Design of a new phase boundary to enhance the piezoelectric properties of lead-free (K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ – Bi_0.5K_0.5HfO₃ ternary ceramics

In this study, a new phase boundary was successfully established in (1-x)(K_0.48Na_0.48Li_0.04)(Nb_0.95Sb_0.05)O₃ –xBi_0.5K_0.5HfO₃ (KNLNS–xBKH) ternary ceramics. Changes in phase structure were verified by the X-ray diffraction (XRD) analysis and Raman scattering measurements of KNLNS–xBKH with varying x contents. The results indicate that KNLNS–xBKH exhibits an orthorhombic phase (Amm2) at x = 0, a mixed orthorhombic-tetragonal phase (Amm2 + P4mm) at x = 0.01–0.02, a mixed rhombohedral-tetragonal (R-T) phase (R3m + P4mm) at x = 0.03–0.04, and ultimately to a single rhombohedral phase (R3m) at x = 0.05. Under optimal conditions, the (1-x)KNLNS–xBKH ceramics demonstrated remarkable electrical properties, including a density of 4.58 g/cm³, a piezoelectric constant (d₃₃) of 238 pC/N, a remanent polarization of (P_r) of 15.4 µC/cm² and a high Curie temperature (T_C) of nearly 310 °C at x = 0.03. These properties are attributed to the establishment of a new rhombohedral (R)–tetragonal (T) phase boundary with microstructures characterized by high density, large grain size, and low porosity. Furthermore, KNLNS–xBKH ceramics with x = 0.05 exhibited a high recoverable energy density (W_rec = 0.26 J/cm³) and a large energy storage efficiency (η = 56.8%) under a low electric field of 20 kV/cm, representing a significant advancement towards the development of practical applications.