Proper Ferroelastic Transition in Piezoelectric Lithium Ammonium Tartrate

Dielectric and Brillouin scattering experiments on a lithium ammonium tartrate crystal have shown that the transition in this crystal is described by the free energy of a piezoelectric crystal, $F=\frac{1}{2}{\left({\chi }^x\right)}^{-1\mathrm{}}{P}^2+aPx+\frac{1}{2}\beta (T-T_0)x^2,$ where the primary order parameter is the homogeneous strain, which gives rise to ferroelectricity through the piezoelectric coupling with the polarization.