The tunable ferroelectricity and piezoelectricity of the KNN piezoceramics by Na concentrations: First-principles calculations

The ferroelectric and piezoelectric properties of orthorhombic K1-xNaxNbO3 (KNN) piezoceramics with various Na concentrations x are studied by the first-principles calculations. The spontaneous polarization of KNN single crystals increases monotonically with Na concentrations, indicating an enhanced ferroelectricity in KNN. Moreover, the averaged shear, transverse and longitudinal piezoelectric coefficient d-15*, d-31* and d-33* of KNN piezoceramics are calculated from the data of KNN single crystals to show that piezoelectricity of KNN piezoceramics positively correlates with Na concentrations and exhibits the best performance at x = 0.5625, which agrees well with the available experimental data. At this concentration, d-15*, d-31* and d-33* of orthorhombic KNN piezoceramics reach maximum, which are 1.13, 1.79 and 0.85 times larger than those of KNbO3 piezoceramics, respectively. This work provides a better understanding to the effect of Na concentrations on the enhanced ferroelectricity and piezoelectricity in the KNN piezoceramics.