Investigation of the structural influence on the properties of functional inorganic oxides

While properties are extremely important from an application point of view, it is crucial to have a detailed understanding of the underlying structural influence. Once a concrete correlation between the structure and the observed property is established, rational design of novel materials with optimized properties can be realized. These optimized materials lead to advancements in technology in a variety of applications, including new building materials, faster electronic devices, and more efficient catalysts. This dissertation examines the structural influence on the observed properties in a series of metal oxide materials for electronic and energy applications. A series of pyrochlores Ag[subscript 1-nx]M[superscript n][subscript x]SbO₃ (M = Na⁺, K⁺, Tl⁺, and Cd²⁺) has been studied to evaluate the structural influence on the samples' photocatalytic activity. A complete solid solution between the anion-deficient pyrochlore Ag₂Sb₂O₆ and the ideal pyrochlore Cd₂Sb₂O₇ has been synthesized through the standard solid state ceramic method. Each composition has been characterized by various different techniques, including powder X-ray diffraction, optical spectroscopy, electron paramagnetic resonance and ¹²¹Sb Mössbauer spectroscopy. Computational methods based on density functional theory complement this investigation. Photocatalytic activity has been studied, and transport properties have been measured on pellets densified by spark plasma sintering. The analysis of the data collected from these various techniques enables a comprehensive characterization of the complete solid solution and revealed an anomalous behavior in the Cd-rich end of the solid solution, which has been proposed to arise from a possible radical O⁻ species in small concentrations. Polycrystalline samples of the pyrochlore series Ag[subscript 1-nx]M[superscript n][subscript x]SbO₃ (M = Na⁺, K⁺ and Tl[superscript +/3+]) have been structurally analyzed through total scattering techniques and evaluated for photocatalytic activity. The upper limits of x obtained are 0.08 for Na, 0.16 for K, and 0.17 for Tl. The Ag⁺ cation occupies a site with inversion symmetry on a 3-fold axis. When the smaller Na⁺ cation substitutes for Ag⁺, it is displaced by about 0.6 Å perpendicular to the 3-fold axis to achieve shorter Na-O bond distances. When the larger Tl⁺ cation substitutes for Ag⁺, it is displaced by about 1.14 Å along the 3-fold axis and achieves an environment typical of a lone pair cation. Some of the Tl³⁺ from the precursor remains unreduced, leading to a formula of Ag₀.₇₇Tl⁺₀.₁₃Tl³⁺₀.₀₄SbO₃.₀₄. The position of the K⁺ dopant was effectively modeled assuming that K⁺ occupied the same site as Ag⁺. The expansion of the lattice caused by substitution of the larger K⁺ and Tl⁺ cations results in longer Ag-O bond lengths, which would reduce the overlap of the Ag 4d and O 2p orbitals that compose the valence band maximum. Substitution of the smaller Na⁺ results in a decrease in the Ag-O bond distance, thus increasing the overlap of the Ag 4d and O 2p orbitals. An increase in the photocatalytic activity has been observed for the nominal composition Ag₀.₈Na₀.₂SbO₃ made through solid state synthesis, and this is attributed to both the slight decrease in the band gap and the increase in pore dimensions compared to the parent compound AgSbO₃. The structural transitions in Cd₂Nb₂O[subscript 7-x]S[subscript x] (x = 0, 0.25, 0.5, and 0.7) have been studied to determine the origin of ferroelectricity in pyrochlore oxides. For x = 0, 0.25, and 0.5 peak splitting indicative of a transition to orthorhombic symmetry is observed below the transition temperature. In the x = 0.7 sample, the evolution of new peaks suggest a cubic space group is retained below the phase transition accompanied by a loss of the face-centering symmetry. The observed lowering of symmetry may be responsible for the transition into a ferroelectric phase, and may be driven by a strong displacement of both the Nb and Cd from the high- to low-symmetry structures. The S content may drive the stability if different ferroelectric phases, as no trend was observed with increasing content in the ferroelectric Curie temperatures of the samples. The structure of the hollandites A[subscript x]Ru₄O₈ (A = K⁺, Rb⁺) has been studied through total scattering techniques upon cation exchange with Na⁺ on the A-site to evaluate the effect on the quasi-one dimensional (Q1D) nature of these materials. It is observed that the A-site of the hollandite structure is not fully occupied when A = K⁺, Rb⁺, and full A-site occupancy is achieved after ion exchange with NaNO₃. All samples exhibit Pauli paramagnetism, and this is primarily due to a large low temperature range of metallic conduction. The double chains of edge-shared RuO₆ octahedra and corner shared double chains found in the channel of the hollandite structure promotes two conduction mechanisms: ρ∥ (intra-chain metallic) and ρ⊥ (inter-chain hopping). The coexistence of ρ∥ and ρ⊥ gives rise to metallic conductivity below T[subscript max] (suppressed hopping at lower temperature) and semiconductivity above T[subscript max] (intra-chain mean free path becomes smaller than the inter-chain hopping distance), exhibiting the Q1D conduction at lower temperatures. The inter-chain distance is larger in the Rb-containing samples, and consequently the region dominated by intra-chain metallic conduction increases, along with an increase in T[subscript max]

