Phonon anomalies and martensitic transitions in bcc materials

Abstract

The general systematics of phonon spectral anomalies are considered for materials with bcc and B2 structures. In a model of central pair-wise interactions, it is shown that anomalies on the TA₂ branch at κ=1/3 (211), the LA phonon branch at κ=2/3 (111), and on the TA branch at κ=1/2 (111) arise simultaneously with the softening of the shear elastic moduli C' and C₄₄. The TA phonon anomaly at κ=1/3 (110) is not due to softening of the elastic moduli, but rather is due to the long range nature of the pair-wise or multiparticle interaction. A model is proposed for the nucleation of the martensitic phase for B2 compounds, the distinctive feature of which is a consideration of the “softening” of the shear moduli, which is modulated by lattice relaxation around ordering defects, along with the possibility of additional defect ordering near the transition.