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Title: Solid state synthesis and pulsed laser deposition of electroceramic materials
Author: Tzitzeklis, Christos
ISNI:       0000 0004 7659 0342
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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The work presented in this thesis is related to two different families of technological materials: optoelectronics and multiferroics. Our focus on these categories of materials was largely driven by an interest in growing highly crystalline epitaxial films by the Pulsed Laser Deposition technique and the anticipated thin film properties of smart materials, suitable for miniaturized electronic and energy devices. The thesis is structured in two parts. In the first part, the experimental investigation of SrZn1-xLixO2 as an optically transparent p-type material is discussed. Our work in this area was initially motivated by computational studies by Gupta et al.i on SrZnO2 as a host for p-type carrier and first-principles doping-dependent calculations indicating Li as the most promising p-type dopant. A range of dopants were experimentally tested by the conventional solid-state reaction and strictly anaerobic conditions and the successful substitution of Li+ for Zn2+ has been confirmed by combined PXRD and ICP studies. High quality ceramic samples of SrZn1-xLixO2 (0 < x < 0.06) were isolated and their structural, compositional and physical properties have been investigated. The p-type behaviour of SrZn1-xLixO2 was validated by thermopower and variable-pO2 resistivity measurements and found to be maintained for 40 days after storage in dry air, confirming the long-term stability of our p-type compounds. The synthesized SrZn1-xLixO2 samples were also found to retain the optical transparency of the parent SrZnO2 having a band gap of 4.27 eV, measured by UV-visible spectroscopy, in agreement with Gupta's calculations predicting a band gap of 3.80 eVi. It is worth noting here that this value supersedes the only previous report of SrZnO2 band gap of 3.41 eVii. Attempts to grow single-phase SrZnO2 films under variable growth conditions by PLD were unsuccessful. The main results of this work are currently under submission. In the second part of this thesis, our attention was focused on (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3-xCaTiO3 (BTFM-CTO), a single-phase room temperature multiferroic stabilized at the morphotropic phase boundary between the rhombohedral [111]p and orthorhombic [001]p phases. The material exemplifies a doping strategy has been developed by Rosseinsky's group towards a lead-free and ambient-pressure bismuth-based phase demonstrating a long-range polarization along both the rhombohedral [111]p and orthorhombic [001]p phases. Investigating the room temperature properties of the x = 0.15 and y = 0.75 composition, Mandal et aliii succeeded to measure polarization of 66 μC/cm2, a saturated magnetization of 0.0097 ???? per Fe and a linear magnetoelectric susceptibility of 0.19(1) ps/m. In this thesis, our work was focused on the epitaxial growth of 0.85BiTi0.1Fe0.80Mg0.1 - 0.15CaTiO3 thin films on single crystal substrates of SrTiO3 and the investigation of their ferroelectric and magnetic properties. The multi-cation nature of the targeting compound required the precise control of the deposition process and the epitaxial growth of morphotropic (1-x)BiTi(1-y)/2FeyMg(1-y)/2O3-xCaTiO3 was only possible in a narrow window of growth conditions, identified by the combined use of elemental analysis (EDX/SEM) and structural (XRD) studies. Small deviations from this optimum set of growth conditions resulted in impurity phases. The coexistence of the two-phase regime in the as-deposited films was initially observed as a peak splitting of their out of plane (00l)PC pseudocubic reflections and later verified by detailed rocking curve measurements revealing the presence of two well-distinct crystalline phases. Reciprocal space maps confirm the coherent morphotropic film growth along different in plane crystallographic directions and their in-plane stretching to the SrTiO3 substrate. The crystalline structure of BTFM-CTO films was also found to be sensitive to epitaxial constraints. A higher lattice mismatch between ultra-thin (20 nm) BTFM-CTO films and the LaAlO3 substrate resulted in the growth of a single tetragonal-like phase, relaxed to a mixture of tetragonal and morphotropic (O+R) phases at higher thickness. To study the electrical properties of BTFM-CTO films parallel-plate capacitors were fabricated with the growth of a SrRhO3 buffer layer as bottom electrode and sputtered with Pt acting as top electrode. Dielectric measurements have shown the insulating (? 1.6 GO) and low losses nature of BTFM-CTO films. Room-temperature PE hysteresis loops measurements revealed the ferroelectric properties of BTFM-CTO films with a remanent ????????(??????)???? polarization exceeding 130 μC/cm2 under an applied voltage up to 25 V. These values were found to be much larger than those measured on bulk BTFM-CTO as a result of the preferential film orientation. The dielectric and ferroelectric properties of BTFM-CTO films were also found to depend on the morphotropic nature of the deposited films and samples failed to grow in the phase transition region have shown a shown a lossy, non-ferroelectric behaviour. Temperature dependent M(T) magnetisation and isothermal field-dependent magnetization M(H) measurements showed a substantially different magnetic behaviour of BTFM-CTO films to that of bulk samples with a strongly magnetized ferromagnetic iron-based impurity dominating the overall magnetic response of the films and hindering the weak ferromagnetic response of BTFM-CTO films.
Supervisor: Rosseinsky, M. J. ; Claridge, J. B. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral