THE FERROELASTIC TRANSITION OF BETAINE FUMARATE

Investigations of polarized Raman spectra of orthorhombic betaine fuma rate crystals revealed the existence of two low-frequency optic modes which soften slightly on approaching the ferroelastic transition tempe rature T-c approximate to 194 K. The eigenvectors of these modes with B-1g- and B-2g-symmetry, respectively, are coupled bilinearly to stram s of corresponding symmetries leading to renormalized elastic moduli. It is shown that these couplings account quantitatively for the temper ature dependences of the moduli c(55) and c(66) and eventually lead to a ferroelastic transition and a spontaneous shear strain e(5s). By op tical and dielectric measurements it is confirmed that the transition is of second order and follows a classical Landau behaviour with a cri tical exponent of the order parameter of 0.5 within experimental error .