Comprehensive analysis of the temperature dependence of the crystal structure of (1-x)K0.5Na0.5NbO3-xBa(Li1/4Nb3/4)O3 piezoelectric ceramics

Much attention has been paid to the characterization of the piezoelectric properties and crystal structure of K0.5Na0.5NbO3 (KNN)-based ceramics with Li substitution for the A-site to date. In contrast, this study describes the influence of Li substitution for the B-site in KNN-based ceramics on the phase stability and of the piezoelectric properties of the ceramics. Thus, the Ba(Li1/4Nb3/4)O-3 (BaLN) perovskite in which Li occupies the B-site was chosen as an additive; the crystal structure and piezoelectric properties of (1-x)KNN-xBaLN (x = 0-0.05) ceramics were investigated in this study. The introduction of a multiphase region consisting of orthorhombic (Amm2) and tetragonal (P4mm) phases was succeeded by BaLN addition. The variations in the peak intensity of X-ray powder diffraction (XRPD) patterns at room temperature and Li-7 solid state NMR spectra implied that the multiphase region was stable at x <= 0.01, whereas the presence of a minor secondary phase was suggested at x > 0.01. The presence of a multiphase region as a function of temperature was also characterized by in situ synchrotron XRPD together with Rietveld analysis, in situ Raman spectroscopy, differential scanning calorimetry (DSC), and measurement of the dielectric constant. The disappearance of the dielectric peak around 200 degrees C, which corresponded to the phase transition from the orthorhombic to tetragonal phase, was observed, suggesting the broadening of the multiphase region at room temperature. Although the multiphase region at x = 0.01 was relatively stable at temperatures below 175 degrees C, a remarkable decrease in the fraction of the orthorhombic phase from 52.5% to 12.5% was observed in the temperature range of 175-200 degrees C where the presence of a DSC peak and the Raman shift of the A(1g) mode were also recognized. The coexistence of orthorhombic and tetragonal phases leads to the enhancement of the piezoelectric constant from 98.4 to 193.7 pC N-1 in the composition range of 0-0.01.