Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 136-151).

Thesis (Ph. D.)--Ohio State University, 2003. / Title from first page of PDF file. Document formatted into pages; contains xxvi, 255 p.; also includes graphics. Includes abstract and vita. Advisor: Patrick M. Woodward, Dept. of Chemistry. Includes bibliographical references (p. 221-228).

Interest in the molybdenum double perovskites has increased since the discovery of high temperature, low field magnetresistance in Sr2FeMoO6. Molybdenum double perovskites of the form Ba2LnMoO6 (where Ln = Sm, Sm0.5Eu0.5, Eu, Gd, Pr) as well as Sr2ScMoO6 have been synthesized by solid-state reactions up to 1200°C. Laboratory X-ray diffraction, variable temperature neutron and synchrotron X-ray diffraction as well as DC-SQUID magnetometry have been employed, in order to investigate the intricate relationship between spin, lattice and orbital degrees of freedom within these compounds. A variable temperature neutron diffraction experiment has been performed on the molybdenum double perovskite Ba2154SmMoO6 between 353 and 877 K in order to further investigate the crystal structure into the high temperature regime. Variable temperature neutron diffraction studies of Ba2PrMoO6 have been performed to investigate the links between structure, spin and orbital order in this material. A synchrotron X-ray diffraction study of Ba2GdMoO6 is detailed. Ba2GdMoO6 exhibits a ferroelastic phase transition below 220 K; the first time this has been reported in a Mo5+ double perovskite. Furthermore, a study of the geometrically frustrated double perovskite Sr2ScMoO6 has been carried out, in order to further investigate the exotic new valence bond glass phase. The results shed new light on the complex relationship between spin, lattice and orbital degrees of freedom in Ba2LnMoO6 double perovskites.

In this research project, iron-containing perovskites with the general formulae A₃Fe₂B'O₉, A₂A'Fe₂B'O₉ and A₂A'FeB'B''O₉ have been synthesised using conventional solid-state reactions. A combination of experimental techniques has been applied to characterise the samples. The main aspects studied include their crystal symmetry, cation ordering pattern and magnetic behaviour. We have shown for the first time that Sr₃Fe₂TeO₉ can adopt a trigonal perovskite-like structure with the Fe³⁺ and Te⁶⁺ cations ordering in a 2:1 sequence. However, the trigonal structure is disrupted by both nanotwinning and regions where the cations order in a 1:1 sequence. These disruptions prevent full antiferromagnetic ordering throughout the sample and the unordered spins form a spin-glass phase that coexists below 80 K. The identification of this disorder has allowed us to account for inconsistencies in the existing literature. Ba₃Fe₂TeO₉ is a 6H perovskite in which the cation sites in the face-sharing octahedra and the vertex-sharing octahedra are occupied by different fractions of Fe³⁺. At a temperature close to 300 K the atomic moments begin to order in an antiferromagnetic manner, and the spins that are unable to take part in the long-range magnetic order form clusters that freeze at 18 K. In the series Sr_xBa_3-xFe₂TeO₉ (x=1~2.5), as x decreases the crystal structure switches from purely pseudo-cubic to purely hexagonal via a biphasic region, illustrating the effect of the radius ratio r_A/r_B on crystal structure. A₂LaFe₂SbO₉ (A=Ba, Sr, Ca) and CaLa₂Fe₂SnO₉ have cation disorder on their A sites. Ca₂LaFe₂SbO₉ and CaLa₂Fe₂SnO₉ both adopt a monoclinic P21/n structure and Sr2LaFe2SbO9 adopts a triclinic P -1 structure, all of which show different levels of B-cation ordering. Ba₂LaFe₂SbO₉ has a Pbnm structure with disordered B sites. These perovskites are not paramagnetic at 300 K, and they all adopt a G-type magnetic structure, which leads to long-range ferrimagnetic or weak ferromagnetic behaviour. The three antimonycontaining compounds contain a minor spin-glass-like phase below 50 K, while CaLa₂Fe₂SnO₉ has a relatively well-developed magnetic backbone at 300 K. We found that the six-coordinate cations in the P2₁/n perovskite SrLa₂FeCoSbO₉ order in a previously unreported manner. The observed cation distribution, with diamagnetic Sb⁵⁺ and magnetic Co²⁺ each partially occupying only one of the six-coordinate sites, results in ferrimagnetism below the Curie temperature of 215 K. CaLa₂FeCoSbO₉ and ALa2FeNiSbO9 (A=Ba, Sr, Ca) were prepared as analogues of SrLa2FeCoSbO9, and they have similar crystal structure and high Curie temperatures. However, TEM revealed the different levels of inhomogeneity present in these four compounds. The inhomogeneity is least significant in CaLa₂FeCoSbO₉, and it is most significant in BaLa₂FeNiSbO₉ where both primitive phase and body-centred phases have been observed in a single crystallite. Consequently, doubt has been cast on the interpretation of the diffraction data for these inhomogeneous samples. A₂LaFe₂NbO₉ (A=Sr, Ca) and CaLa₂Fe₂TaO₉ were prepared with d⁰ cations for comparison with the perovskites containing d¹⁰ B-cations. Ca₂LaFe₂NbO₉ and CaLa₂Fe₂TaO₉ adopt the P2₁/n structure, and Sr₂LaFe₂NbO₉ adopts the P -1 structure. These three perovskites show less well-developed ordering pattern than their antimony analogues due to the smaller difference in size of B-cations. They are not simple paramagnets at 300 K, and they all adopt a G-type magnetic structure with long-range ferrimagnetism. However, the formation of magnetic backbone is significantly slowed down from that in the d¹⁰ compounds and the temperature for the paramagnetic spins to freeze is lowered below 20 K. We propose that in these materials a J₃ interaction occurs via the Fe³⁺ - O - Nb⁵⁺/Ta⁵⁺ - O - Fe³⁺ pathway to compete with the dominant J₁ interaction. The J₃ interaction is more significant when d⁰ cations are present because hybridisation of the empty d orbitals and the anion p orbitals facilitates virtual electron transfer.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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.

Metal halide perovskites have rapidly gained researchers’ attention and become one of the most promising materials today, with exciting properties and multiple optoelectronic applications. Regardless of the shape into which perovskite materials have been fashioned, the nanocrystals that constitute these materials have been the object of extensive research. Perovskite nanocrystals (PNCs) have rapidly developed due to their favorable optical and electronic properties, including high photoluminescence quantum yield, narrow emission bands, and tunable optical band gap. This dissertation discusses several chemical approaches to enhance optical performance properties of PNCs, including their photoluminescence quantum yield and stability.

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Thesis (Ph. D.)--Oregon State University, 2009. / Printout. Includes bibliographical references. Also available on the World Wide Web.

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