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University of Southampton Research Repository Eprints Soton University of Southampton Research Repository ePrints Soton Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g. AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination http://eprints.soton.ac.uk UNIVERSITY OF SOUTHAMPTON FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS School of Chemistry Structural studies of layered transition metal oxides by Peter James Hickey Thesis for the degree of Doctor of Philosophy October 2009 UNIVERSITY OF SOUTHAMPTON ABSTRACT FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS SCHOOL OF CHEMISTRY Doctor of Philosophy Structural studies of layered transition metal oxides By Peter Hickey A number of transition metal oxide materials, both new and previously reported, have been synthesised, their structures based on the n = 2 Ruddlesden-Popper layered perovskites. Their structures were determined through Rietveld refinement against X-ray and neutron powder diffraction, supported by vibrating sample magnetometry and thermal analysis. The structure of the double layered phase Gd2SrCo2O7 was determined in the temperature range 300 – 973 K by refinement of powder neutron diffraction data. The room temperature structure is tetragonal, and displays small tilts of the oxygen sublattice that are described by a 2a0 2a0 x c supercell model in the space group P42/mnm rather than the idealised Sr3Ti2O7 I4/mmm structure as previously reported. A structural transition at ~ 580 K is characterised by reduction in the unit cell symmetry and distortion of CoO6 octahedra; the behaviour is concurrent with a spin crossover in the Co (III) ions from intermediate to high spin. The structure of the new phase Nd2BaCo2O7 was determined in the temperature range 298 – 873 K by refinement of powder neutron diffraction data. At all temperatures, the structure is metrically tetragonal, however the octahedral rotations are best described in the orthorhombic space group Bbcb, with a = b = 2a0. The structures of the phases Ln2SrFe2O7 (Ln = La, Nd, Gd) have been determined by refinement of variable temperature powder neutron diffraction data. At room temperature all the materials crystallise in the tetragonal space group P42/mnm, isostructural with the phase Tb2BaFe2O7. On heating, the gadolinium and neodymium phases undergo transitions to lower symmetry; the octahedral tilts of the high temperature phases are best described in the space group Bbmm. The new phases Nd2-xSr1+xFe2-xO7 have been prepared and their structures determined by Rietveld refinement of room temperature powder neutron diffraction. All the materials crystallise in the tetragonal space group P42/mnm. The degree of octahedral tilting decreases as x increases, indicating a reduction of compression of the perovskite lattice as the average A-site cationic radius increases. The phase Sr3CoNbO7 has been prepared and its structure determined by refinement of powder neutron diffraction data. At room temperature and 3.4 K there is no evidence of magnetic ordering, B-site cationic ordering or octahedral tilting. The material is isostructural with the undistorted Sr3Ti2O7, which implies that the spin state of Co (III) in the material is low spin. The material Sr3Ti2O7 was prepared and its structure determined in the temperature range 298 – 1073 K. At all temperatures, the structure is tetragonal, with no rotations of octahedra. Unit cell parameters and bond lengths expand smoothly over the temperature range. i Table of contents Chapter 1 Chapter 1 – Introduction ............................................................................................................. 1 Transition metal oxide materials ......................................................................................................... 1 Perovskites .......................................................................................................................................... 1 Properties of materials based on the perovskite structure ................................................................... 3 Superconductivity ........................................................................................................................................... 3 Cuprate superconductors ................................................................................................................................ 3 Superconductivity in iron arsenides ................................................................................................................ 7 Magnetism and magnetoresistance in perovskite derivatives ......................................................................... 8 Ruddlesden-Popper phases ................................................................................................................ 12 A3M2X7 ......................................................................................................................................................... 13 A = K, Rb ..................................................................................................................................................... 13 A = La ........................................................................................................................................................... 14 A = Ca, Sr, Ba ............................................................................................................................................... 16 (AA’)3M2X7 .................................................................................................................................................. 24 Scope of this work ............................................................................................................................. 25 References ......................................................................................................................................... 26 Chapter 2 – Experimental methods .......................................................................................... 30 Introduction ....................................................................................................................................... 30 Synthesis methods ............................................................................................................................. 30 Calcination .................................................................................................................................................... 30 Sol-gel reactions ........................................................................................................................................... 31 Characterisation methods – diffraction methods ............................................................................... 32 X-ray diffraction ........................................................................................................................................... 32 Powder X-ray diffraction .............................................................................................................................. 36 Instrumentation ............................................................................................................................................. 37 D5000 diffractometer .................................................................................................................................... 39 D8 diffractometer.......................................................................................................................................... 39 Variable temperature powder diffraction ...................................................................................................... 40 Structural refinement .................................................................................................................................... 40 Neutron diffraction ....................................................................................................................................... 42 Reactor sources ............................................................................................................................................. 43 D2B and D20 ................................................................................................................................................ 43 Spallation sources ......................................................................................................................................... 44 POLARIS...................................................................................................................................................... 46 HRPD ........................................................................................................................................................... 47 Characterisation methods – non-diffraction methods ......................................................................
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