  Many ferroelastic crystals display at sufficiently low measurement
frequencies a huge elastic softening below Tc which is caused by domain wall
motion. Materials range from perovskites to iron based superconductors and
shape memory materials. We present a model - based on Landau-Ginzburg theory
including long range elastic interaction between needle shaped ferroelastic
domains - to describe the observed superelastic softening. The theory predicts
that the domain wall contribution to the elastic susceptibility is different
for improper and proper ferroelastic materials. A test of the theory against
experimental data on SrTiO3, KMnF3, LaAlO3, La1-xNdxP5O14 and NH4HC2O4.1/2H2O
yields excellent agreement.
