Baker, Daniel William (2010) Lead-free piezoelectric niobate perovskites. PhD thesis, University of Warwick.

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Abstract

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.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Niobates, Piezoelectric materials, Perovskite
Official Date:	May 2010
Dates:	Date Event May 2010 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Thomas, Pam A.
Extent:	xv, 164, [19] leaves
Language:	eng

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