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Title: An investigation of the structure and properties of 4d transition metal perovskite oxides
Author: Hopper, Harriet A.
ISNI:       0000 0004 6346 5120
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2017
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Perovskite-type materials have been widely studied in the literature as a result of the plethora of properties they have been found to exhibit. This is largely down to their versatile nature, which allows the substitution of a wide variety of different elements into the crystallographic sites. In addition to this the presence of 4d and 5d transition metal elements enables an even wider range of potential properties to be considered. The solid solution Sr1-xBaxMoO3 (x = 0.000, 0.025, 0.050, 0.075, 0.100 and 1.000) has been synthesised. Examination of the X-ray diffraction data via Rietveld refinement showed the materials crystallised with cubic Pm-3m symmetry, and there was a miscibility gap from x = 0.1 – 1.0. Examination of the optical properties showed that increasing x from 0 to 1 reduced the measured band gap, which was attributed to the electronic transition from the Mo 4d t2g band to the eg band, from 2.20 eV to 2.07 eV, as the ligand field splitting energy is closely related to the extent of hybridisation between Mo dx2-y2 and dz2 and the O 2p orbitals and the larger radius of Ba2+ compared to Sr2+ leads to longer Mo-O bonds and therefore weaker orbital mixing. The materials were examined as potential water-splitting photocatalysts but no evidence of hydrogen or oxygen evolution was found. In a similar fashion the solid solution Sr1-xCaxMoO3 (x = 0.00, 0.05, 0.10, 0.13, 0.15 and 0.17) was synthesised, and structural phase transitions were found to occur as x increased, from cubic Pm-3m to tetragonal I4/mcm to orthorhombic Imma. Discontinuities were observed in the cell parameters, bond lengths and angles at the transition from tetragonal to orthorhombic as a result of the switching of the octahedral rotation axis at the tetragonal to orthorhombic transition. The band gap was also found to decrease from 2.20 eV to 2.10 eV as x increased, which was further attributed to the octahedral tilting. The magnetic, electrical and structural properties of the Ruddlesden-Popper material – a variation on the perovskite structure – Sr3CoRuO7 were examined, and showed no structural changes down to 5 K, and no evidence of long-range magnetic order. A broad antiferromagnetic transition was observed at ~160 K which was attributed to short-range magnetism. The material was found to be semiconducting, and displayed Mott variable-range hopping behaviour below 240 K. The novel hexagonal perovskite series Ba3AMo2O9 (A = Sr, Ca, Nd and Pr0.5Nd0.5) was successfully synthesised, and attempts were made to synthesise the material Ba4Mo2O9, which was obtained mostly phase pure, with some minor impurities which were identified as polymorphs of the material and small amounts of Ba6Nb3O13.5 and Ba5.75Nb2.25O11.38. Examination of the magnetic properties revealed what appeared to be a transition at ~100 K in the Ba4Mo2O9, Ba3SrMo2O9 and Ba3CaMo2O9 materials, and spin gap formation was suspected below 100 K. The reduction in susceptibility was a possible indicator of spin dimer formation. Curie-Weiss fits were obtained for Ba4Mo2O9, Ba3CaMo2O9 and Ba3Pr0.5Nd0.5Mo2O9.
Supervisor: Not available Sponsor: University of Aberdeen ; Royal Society of Chemistry
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Perovskite