| dc.identifier.uri | http://hdl.handle.net/11401/77071 | |
| dc.description.sponsorship | This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree. | en_US |
| dc.format | Monograph | |
| dc.format.medium | Electronic Resource | en_US |
| dc.language.iso | en_US | |
| dc.publisher | The Graduate School, Stony Brook University: Stony Brook, NY. | |
| dc.type | Dissertation | |
| dcterms.abstract | Transition metal oxides possess a number of useful physical properties that have allowed compounds belonging to this class to serve as functional oxides in a variety of high-tech applications. The range of physical properties among this family is strongly related to the transition metal d orbital splitting, predicted by ligand field theory, and the nature of the transition metal d electrons. This dissertation is focused on the specific subset of transition metal oxides that contain direct metal-metal (M-M) bonds. To date, transition metal oxides with M-M bonds have been largely explored in terms of crystal structure characterization, with little work done to characterize the effects of M-M bond formation on the electronic structure and resultant physical properties of these compounds. M-M bonding will lead to d orbital splitting that deviates from the orbital splitting predicted by ligand field theory, and result in electronic structure configurations that more closely resemble narrow f electron states. In turn, this means compounds with M-M bonds should have physical properties that differ from the same compounds without M-M bonds. M-M bonding is characterized by a M-M distance shorter than the distance between atoms in the pure metal, with Mo known to form the largest number of clusters and nuclearities. Due to the large number of known lanthanum molybdenum oxides with Mo-Mo bonds that could be explored further, study began with attempts to synthesize and investigate known lanthanum molybdates. Work extended to include other known and novel rare earth molybdenum oxides with Mo-Mo bonds using a comprehensive approach that incorporated the study of relationships between crystal structures, electronic structures, and physical properties in these systems. The tetragonal compound La4Mo2O11 is a rare example of a solid state oxide with direct metal-metal bonding, as reflected in the short Mo-Mo bond distance of 2.59 Å. The Mo cations are located in isolated Mo2O10 octahedral dimers, which represent one of the simplest geometries in which to study metal-metal bonding. Although a d1 electron configuration is expected for the pentavalent Mo cations based on electron counting arguments, this compound is found to be diamagnetic (S = 0) in magnetic susceptibility measurements and non-metallic in electronic transport measurements. Diffuse reflectance measurements show that La4Mo2O11 absorbs light well into the infrared spectrum (observed onset energy of ~ 0.5 eV), though the present data do not permit the precise determination of a band gap due to the many component features observed in the optical spectra. When the band gap of La4Mo2O11 is investigated by density function theory (DFT) studies, a band gap of 0.9 eV is predicted by LDA methods (which are known to underestimate band gaps) and a 2.2 eV band gap is predicted using HSE methods typically produce more accurate estimates of band gaps. Both of these DFT-calculated gaps are significantly larger than the activation energy obtained by fitting electronic transport data. This suggests that electronic conduction likely occurs through a polaronic or other defect-driven mechanism with activation energy of 0.25 eV, and is very consistent with the transport behavior previously observed for the closely related compound La2MoO5. Furthermore, the DFT results indicate that the strong infrared absorption observed in experimental measurements is associated with phenomena other than just simple excitations across the band gap. It is demonstrated that the typical t2g and eg states expected for isolated MoO6 octahedral are split by the Mo-Mo bonding in La4Mo2O11 into ten non-degenerate states. These states can be readily rationalized using a molecular orbital approach, and are believed to be the source of the observed diamagnetism and small band gap behavior of this phase. The structure of the novel compound La2MoO5 has been solved from powder X-ray and neutron diffraction data and belongs to the tetragonal space group P4/m (no. 83) with a =12.6847(3) Å and c = 6.0568(2) Å and with Z = 8. It consists of equal proportions of bioctahedral (Mo2O10) and square prismatic (Mo2O8) dimers, both of which contain direct Mo−Mo bonds and are arranged in 1D chains. The Mo−Mo bond length in the Mo2O10 dimers is 2.684(8) Å, while there are two types of Mo2O8 dimers with Mo−Mo bonds lengths of 2.22(2) and 2.28(2) Å. Although the average Mo oxidation state in La2MoO5 is 4+, the very different Mo−Mo distances reflect the fact that the Mo2O10 dimers contain only Mo5+ (d1), while the prismatic Mo2O8 dimers only contain Mo3+ (d3), a result directly confirmed by density function theory calculations. This is due to the complete disproportionation of Mo4+, a phenomenon which has not previously been observed in solid-state compounds. La2MoO5 is diamagnetic, behavior which is not expected for a nonmetallic transition-metal oxide whose cation sites have an odd number of d electrons. The resistivity displays the Arrhenius-type activated behavior expected for a semiconductor with a band gap of 0.5 eV, exhibiting an unusually small transport gap relative to other diamagnetic oxides. Diffuse reflectance studies indicate that La2MoO5 is a rare example of a stable oxide semiconductor with strong infrared absorbance. It is shown that the d orbital splitting associated with the Mo2O8 and Mo2O10 dimeric units can be rationalized using simple molecular orbital bonding concepts. Among oxide compounds with direct metal-metal bonding, the A5B2O12 family of compounds has a particularly