The intrinsic elasticity of twin walls: Ferrielectric twin walls in ferroelastic CaTiO ₃

Computer simulation of ferrielectric domain walls in ferroelastic CaTiO ₃ demonstrate that twin walls possess almost the same compressibility as the bulk. The Ti-Ti repetition length inside the wall is approximately 0.2% greater than the equivalent lattice parameter of the bulk. A slight reduction in the uniaxial compressibility perpendicular to the twin wall is compared with predictions of various theoretical models which give either large softening or large hardening. Simple spring models predict correctly that any change is expected to be small for relevant potentials far from the ferroelastic transition point.