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• 1. [Article] Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2}

  We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 ...

  Citation × Citation Citation Abstract Copy Title: Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2} Author: Stetzer, M. R., Heiney, Paul A., Stephens, Peter W., Dinnebier, R. E., Zhu, Q., McGhie, Andrew R., Strongin, Robert M., Brandt, B. M., Smith, Amos B., III Year: 2000 We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 cyclopropane molecules, like those of the 6,5-annulene isomer and C60O epoxide, are orientationally disordered and crystallize on a face-centered-cubic lattice such that their methylene groups are statistically disordered among the octahedral voids. Unlike 6,5−C61H2 and C60O, the low-temperature structure is not Pa3¯, but rather a low-symmetry orthorhombic lattice in which a≈b Title: Structure and Phase Transitions of the 6, 6-Cyclopropane Isomer of C_ {61} H_ {2} Author: Stetzer, M. R., Heiney, Paul A., Stephens, Peter W., Dinnebier, R. E., Zhu, Q., McGhie, Andrew R., Strongin, Robert M., Brandt, B. M., Smith, Amos B., III Year: 2000 We have used x-ray powder diffraction and differential scanning calorimetry to study the crystalline structures and thermal behavior of the 6,6-cyclopropane isomer of C61H2. At room temperature, the C61H2 cyclopropane molecules, like those of the 6,5-annulene isomer and C60O epoxide, are orientationally disordered and crystallize on a face-centered-cubic lattice such that their methylene groups are statistically disordered among the octahedral voids. Unlike 6,5−C61H2 and C60O, the low-temperature structure is not Pa3¯, but rather a low-symmetry orthorhombic lattice in which a≈b Close

• 2. [Article] 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 ...

  Citation × Citation Citation Abstract Copy Title: Investigation of the structural influence on the properties of functional inorganic oxides Author: Laurita-Plankis, Geneva 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]. Title: Investigation of the structural influence on the properties of functional inorganic oxides Author: Laurita-Plankis, Geneva 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]. Close

• 3. [Article] Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs).

  X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit ...

  Citation × Citation Citation Abstract Copy Title: Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Author: Zhu, Q., Zhou, O., Fischer, J. E., McGhie, Andrew R., Romanow, W. R., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1993 X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60. Title: Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Author: Zhu, Q., Zhou, O., Fischer, J. E., McGhie, Andrew R., Romanow, W. R., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1993 X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60. Close

• 4. [Article] Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2}

  We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered ...

  Citation × Citation Citation Abstract Copy Title: Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2} Author: Lommen, Andrea N., Heiney, Paul A., Vaughan, Gavin B. M., Stephens, Peter W., Liu, Dengfa, Li, Dayin, Smith, Allan L., McGhie, Andrew R., Strongin, Robert M., Brard, Laurent, Smith, Amos B., III Year: 1994 We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60O and higher still than in C60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases. Title: Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2} Author: Lommen, Andrea N., Heiney, Paul A., Vaughan, Gavin B. M., Stephens, Peter W., Liu, Dengfa, Li, Dayin, Smith, Allan L., McGhie, Andrew R., Strongin, Robert M., Brard, Laurent, Smith, Amos B., III Year: 1994 We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60O and higher still than in C60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases. Close

• 5. [Article] Solid Solution Studies of Layered Honeycomb-Ordered Phases O3-Na3M2SbO6 (M = Cu, Mg, Ni, Zn)

  This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-solid...

  Citation × Citation Citation Abstract Copy Title: Solid Solution Studies of Layered Honeycomb-Ordered Phases O3-Na3M2SbO6 (M = Cu, Mg, Ni, Zn) Author: Berthelot, Romain, Sleight, A. W., Subramanian, M. A., Schmidt, Whitney This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-solid-state-chemistry/. Title: Solid Solution Studies of Layered Honeycomb-Ordered Phases O3-Na3M2SbO6 (M = Cu, Mg, Ni, Zn) Author: Berthelot, Romain, Sleight, A. W., Subramanian, M. A., Schmidt, Whitney This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/journal-of-solid-state-chemistry/. Close

• 6. [Article] Effects of doping, cation stoichiometry, and the processing conditions on the dielectric properties of high-K CaCu₃Ti₄O₁₂ ceramics

  Since its discovery, the anomalous dielectric behavior of CaCu₃Ti₄O₁₂ (CCTO) has drawn a great deal of attention for many possible applications in electronic devices. The origin of the giant dielectric ...

