Ferroelasticity in Aniline-HBr

Abstract

The orthorhombic to monoclinic phase transition of aniline-HBr crystal has been investigated with X-ray scattering technique to clarify the mechanism of the phase transition. The temperature dependence of monoclinic angle below the transition indicates the occurrence of ferroelasticity in this substance. The temperature dependence and the q-dependence of the X-ray critical scattering show that a transverse optical mode as well as the uniform transverse acoustic mode becomes unstable at the transition. The phase transition is discussed in terms of the coupling of the three systems: NH₃⁺ ion having an order-disorder freedom of motion, shear strain, and internal displacement of ion described by the TO-mode.