Abstract
In the frame of thermodynamic approach, it is shown that all the admissible values of the d33, d32, and d31 piezoelectric moduli of any orthorhombic (mm2) single crystal in the 3-dimensional (d33, d32, d31) space lie within a restricted area having the form of ellipsoid. The main parameters of this ellipsoid (the aspect ratios and the angles characterizing its orientation) depend only on the elastic constants of the crystal. Using the developed approach, the areas of the allowable values of piezoelectric moduli have been determined for six crystals: single-domain KNbO3 and 0.27Pb(In0.5Nb0.5)O3-0.40Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 crystals possessing true orthorhombic symmetry and four polydomain macroscopically pseudoorthorhombic crystals—tetragonal BaTiO3 and PbTiO3 crystals with laminate 90° domain structure as well as rhombohedral 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 and 0.93Pb(Zn1/3Nb2/3)O3–0.07PbTiO3 crystals, poled along the [011] direction of the prototype cubic cell. Four crystals from this set possess negative values of the longitudinal Poisson's ratio and are thus partially auxetics.
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