zeta, there is poor agreement between the neutron measurements and the extrapolation of the line of initial slope determined from an ultrasonic velocity measurements. Phonon frequencies have been measured for wave vectors to within q = 0.033 (..sqrt..2 2..pi../a) of the zone centre, and, contrary to the phonon softening with decreasing temperature, which might be expected from the known decrease in (C/sub 11/ - C/sub 12/)/2, phonon frequencies showed the normal trend of frequency increasing (hardening) as temperature was decreased in the premartensitic regime. To reconcile the disagreement between the extrapolation of the ultrasonic data and the measured (zeta zeta0)(zeta anti zeta 0) phonon frequencies, there must exist a positive curvature in the dispersion relation at q < 0.03 (..sqrt..2 2..pi../a). This is consistent with one recent theory for martensite nucleation. 10 refs., 2 figs.