Piezoelectric and dielectric properties of Sn-doped (Na_0.5K _0.5)NbO₃ ceramics processed under low oxygen partial pressure atmosphere

Sn-doped (Na_0.5K_0.5)NbO₃ (Sn-NKN) ceramics fired under various oxygen partial pressure (pO₂) conditions have been investigated and discussed in terms of bulk piezoelectric and dielectric properties. X-ray diffraction measurements and Rayleigh analysis indicate that the substitution site of the Sn cations depend on the pO₂ atmosphere in the firing process. For pO₂ higher than 1.0 ' 10%10 atm, Sn cations mainly substitute as Sn4+ at the B-site of perovskite NKN, whereas Sn2+ A-site substitution is favored under a low-pO₂ atmosphere. Low-pO₂ fired Sn-NKN ceramics exhibit higher relative permittivity, Curie temperature, and piezoelectric coefficient (d₃₃). Sn ²⁺ at A-site acts as a donor and reduces the p-type carrier concentrations that result from an electronic compensation of metal vacancies created through the high volatility of Na and K suboxides. The higher piezoelectricity and resistivity in low-pO₂ fired Sn-NKN ceramics make this material suitable for base-metal cofired devices such as Ni-innerelectrode multilayer capacitors and actuators.