Thermal and electromechanical properties of melilite-type piezoelectric single crystals

Melilite-type crystals, including Ca₂Ga₂SiO₇, Ca₂MgSi₂O₇, and CaNdGa₃O₇, were successfully grown by the Czochralski technique. Thermal properties were investigated and full matrices of electromechanical constants of these melilite crystals were evaluated by impedance method, with d₁₄ being on the order of 5.3-9.3 pC/N. The relationship between microstructure and electromechanical properties of the three kinds of crystals was established, in order to explore the piezoelectric origin and further optimize the piezoelectric properties. The distortion of eight-fold antiprisms and the distortion/rotation of tetrahedrons were thought to contribute to the piezoelectric d₁₄ and d₃₆, respectively. In addition, the layered structure leads to strong anisotropic behavior, accounts for the high resistivity along Z direction in Ca₂MgSi₂O₇, while the disordered structure accounts for the degraded resistivity in CaNdGa₃O₇. The properties at elevated temperature were investigated, where the elastic constants show high thermal stability over the range of 25-500 °C, with minimal variation of 6%.