Structural modification and piezoelectric properties in Bi_0.5Na_0.5TiO₃–BaTiO₃–SrTiO₃ thin films

The structure and piezoelectric properties of the (0.935−x)Bi_0.5Na_0.5TiO₃–0.065BaTiO₃–xSrTiO₃ (BNT–BT–ST) thin films as a function of ST substitution were investigated. The ST substitution was found to induce downward shift of the depolarization temperature (T _d). The BNT–BT–ST thin films underwent a transition from ferroelectric phase to relaxor pseudocubic phase with the increase of ST content. The increased polarization of the compositions at around x = 0.18–0.20 was related to three possible mechanisms: polarization extension favoured by the simultaneous presence of polar and non-polar phases; the occurrence of electric field-induced transitions from weakly polar relaxor to ferroelectric polar phase; and the enhanced polarizability of the crystal structure induced by the weakening of the Bi–O bond. The enhanced piezoelectric coefficient d _33,f of 120 pm/V was obtained for the compositions at around x = 0.18–0.20. It was attributed to the phase transition of ferroelectric to relaxor pseudocubic being at near room temperature.