Elastic soft mode in perovskite-type layer-structure materials

Measurements of the elastic mode c₆₆ in the layer-structure materials (CH₃NH₃)₂ MnCl₄ and (CH₃NH₃)₂ CdCl₄ 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 εxy 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.