Strategies and Considerations for Least-Squares Analysis of Total Scattering Data

The process of least-squares analysis has been applied for decades in the field of crystallography. Here, we discuss the application of this process to total scattering data, primarily in the combination of least-squares Rietveld refinements and fitting of the atomic pair distribution function (PDF). While these two approaches use the same framework, the interpretation of results from least-squares fitting of PDF data should be done with caution through carefully constructed analysis approaches. We provide strategies and considerations for applying least-squares analysis to total scattering data, combining both crystallographic Rietveld and fitting of PDF data, given in context with recent examples from the literature. This perspective is aimed to be an accessible document for those new to the total scattering approach, as well as a reflective framework for the total scattering expert

Vacancy Tuning in Li,V-Substituted Lyonsites

© 2020 Taylor & Francis Group, LLC. The lyonsite structure, characterized by the formula A 4 M 3O12, is a relatively understudied tunneled crystal structure. This host structure is known to be compositionally flexible, able to incorporate a number of cations in various oxidation states into the A site. In the parent compound Co3.75V1.5Mo1.5O12, it is apparent that a stoichiometric vacancy of 0.25 is unavoidable as a result of Coulombic repulsion. This work focuses on the systematic elimination of vacancies by chemically introducing guest Li ions while maintaining host integrity. A full solid solution was found to exist with the formula □0.25–1/8xLi x Co3.75–7/8xV1.5–3/4xMo1.5+3/4xO12 (0 ≤ x ≤ 2), terminating at the known end member Li2Co2Mo3O12. Lattice refinements on PXRD data confirmed the isostructural nature of the whole series, and detailed structural analysis revealed that competition between Li and Co in the same crystallographic site is unequal, with Li exhibiting a stronger site preference for larger interstitial sites. Diffuse reflectance analysis revealed that the optical band gap is directly tunable with x, and supporting structure-property relationships were also explored via magnetometry and dielectric measurements

Evidence of paracrystalline cation order in the Ruddlesden-Popper phase LaSr3NiRuO8 through total scattering techniques

Cation ordering in perovskite-derived phases can lead to a wealth of tunable physical properties. Ordering is typically driven by a large difference between the cation size and charge, but many Ruddlesden–Popper phases An+1BnO3n+1 appear to lack such B-site ordering, even when these differences are present. One such example is the “double” Ruddlesden–Popper n = 1 composition LaSr3NiRuO8. In this material, a lack of B-site ordering is observed through traditional crystallographic techniques, but antiferromagnetic ordering in the magnetism data suggests that B-site cation ordering is indeed present. Neutron total scattering, particularly analysis of the neutron pair distribution function, reveals that the structure is locally B-site-ordered below 6 Å but becomes slightly disordered in the midrange structure around 12 Å. This provides evidence for paracrystalline order in this material: cation ordering within a single perovskite sheet that lacks perfect registry within the three-dimensional stack of sheets. This work highlights the importance of employing a structural technique that can probe both the local and midrange order in addition to the crystallographic structure and provides a structural origin to the observed magnetic properties of LaSr3NiRuO8. Further, it is proposed that paracrystalline order is likely to be common among these layered-type oxides

Leggett Modes Accompanying Crystallographic Phase Transitions

Higgs and Goldstone modes, well known in high-energy physics, have been realized in a number of condensed matter physics contexts, including superconductivity and magnetism. The Goldstone-Higgs concept is also applicable to and gives rise to new insight on structural phase transitions. Here, we show that the Leggett mode, a collective mode observed in multiband superconductors, also has an analog in crystallographic phase transitions. Such structural Leggett modes can occur in the phase channel as in the original work of Leggett [Prog. Theor. Phys. 36, 901 (1966)PTPKAV0033-068X10.1143/PTP.36.901]. That is, they are antiphase Goldstone modes (antiphasons). In addition, a new collective mode can also occur in the amplitude channel, an out-of phase (antiphase) Higgs mode, that should be observable in multiband superconductors as well. We illustrate the existence and properties of these structural Leggett modes using the example of the pyrochlore relaxor ferroelectric Cd2Nb2O7

Lattice Anharmonicity of Stereochemically Active Lone Pairs Controls Thermochromic Band Gap Reduction of PbVO\u3csub\u3e3\u3c/sub\u3eCl

