Experimental study of the ferroelastic phase transition in urea/n-heptadecane composite.

An experimental study of the ferroelastic phase transition in urea/n-heptadecane CO(NH2)2/C17H40 composite around the structural phase transition undergone by this crystal at 159 K is presented. The temperature dependence of the macroscopic spontaneous strain and the optical birefringence around this temperature has been determined. A phenomenological model limited to the hexagonal-orthorhombic change of the urea sublattice leads to a linear relation between these quantities and the phase transition entropy. The experimental data agree with this description exception made of the near vicinity of the phase transition, where the influence of the alkane chain ordering processes cannot be excluded.