Composition Dependence of Crystal Structure, Electronic Structure, Ferroelectric and Piezoelectric Properties of (K,Na,Li)NbO3

Abstract

INTRODUCTION Pb(Zr,Ti)O3-based piezoelectric ceramics have been widely used in electronic devices because of their excellent piezoelectric properties. Recently, owing to growing concern about environmental problems, the development of alternative piezoelectric ceramics is highly expected because Pb(Zr,Ti)O3 (PZT) contain a toxic element. (K,Na)NbO3(KNN) is one of the promising candidates.1) Superior characteristics of PZT is considered to be due to the morphotropic boundary (MPB), and KNN is also known to have a phase boundary similar to MPB. However, the characteristics and the crystal structure in the phase boundary are not well understood at the moment. It was also reported that substituting Li to the A-site of the perovskite structure ABO3 induces phase transition to tetragonal the KNN.2) From such backgrounds, we synthesized (K0.45Na0.55Lix)NbO3 and tried to reveal the composition dependence of the properties, the crystal and electronic structures in the phase boundary. In addition, we replaced the B-site by Ta, V and Mo partially, and then examined effects of the substitutions on the properties. EXPERIMENTAL Starting materials were wet-mixed in appropriate proportions, and then calcined in air for 2 h at 900 ℃. Thereafter, the samples were ground by a ball mill, and then sintered by the spark plasma sintering (SPS) method under a uniaxial pressing at 50 MPa for 5 min at 1050 ℃ in vacuum. For the obtained products, an recovery annealing treatment was performed at 900 ℃ for 4 h in O2. The identification of the phases and the evaluation of the lattice parameters were performed by XRD. Metal compositions of the ceramics were measured by ICP. Relative densities of the pellets were estimated by the Archimedes method. P-E hysteresis loop measurements were carried out at various frequencies using a TF-2000FE device (aixACCT) with the virtual ground mode. The surface and cross-section morphologies of the samples were observed by SEM. The temperature dependences of the dielectric permittivity (ε s ) and dielectric losses (tanδ) of the samples were measured by a LCR meter (HP-4284A). For average crystal structure analysis, we performed Rietveld analysis (RIETAN-FP) using synchrotron X-ray diffraction patterns.(BL19B2, spring-8) The electron densities were determined by the maximum entropy method (MEM) using the Dysnomia program. RESULTS AND DISCUSSION Figure 1 shows XRD patterns of (K0.45Na0.55Lix)NbO3 (x=0~0.07). These samples could be identified as the perovskite structures with the tetragonal (P4mm) and/or the orthorhombic (Amm2) symmetries. According to the crystal structure analysis by Rietveld method, in the composition of x=0.04~0.07, good fit can be obtained in a two-phase analysis of Amm2 and P4mm. These results indicate that there is a phase boundary around the composition range. It was found that all of the sample were sufficiently dense by the Archimedes method and the SEM observation. Measurements of P-Ehysteresis loop, the relative dielectric constant and dielectric loss demonstrated that the properties were improved in the x = 0.05~ 0.07. Such a change of the properties were examined from the electronic structure analysis by MEM and crystal structure analysis by Rietveld method. Further, we also investigated B-site substitution with a phase boundary, and evaluated their effect of the ferroelectric properties. REFERENCE 1)Y. Saito, et al. Nature, 432,84 (2004). 2) Hongliang Du, et al. Appl. Phys. Lett. 91, 182909 (2007). Figure 1