Electromechanical properties of different phases in ferroelectric crystals regulated by variously oriented electric fields

Electric fields offer a convenient and tunable way to induce phase transitions for regulating the electromechanical properties of ferroelectrics. However, regulating the electromechanical properties by using electric fields in various directions for different ferroelectric phases has yet to be systematically investigated, especially for lead-free material KNbO₃. Based on the nonlinear thermodynamics analysis, the electric field-temperature phase diagrams of KNbO₃ single crystals under different electric field directions (E_[001], E_[011], E_[111]) have been constructed, along with the electric-field-induced electromechanical responses. The results show that the phase diagrams are markedly different under different electric field directions. Specifically, the electric field-temperature phase diagram appears as a "line"-shaped phase boundary under E_[001], while it appears as a "U"-shaped phase boundary under E_[011], and an arrowhead-shaped phase boundary under E_[111]. It is also found that there are excellent electromechanical responses near both "U"-shaped and arrowhead-shaped phase boundaries due to the significant alterations in polarization slopes near the phase boundaries, offering an alternative pathway to regulate and enhance the electromechanical properties in ferroelectrics.