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Title: Investigation of the structure of the perovskite series (Na1-xKx)1/2Bi1/2TiO3
Author: O'Brien, Aoife
ISNI:       0000 0004 2749 1815
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; QE Geology