Optic soft mode in ferroelastic betaine squarate

Abstract

The ferroelastic transition of betaine squarate at Tc ≈ 52·7 K is shown to be driven by an optic mode. Raman spectroscopic investigations reveal a soft mode of A 2 symmetry, the eigenvector of which couples bilinearly with the shear strain e 5, which represents the order parameter. This coupling renormalizes the elastic stiffness c 55 and eventually leads to the transition when c 55(T) becomes zero at Tc .