  Many proper and improper ferroelastic materials display (at sufficiently low
measurement frequencies) a huge elastic softening below Tc. This giant elastic
softening, which can be suppressed with uniaxial stress, is caused by domain
wall motion. Here we shortly review our results on frequency and temperature
dependent elastic measurements of some perovskites which exhibit improper
ferroelastic phase transitions. We also present a new model - based on
Landau-Ginzburg theory including long range interaction of needle shaped
ferroelastic domains - which describes superelastic softening observed in some
of the perovskite systems very well. We also show, howthe theory can be
extended to describe proper ferroelastic materials and apply the theory to
describe the elastic behaviour of the proper ferroelastic material
La1-xNdxP5O14 (LNPP).
