Observation of static and dynamic pretransitional phenomena near the martensitic bcc-hcp transformation in sodium

Abstract

Pretransitional phenomena connected with the bcc-hcp martensitic transformation in Na have been observed by neutron scattering techniques. In the bcc phase a softening of the low-lying transverse-acoustic [110] phonons polarized along [1?10] is found when the transition temperature is approached. Moreover, a $q$-independent diffuse scattering showing a marked hysteresis effect is observed. Finally, the microscopic Gr\"uneisen parameters of the transverse-acoustic phonons in the [110] direction polarized along [1?10] have been determined at room temperature, yielding only small positive values near the zone boundary---in disagreement with present theories.