  Citation × Citation Citation Abstract Copy Title: Effects of doping, cation stoichiometry, and the processing conditions on the dielectric properties of high-K CaCu₃Ti₄O₁₂ ceramics Author: Kwon, Seunghwa Since its discovery, the anomalous dielectric behavior of CaCu₃Ti₄O₁₂ (CCTO) has drawn a great deal of attention for many possible applications in electronic devices. The origin of the giant dielectric constant in CCTO was explained via an internal barrier layer capacitor model, and it has been reported that the dielectric properties of CCTO are sensitive to its microstructure which depends upon the processing conditions. To further explore its unusual dielectric phenomena, the current study focuses on the process-property-structure relationship of the high-K CCTO via doping, cation non-stoichiometry, and sintering conditions. A variety of CCTO pellets were obtained by utilizing the different processing parameters via conventional solid-state synthesis methods. For the doping study, three types of dopants were selected with the variation of doping concentration. The study of undoped CCTO ceramics was carried out by the modification of the CuO and TiO₂ content in CCTO to create the stoichiometric formula CaCu₃+xTi₄+yO₁₂ (x = -0.06, 0, - 0.06; y = 0.08, 0, -0.08). Different processing factors including heating and cooling rates, sintering temperature, and sintering time were applied for the study of stoichiometric CCTO. X-ray diffraction, dielectric measurements, impedance spectroscopy, thermal analysis, and electron microprobe analysis were used to determine the existence of CuO and Cu₂O secondary phases, dielectric constant and loss tangent, electrical resistivity of grains and boundaries, decomposition reactions, and the microstructural changes of CCTO ceramics. It was revealed that the doping method improved the dielectric constant and loss tangent. The similar improvement in dielectric properties was also found in the Cu-deficient and Ti-deficient CCTO. The measurement and characterization results of stoichiometric CCTO clearly indicated that the dielectric properties, evolution of secondary phases, and microstructures were strongly dependent upon the processing parameters. Further, CCTO became Cu-deficient and Ti-excessive regardless of sintering conditions, which provides evidence for its complex behavior created by the reduction/oxidation reactions and the defect chemistry. Title: Effects of doping, cation stoichiometry, and the processing conditions on the dielectric properties of high-K CaCu₃Ti₄O₁₂ ceramics Author: Kwon, Seunghwa Since its discovery, the anomalous dielectric behavior of CaCu₃Ti₄O₁₂ (CCTO) has drawn a great deal of attention for many possible applications in electronic devices. The origin of the giant dielectric constant in CCTO was explained via an internal barrier layer capacitor model, and it has been reported that the dielectric properties of CCTO are sensitive to its microstructure which depends upon the processing conditions. To further explore its unusual dielectric phenomena, the current study focuses on the process-property-structure relationship of the high-K CCTO via doping, cation non-stoichiometry, and sintering conditions. A variety of CCTO pellets were obtained by utilizing the different processing parameters via conventional solid-state synthesis methods. For the doping study, three types of dopants were selected with the variation of doping concentration. The study of undoped CCTO ceramics was carried out by the modification of the CuO and TiO₂ content in CCTO to create the stoichiometric formula CaCu₃+xTi₄+yO₁₂ (x = -0.06, 0, - 0.06; y = 0.08, 0, -0.08). Different processing factors including heating and cooling rates, sintering temperature, and sintering time were applied for the study of stoichiometric CCTO. X-ray diffraction, dielectric measurements, impedance spectroscopy, thermal analysis, and electron microprobe analysis were used to determine the existence of CuO and Cu₂O secondary phases, dielectric constant and loss tangent, electrical resistivity of grains and boundaries, decomposition reactions, and the microstructural changes of CCTO ceramics. It was revealed that the doping method improved the dielectric constant and loss tangent. The similar improvement in dielectric properties was also found in the Cu-deficient and Ti-deficient CCTO. The measurement and characterization results of stoichiometric CCTO clearly indicated that the dielectric properties, evolution of secondary phases, and microstructures were strongly dependent upon the processing parameters. Further, CCTO became Cu-deficient and Ti-excessive regardless of sintering conditions, which provides evidence for its complex behavior created by the reduction/oxidation reactions and the defect chemistry. Close