Understanding complex magnetic order in disordered cobalt hydroxides through analysis of the local structure

In many ostensibly crystalline materials, unit-cell-based descriptions do not always capture the complete physics of the system due to disruption in long-range order. In the series of cobalt hydroxides studied here, Co(OH)$_{2-x}$(Cl)$_x$(H$_2$O)$_{n}$, magnetic Bragg diffraction reveals a fully compensated N\'eel state, yet the materials show significant and open magnetization loops. A detailed analysis of the local structure defines the aperiodic arrangement of cobalt coordination polyhedra. Representation of the structure as a combination of distinct polyhedral motifs explains the existence of locally uncompensated moments and provides a quantitative agreement with bulk magnetic measurements and magnetic Bragg diffraction

Solid state chemistry of platinum group and noble metal oxides: Implications for heterogeneous catalysis

The substitution of precious metal ions in oxide hosts represents an attractive alternative to conventional metal-particle-on-oxide catalysts, potentially providing improvements in efficacy and lifetime while simultaneously reducing the loading of these scarce and expensive elements. This dissertation addresses a range of topics within a theme of understanding the solid state chemistry of platinum group and noble metal ions, with implications for their role in heterogeneous catalysis. To inform an understanding of the crystal chemistry of precious metal-substituted oxides, we begin by examining stoichiometric complex oxide compounds of isoelectronic Au3+ and Pd2+. La4LiAuO8 and La2BaPdO5 display exceptional resistance towards thermal dissociation and chemical reduction, serving as model compounds for understanding the crystal chemical conditions that stabilize these oxophobic elements in oxide environments. This understanding is further borne out in a neutron powder diffraction study of La4LiAuO8 – the first such study of a stoichiometric (rather than lightly substituted) oxide compound with gold. Recent interest in platinum group metal- (PGM) substitution has been stimulated by findings that certain substituted complex oxides can reversibly extrude and reincorporate the PGM in response to redox changes in the atmosphere to which they are exposed. Of the PGM-substituted complex oxides studied, almost all have been perovskites. Here we present an investigation of Pd 2+/Pd0 redox cycling in a non-perovskite host, hexagonal YMnO3-type YMn0:5Fe0:5O3. YMn0.5Fe 0.5-xPdxO3-δ is only modestly tolerant to removal and redistribution of Pd2+; after only a few cycle cycles PdO is formed, rather than Pd2+ being reintegrated into the host. The final study presented here is a computational investigation of electronic structural features in compounds spanning the range of formal oxidation states adopted by gold in extended solids. Guided by the well-established importance of the position at which the transition element d band is centered (relative to the Fermi energy), we find an absence of any correlation between the charge on or oxidation state of Au and the position of the Au 5d-band center. Instead of correlating with the Au charge or oxidation state, the location of the d-band center shows a strong dependence on the degree of its filling

Hybrid Inorganic–Organic Materials with an Optoelectronically Active Aromatic Cation: (C₇H₇)₂SnI₆ and C₇H₇PbI₃

Inorganic materials with organic constituentshybrid materialshave shown incredible promise as chemically tunable functional materials with interesting optical and electronic properties. Here, the preparation and structure are reported of two hybrid materials containing the optoelectronically active tropylium ion within tin- and lead-iodide inorganic frameworks with distinct topologies. The crystal structures of tropylium tin iodide, (C₇H₇)₂SnI₆, and tropylium lead iodide, C₇H₇PbI₃, were solved using high-resolution synchrotron powder X-ray diffraction informed by X-ray pair distribution function data and high-resolution time-of-flight neutron diffraction. Tropylium tin iodide contains isolated tin­(IV)-iodide octahedra and crystallizes as a deep black solid, while tropylium lead iodide presents one-dimensional chains of face-sharing lead­(II)-iodide octahedra and crystallizes as a bright red-orange powder. Experimental diffuse reflectance spectra are in good agreement with density functional calculations of the electronic structure. Calculations of the band decomposed charge densities suggest that the deep black color of tropylium tin iodide is attributed to iodide ligand to tin metal charge transfer, while the bright red-orange color of tropylium lead iodide arises from charge transfer between iodine and tropylium states. Understanding the origins of the observed optoelectronic properties of these two compounds, with respect to their distinct topologies and organic–inorganic interactions, provides insight into the design of tropylium-containing compounds for potential optical and electronic applications

