ACOUSTIC PHONON-DISPERSION IN NITE2

The dispersion laws for acoustic phonons propagating along ( zeta 00), ( zeta zeta 0) and (00 zeta ) in nickel ditelluride have been measured by neutron spectrometry. In spite of the relatively small value of the unit-cell dimension ratio c/a in this crystal, a tendency to exhibit quasi-two-dimensional character can be seen in the dispersion curves. A small kink, reminiscent of a Kohn anomaly, is found in the longitudinal branch in the (100) and (110) directions. The elastic constants and sound velocities have been determined from the initial slopes of the curves.