Structure and electrical properties of MnO2-doped Sr2−xCaxNaNb5O15 lead-free piezoelectric ceramics

► MnO2 doped lead-free piezoelectric ceramics Sr2−xCaxNaNb5O15 (x = 0.05-0.35) were synthesized. ► Mn ions and Ca2+ entered into the Sr2NaNb5O15 lattices to from a solid solution. ► The substitution of Ca2+ decreases d33, kp and kt. The “hard” and “soft” effects of MnO2 attributed to the strong dielectricity and high piezoelectricity of the ceramics.