Ultrasonic study of the martensitic phase transformation in sodium

A gated coherent phase-sensitive ultrasonic technique was employed to measure the ultrasonic velocity and attenuation in single crystals of sodium in the vicinity of the martensitic phase transformation temperature. In order to minimize strains, electromagnetic acoustic transducers were used to generate and receive the acoustic signals. Acoustic waves associated with the elastic constants $C_{44}$, $C_{11}$, C’=($C_{11}$-$C_{12}$)/2, and $C_n$=($C_{11}$+$C_{12}$+2$C_{44}$)/2 were studied for temperatures between 4.2 and 90 K and for frequencies between 6 and 8 MHz. No premartensitic effects were observed for any of the modes studied. The transition was abrupt and was always accompanied by a large increase in the attenuation. The velocity of the waves associated with the elastic constants $C_{44}$ and $C_n$ decreased at the transformation temperature while the velocity associated with C’ and $C_{11}$ increased as a result of the transformation. During the reversion process, the crystals underwent a structural reorientation. The results are compared with neutron scattering studies in sodium and previous ultrasonic measurements in lithium.