Intriguing dielectric phenomena in (K,Na)1-xLixNbO3 ceramics

The (K,Na)NbO₃-based ceramics are a very promising type of lead-free piezoelectric materials. In spite of the scientific and technological importance, systematical investigations on the dielectric spectra had been rarely performed so far. This paper reports our study results obtained in the (K_0.50Na_0.50)_1-xLi_xNbO₃ ceramics with either x = 0.03 or x = 0.065 (denoted as KNLN-030 and KNLN-065 ceramics, respectively) in the unpoled state and the poled state, respectively. Before poling, the KNLN-030 ceramic shows a dielectric spectrum of strong frequency dependence with a broad peak of dielectric loss around 1 MHz, while the KNLN-065 one has a dielectric spectrum of weak frequency dependence below 60 MHz at room temperature. After poling, piezoelectric resonance peaks appear and are accompanied by a large step-like reduction of dielectric permittivity ε′ in the dielectric spectra of both two ceramics. More interestingly, the KNLN-065 ceramic exhibits an abnormal phenomenon that there is a marked increases in low-frequency ε′ upon poling, whereas the KNLN-030 one shows normally a large decrease of low-frequency ε′. Further, the KNLN-065 ceramic displays the larger low-frequency ε′ in the poled state than in the unpoled state over a considerably wide temperature range. By contrast, such abnormal phenomenon is recognized only in a limited region above the orthorhombic-tetragonal phase transition in the KNLN-030 ceramic. The analysis of domain structure suggests that the observed abnormal dielectric phenomenon possibly arises from the enhancement of domain-wall mobility.