Copyright © 2020 American Chemical Society. Stereochemically active lone pairs of electrons play an important role in a diverse range of physical phenomena in many materials, ranging from semiconducting halide perovskites to thermochromic inorganic-organic hybrids. In this paper, we demonstrate the importance of the 6s2 lone pair of Pb on the reversible thermochromic transition in the mixed-anion inorganic compound, PbVO3Cl. This 6s2 stereochemically active lone pair results in subtle structural distortions upon heating while maintaining its overall orthorhombic structure. These distortions result in competing interactions with the Pb 6s2 lone pair and ultimately, a pronounced change between yellow and red at ∼200 °C. X-ray diffraction analyses of PbVO3Cl demonstrate two-dimensional features in contrast to the three-dimensional network in isostructural BaVO3Cl. X-ray and neutron pair distribution function experiments reveal that Pb-O interatomic distances decrease upon heating, while Pb-Cl distances are only affected by thermal motion. X-ray photoelectron spectroscopy measurements provide experimental evidence of the presence of the 6s2 lone pair at the valence band maximum, which are corroborated by first-principles calculations. The results demonstrate a broadly generalizable mechanism for using repulsions between lone-pair electrons of p-block cations to drive discontinuous changes of local symmetry and electronic structure

Covalency-driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7-xSx

The arrangement of cations on the triangular pyrochlore lattice leads to a wealth of interesting physical phenomena influenced by geometric frustration. Although uncommon, several pyrochlore materials overcome this frustration and exhibit polar structures. Unraveling the origin of such behavior is key to understanding how broken inversion symmetry arises in complex crystal structures. Here, we investigate the effect of varying degrees of covalency in the pyrochlore lattice through a detailed structural and lattice dynamical analysis of the pyrochlore oxysulfide series Cd2Nb2O7-xSx above and below the ferroelectric transition temperatures (TC) using synchrotron X-ray diffraction and first principles calculations. All compositions exhibit the cubic Fd3 m pyrochlore aristotype above TC, whereas the amplitude and character of various structural distortions are found to be composition-dependent below TC. For x = 0, large Cd and Nb cation displacements occur to produce the polar Ima2 structure accompanied by a change in translational symmetry. Our symmetry and lattice dynamical calculations indicate that Cd2Nb2O7 undergoes a proper ferroelectric transition through TC. Analysis of the sulfur-substituted niobates indicates that although the polar space group Fdd2 is adopted by the nominal x = 0.25 sample, the transition into the polar phase is improper. For the nominally x = 0.7 composition, the lattice remains nearly cubic, but exhibits a high degree of structural disorder in the pyrochlore channel, with a deviation from the linear Cd-X′-Cd bond by nearly 15° to accommodate the large size of S while preventing extreme stretching of the Nb-O bond. This highly distorted Cd-X′ network is accompanied by a highly distorted NbO6 network, which is accommodated by the polarizable NbO6 coordination environment. This sheds light on the limited existence of oxysulfide pyrochlores; for example, the lack of reported S substitution in the case of the similar yet less-polarizable Cd2Ta2O7. Our work provides a new understanding of how inversion-symmetry lifting displacements arise and how anion substitution, which tunes covalent cation-anion interactions, is a useful strategy for manipulating polar behavior in a pyrochlore lattice

From Ag₂Sb₂O₆ to Cd₂Sb₂O₇: Investigations on an anion-deficient to ideal pyrochlore solid solution

A complete solid solution between the anion-deficient pyrochlore Ag₂Sb₂O₆ and the ideal pyrochlore Cd₂Sb₂O₇ has been synthesized through the standard solid state ceramic method. Each composition has been characterized by various different techniques, including powder X-ray diffraction, optical spectroscopy, electron paramagnetic resonance and ¹²¹Sb Mössbauer spectroscopy. Computational methods based on density functional theory complement this investigation. Photocatalytic activity has been studied, and transport properties have been measured on pellets densified by spark plasma sintering. The analysis of the data collected from these various techniques enables a comprehensive characterization of the complete solid solution and revealed an anomalous behavior in the Cd-rich end of the solid solution, which has been proposed to arise from a possible radical O⁻ species in small concentrations.Keywords: Cd₂Sb₂O₇, Thermogravimetric analysis, Ag₂Sb₂O₆, ¹²¹Sb Mössbauer spectroscopy, Optical properties, Pyrochlor

Uncorrelated Bi off-centering and the insulator-to-metal transition in ruthenium A(2)Ru(2)O(7) pyrochlores

The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionally-controlled insulator-to-metal transition in the solid solution oxide pyrochlore (Pr1-xBix)2Ru2O7. Prior work has established site disorder by the Bi3+ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the crossover from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state