Polymorph Selectivity of Superconducting CuSe₂ Through Kinetic Control of Solid-State Metathesis

Rational preparation of materials by design is a major goal of inorganic, solid-state, and materials chemists alike. Oftentimes, the use of nonmetallurgical reactions (e.g., chalcogenide fluxes, hydrothermal syntheses, and in this case solid-state metathesis) alters the thermodynamic driving force of the reaction and allows new, refractory, or otherwise energetically unfavorable materials to form under softer conditions. Taking this a step further, alteration of a metathesis reaction pathway can result in either the formation of the equilibrium marcasite polymorph (by stringent exclusion of air) or the kinetically controlled formation of the high-pressure pyrite polymorph of CuSe₂ (by exposure to air). From analysis of the reaction coordinate with <i>in situ</i> synchrotron X-ray diffraction and pair distribution function analysis as well as differential scanning calorimetry, it is clear that the air-exposed reaction proceeds via slight, endothermic rearrangements of crystalline intermediates to form pyrite, which is attributed to partial solvation of the reaction from atmospheric humidity. In contrast, the air-free reaction proceeds via a significant exothermic process to form marcasite. Decoupling the formation of NaCl from the formation of CuSe₂ enables kinetic control to be exercised over the resulting polymorph of these superconducting metal dichalcogenides

Hybrid Inorganic–Organic Materials with an Optoelectronically Active Aromatic Cation: (C₇H₇)₂SnI₆ and C₇H₇PbI₃

Inorganic materials with organic constituentshybrid materialshave shown incredible promise as chemically tunable functional materials with interesting optical and electronic properties. Here, the preparation and structure are reported of two hybrid materials containing the optoelectronically active tropylium ion within tin- and lead-iodide inorganic frameworks with distinct topologies. The crystal structures of tropylium tin iodide, (C₇H₇)₂SnI₆, and tropylium lead iodide, C₇H₇PbI₃, were solved using high-resolution synchrotron powder X-ray diffraction informed by X-ray pair distribution function data and high-resolution time-of-flight neutron diffraction. Tropylium tin iodide contains isolated tin­(IV)-iodide octahedra and crystallizes as a deep black solid, while tropylium lead iodide presents one-dimensional chains of face-sharing lead­(II)-iodide octahedra and crystallizes as a bright red-orange powder. Experimental diffuse reflectance spectra are in good agreement with density functional calculations of the electronic structure. Calculations of the band decomposed charge densities suggest that the deep black color of tropylium tin iodide is attributed to iodide ligand to tin metal charge transfer, while the bright red-orange color of tropylium lead iodide arises from charge transfer between iodine and tropylium states. Understanding the origins of the observed optoelectronic properties of these two compounds, with respect to their distinct topologies and organic–inorganic interactions, provides insight into the design of tropylium-containing compounds for potential optical and electronic applications

Circumventing Diffusion in Kinetically Controlled Solid-State Metathesis Reactions

Solid-state diffusion is often the primary limitation in the synthesis of crystalline inorganic materials and prevents the potential discovery and isolation of new materials that may not be the most stable with respect to the reaction conditions. Synthetic approaches that circumvent diffusion <i>in solid-state reactions</i> are rare and often allow the formation of metastable products. To this end, we present an <i>in situ</i> study of the solid-state metathesis reactions MCl₂ + Na₂S₂ → MS₂ + 2 NaCl (M = Fe, Co, Ni) using synchrotron powder X-ray diffraction and differential scanning calorimetry. Depending on the preparation method of the reaction, either combining the reactants in an air-free environment or grinding homogeneously in air before annealing, the barrier to product formation, and therefore reaction pathway, can be altered. In the air-free reactions, the product formation appears to be diffusion limited, with a number of intermediate phases observed before formation of the MS₂ product. However, grinding the reactants in air allows NaCl to form directly without annealing and displaces the corresponding metal and sulfide ions into an amorphous matrix, as confirmed by pair distribution function analysis. Heating this mixture yields direct nucleation of the MS₂ phase and avoids all crystalline binary intermediates. Grinding in air also dissipates a large amount of lattice energy via the formation of NaCl, and the crystallization of the metal sulfide is a much less exothermic process. This approach has the potential to allow formation of a range of binary, ternary, or higher-ordered compounds to be synthesized in the bulk, while avoiding the formation of many binary intermediates that may otherwise form in a diffusion-limited reaction