Low-temperature martensitic phase transition of bcc lithium

The low-temperature martensitic phase transition of bcc lithium has been investigated by means of neutron scattering techniques. A softening of the low-lying transverse-acoustic [110] phonon branch polarized along [11{}0] is found when the transition temperature is approached. Moreover, a diffuse scattering unmodulated in reciprocal space is observed showing a hysteresis in the vicinity of the transition. The low-temperature phase appears at 74 K where an orientation relation (110)bcc{∥}(002)hcp is observed. Finally, connected with the appearance of the new phase a streaked intensity extending from the (110)bcc toward the (220)bcc reflection is found indicating the presence of stacking faults on (110)bcc planes. The results corroborate a model for the phase transformation recently put forward for the related transition in Na and describing the basic feature of the transition as a short-wavelength shear instability occurring on (110)bcc planes.