Measurements of the elastic mode $c_{66}$ in the layer-structure materials ${\left(\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\right)}_2$ Mn${\mathrm{Cl}}_4$ and ${\left(\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\right)}_2$ Cd${\mathrm{Cl}}_4$ indicate an almost complete softening of this mode in the tetragonal low-temperature phase. We interpret this temperature dependence with a linear coupling of the ${\epsilon }_{\mathrm{xy}}$ strain to a soft-order-parameter fluctuation mode. This softening implies that a low-temperature orthorhombic phase not yet observed experimentally has to be taken into account for explaining the corresponding phase transition. Our results further indicate that the strong coupling of the order-parameter fluctuation mode to the acoustic mode prevents the low-temperature orthorhombic phase from becoming stable and, thus, from being observed experimentally.

• Received 28 March 1980

DOI:https://doi.org/10.1103/PhysRevB.22.3452