Large electromechanical strain and electrostrictive effect in (1 − x)(Bi_0.5Na_0.5TiO₃–SrTiO₃)–xLiNbO₃ ternary lead-free piezoelectric ceramics

Lead-free (1 − x)(0.8Bi_0.5Na_0.5TiO₃–0.2SrTiO₃)–xLiNbO₃ (BNST–xLN, x = 0–0.08) piezoelectric ceramics were fabricated by a solid-state sintered technology. The effects of LN-doping on the structural and electrical properties of the BNST–xLN system were systematically investigated. The results of Raman spectroscopy revealed that the substitution of LN softens the phonon vibrations in the BNST–xLN system, in accordance with the remarkable reduction in the phase transition temperature (T_F−R), remnant polarization (P_r), negative strain (S_neg) and piezoelectric coefficient (d₃₃). However, the degradation of the long-range ferroelectric orders was accompanied by a significant increase in the electric field–induced strain response. At x = 0.04, a maximum unipolar strain of ~ 0.36% with a corresponding normalized strain (S_max/E_max) of ~ 600 pm/V was obtained at room temperature, which should be mainly ascribed to the reversibly electric field-induced phase transition between the ergodic relaxor and ferroelectric phases due to their comparable free energies in the two-phase coexistence region. Moreover, it was also found that the BNST–xLN system processes predominant electrostrictive behaviors with relatively high electrostrictive coefficient (Q₃₃) and excellent temperature stability when the field-induced phase transition cannot be trigged by the applied electric field, as evidenced by a fact that the Q₃₃ value of BNST–0.08LN ceramic keeps almost constant as high as ~ 0.028 m⁴/C² in the temperature range from room temperature to 120 °C.