Phonons and martensitic phase transitions in pure bcc Ti and bcc Zr

Phonons have been measured in -Ti and -Zr over the whole range of existence of the high-temperature bcc phase. Low-frequency and strongly damped longitudinal phonons at q=(2/3(111) produce the necessary displacements for the martensitic - transition. The displacements needed for the martensitic - transition are produced by low-frequency and damped transverse phonons at q=(1/2(110), which on approaching the phase transition soften further but not completely. The low-frequency and strongly damped phonons are precursor fluctuations of the or phase within the phase.