Lead-free piezoelectric niobate perovskites

• Main Author: Baker, Daniel William
• Published: University of Warwick 2010
• Subjects: 548; QC Physics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526241

The structure and physical properties of the lead-free piezoelectric niobate perovskite material sodium potassium niobate, KxNa1-xNbO3 (KNN) have been investigated via a variety of experimental techniques. High- and low-temperature X-ray and neutron powder diffraction has been performed on polycrystalline samples of KNN. Rietveld refinement has been carried out on data for samples of KNN from x = 0:1 to x = 0:85, with the structure determined for each sample. The phase diagram from x=0:1 to x=1 has been determined, with particular attention paid to the oxygen octahedra tilt systems. The proposed compositionally-driven phase transitions were also investigated, and it has been found that changes in the tilt system occur at x = 0:2 and x = 0:4 at room temperature, which is a correction to previously published work. The first-order phase transition at x = 0:5 from a monoclinic structure to an orthorhombic structure has also been confirmed. To complement the polycrystalline structural analysis, single-crystal samples of KNN have been fabricated to confirm the exact nature of the tilt systems. It has been found that all crystals possess the same merohedral twinning that is present for other perovskite systems that follow a similar phase transition path from the prototypic cubic phase. The tilt systems above and below the the x=0:2 boundary have been found to be of the type a0b+c0 and a-b+c- respectively. Low-temperature single-crystal diffraction also confirmed the structure of the low-temperature trigonal (rhombohedral) phase. To complement the diffraction work, the short-range structure of KNN has been investigated with 1D and 2D 23Na NMR experiments. It has been found that chemical shift dispersion is present for all samples of KNN because of the shared A-site occupancy of potassium and sodium atoms. The quadrupolar parameters have been determined, and an increase in the electric field gradient for one sodium site can be seen to coincide with an increase in the distortion in the structure because of the tilting of the oxygen octahedra. Birefringence data have been collected for single-crystal samples of KNN using the Metripol technique. No obvious domain structure was seen. The nature of the phase transition from the prototypic cubic phase and the tetragonal phase was investigated, and the analysis suggests that the transition is at a tricritical point between a first-order and a second-order phase transition. Finally, an optical study was performed on samples of KNbO3 (KN) using the Metripol technique. Large self-organised domains similar to those previously observed in barium titanate were seen only on the first heating run. Very small (< 5μm) domains were observed in the samples, and remained in the orthorhombic phase after several heating runs. Single-crystal X-ray diffraction was implemented to determine the structure of the sample. It was also found that same twin law as in barium titanate is present for KN.