O'Brien, A. (Aoife) (2013) Investigation of the structure of the perovskite series (Na1-xKx)1/2Bi1/2TiO3. PhD thesis, University of Warwick.

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Abstract

The structures of the lead-free perovskites in the solid solution range
(KxNa1-x)0.5Bi0.5TiO3 (KNBT100x) have been investigated using high-resolution x-ray
diffraction (XRD) on polycrystalline samples. The room-temperature structure of
Na0.5Bi0.5TiO3 (NBT) has been studied, and identified as having monoclinic Cc symmetry,
with distortion very close to that of rhombohedral R3c. High-temperature XRD has been
used to identify a phase transition between the pseudo-rhombohedral phase and a
tetragonal P4bm phase at 300 °C. This transition has been found to occur via a pseudocubic
structure, although local variations in transition temperatures lead to the coexistence
of both phases, with a hysteresis of ~ 30 °C. The Curie temperature, TC, has been found to
lie in the range 450 °C – 530 °C. The room-temperature structures of compositions from
NBT to (K0.65Na0.35)0.5Bi0.5TiO3 have been studied, and a phase boundary identified at x ~ 0.2.
Compositions from NBT – KNBT20 refine as monoclinic Cc, with pseudo-rhombohedral
symmetry, whilst KNBT25-65 were found to have equilibrium phase P4mm, with a stressinduced
R3m phase also present. A relationship was observed between the spontaneous
tetragonal strain and the proportion of tetragonal phase present in these samples. KNBT10
and KNBT40 did not show evidence of the high-temperature P4bm phase; instead, TC values
were found to be ~ 225 °C and ~ 300 °C respectively. Experimental data have been
combined with data from published literature to construct a phase diagram of the KNBT
system.
KNBT synthesised via a hydrothermal method has been investigated. Hydrothermal
NBT, once heat-treated, has been found to show similar structure to solid state NBT and
undergoes the pseudo-rhombohedral to tetragonal transition at 305 °C, with a hysteresis of
~ 20 °C between heating and cooling. TC was found to be higher, in the range 530 °C –
700 °C. The XRD patterns of non-heat-treated hydrothermal KNBT have broader peaks and
a more undulating background, indicative of smaller crystallite size and diffuse scattering
from a more disordered material. The proposed source of this disorder is the presence of
oxygen vacancies, OH- groups, or A-site clustering. Significant peak splitting was observed
in the XRD patterns of hydrothermal K0.5Bi0.5TiO3 (KBT) and tetragonal KNBT. This was not
the case for solid state KNBT, showing a greater tetragonal distortion in materials prepared
via the hydrothermal method. Heat-treating this material was found to effect a significant
structural change, with single peaks remaining after cooling. It is proposed that this could
be caused by the release of OH- groups, recrystallisation, and mixing of the A-site species.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QD Chemistry Q Science > QE Geology
Library of Congress Subject Headings (LCSH):	Perovskite -- Structure, Phase rule and equilibrium, Solutions, Solid, High temperature chemistry, X-rays -- Diffraction
Official Date:	March 2013
Dates:	Date Event March 2013 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	[not supplied]
Extent:	xv, 144 leaves : illustrations.
Language:	eng

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