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High-density (Bi0:5Na0:5TiO3)0:94(BaTiO3)0:06 (BNBT-6) ceramics were fabricated under varying oxygen partial pressure conditions to develop useful lead-free piezoelectric materials with high mechanical quality factors (Qm). In uence of oxygen partial pressure-controlled sintering on physical properties of BNBT-6 ceramics was investigated. A liquid phase occurred in the sample sintered under logP(O2) = 3.50 atm because part of the Bi, a highly volatile element, evaporated from the sample surface during sintering. The mechanical quality factor, Qm, for the radial vibration mode exhibited a maximum (= 270) at logP(O2) = 3.50 atm that was 1.4 times as large as that of the sample sintered at logP(O2) = 0.12 atm. The temperature coecients of resonant and antiresonant frequencies (TCFfr1 and TCFfa1) of the BNBT-6 sample sintered under logP(O2) = 3.50 atm were almost constant: TCFfr1 = 465 ppm/C for the fundamental resonant frequency and TCFfa1 = 381 ppm/C for the fundamental antiresonant frequency. The current technology produces a substantial improvement in Qm characteristics without lowering other piezoelectric parameters, such as relative permittivity, "r, and electromechanical coupling factors, kp and kt, and can be expected to extend the selectivity of electrode materials used in multilayer ceramic actuators.