Dielectric, piezoelectric properties of MnO₂-doped (K_0.5Na_0.5)NbO₃–0.05LiNbO₃ crystal grown by flux-Bridgman method

Highlights: • KNN–0.05LN based single crystals were grown by flux-Bridgman method. • Dielectric, piezoelecrc and ferroelectric properties were studied. • The effect of MnO₂ doping on the crystals' properties. • Dielectric and other properties were improved due to MnO₂ doping. - Abstract: Lead-free potassium sodium niobate piezoelectric single crystals substituted with lithium and then doped with MnO₂ (K_0.5Na_0.5)NbO₃–0.05LiNbO₃–yMnO₂ (y = 0%, 1.0% and 1.5%) (abbreviated as KNN–0.05LN–yMnO₂) have been grown by flux-Bridgman method using KCl–K₂CO₃ eutectic composition as the flux. Their actual composition as well as the dielectric and piezoelectric properties were studied. Their actual composition deviated from the ratio of the raw materials due to different segregation coefficients of K and Na. The orthorhombic–tetragonal (T_o–t) and tetragonal–cubic phase transition temperature (the Curie temperature T_c) of the single crystal appears at 186 °C and 441 °C, respectively, for KNN–0.05LN–1.0%MnO₂, shift to higher temperatures compared with that of pure KNN–0.05LN crystals, according to the dielectric permittivity versus temperature loops. The KNN–0.05LN–1.0%MnO₂ (001) plate shows higher piezoelectric coefficient d₃₃ and dielectric permittivity ε_r when compared with pure KNN–0.05LN crystal, being on the order of 226 pC/N and 799 (161 pC/N and 530 for KNN–0.05LN), respectively. These excellent properties show that MnO₂ dopant is effective in improving KNN–0.05LN based piezoelectric crystals