Brillouin scattering in Hg2Cl2

Hg₂Cl₂ undergoes a second order phase transition around 185 K. All the elastic constants have been measured above (tetragonal phase) and below (orthorhombic phase) the transition point. The results support the hypothesis that the transition is induced by the condensation of a transversal acoustical mode at an X point of the first Brillouin zone in the tetragonal phase. The coefficients of the coupling terms in the free energy are evaluated and the relative values of the spontaneous deformations are evaluated.