intriguing low-dimensional structure due to the arrangement of bioctahedral dimers into one-dimensional edge-sharing chains along the direction of the metal-metal bonds. Furthermore, these compounds can have a local magnetic moment due to the non-integer oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when compounds with direct metal-metal bonding have integer oxidation states (due to the lifting of orbital degeneracy typically induced by the metal-metal bonding). Although a monoclinic C2/m structure has been previously proposed for Ln5Mo2O12 (Ln = La-Lu, Y) members of this family based on prior single crystal diffraction data, it is found that the average structural model misses many important structural features. Based on synchrotron powder diffraction data, it is shown that the C2/m monoclinic unit cell represents a superstructure relative to the base orthorhombic Immm subcell, and that the superstructure derives from the ordering of nearly-interchangeable Mo2O10 and LaO6 building blocks. The superstructure for this reason is typically highly faulted, indicated by the breadth of superstructure diffraction peaks. Finally, it is shown that oxygen vacancies can occur when Ln = La, resulting in an oxygen deficient stoichiometry of La5Mo2O11.55 and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, as confirmed by magnetic susceptibility measurements. These oxygens are removed from the bioctahedral Mo2O10 dimers, resulting in an atypical local coordination environment for the Mo cations involved in Mo-Mo bonding. Based on the results of DFT calculations, the lifting of the d orbital degeneracy of Ln5Mo2O12 compounds is explained within a simple molecular orbital picture. It is found that this absorption gives rise to strong visible light absorption and infrared absorption that is not typically seen for 4d transition metal oxides. | |
| dcterms.abstract | Transition metal oxides possess a number of useful physical properties that have allowed compounds belonging to this class to serve as functional oxides in a variety of high-tech applications. The range of physical properties among this family is strongly related to the transition metal d orbital splitting, predicted by ligand field theory, and the nature of the transition metal d electrons. This dissertation is focused on the specific subset of transition metal oxides that contain direct metal-metal (M-M) bonds. To date, transition metal oxides with M-M bonds have been largely explored in terms of crystal structure characterization, with little work done to characterize the effects of M-M bond formation on the electronic structure and resultant physical properties of these compounds. M-M bonding will lead to d orbital splitting that deviates from the orbital splitting predicted by ligand field theory, and result in electronic structure configurations that more closely resemble narrow f electron states. In turn, this means compounds with M-M bonds should have physical properties that differ from the same compounds without M-M bonds. M-M bonding is characterized by a M-M distance shorter than the distance between atoms in the pure metal, with Mo known to form the largest number of clusters and nuclearities. Due to the large number of known lanthanum molybdenum oxides with Mo-Mo bonds that could be explored further, study began with attempts to synthesize and investigate known lanthanum molybdates. Work extended to include other known and novel rare earth molybdenum oxides with Mo-Mo bonds using a comprehensive approach that incorporated the study of relationships between crystal structures, electronic structures, and physical properties in these systems. The tetragonal compound La4Mo2O11 is a rare example of a solid state oxide with direct metal-metal bonding, as reflected in the short Mo-Mo bond distance of 2.59 Å. The Mo cations are located in isolated Mo2O10 octahedral dimers, which represent one of the simplest geometries in which to study metal-metal bonding. Although a d1 electron configuration is expected for the pentavalent Mo cations based on electron counting arguments, this compound is found to be diamagnetic (S = 0) in magnetic susceptibility measurements and non-metallic in electronic transport measurements. Diffuse reflectance measurements show that La4Mo2O11 absorbs light well into the infrared spectrum (observed onset energy of ~ 0.5 eV), though the present data do not permit the precise determination of a band gap due to the many component features observed in the optical spectra. When the band gap of La4Mo2O11 is investigated by density function theory (DFT) studies, a band gap of 0.9 eV is predicted by LDA methods (which are known to underestimate band gaps) and a 2.2 eV band gap is predicted using HSE methods typically produce more accurate estimates of band gaps. Both of these DFT-calculated gaps are significantly larger than the activation energy obtained by fitting electronic transport data. This suggests that electronic conduction likely occurs through a polaronic or other defect-driven mechanism with activation energy of 0.25 eV, and is very consistent with the transport behavior previously observed for the closely related compound La2MoO5. Furthermore, the DFT results indicate that the strong infrared absorption observed in experimental measurements is associated with phenomena other than just simple excitations across the band gap. It is demonstrated that the typical t2g and eg states expected for isolated MoO6 octahedral are split by the Mo-Mo bonding in La4Mo2O11 into ten non-degenerate states. These states can be readily rationalized using a molecular orbital approach, and are believed to be the source of the observed diamagnetism and small band gap behavior of this phase. The structure of the novel compound La2MoO5 has been solved from powder X-ray and neutron diffraction data and belongs to the tetragonal space group P4/m (no. 83) with a =12.6847(3) Å and c = 6.0568(2) Å and with Z = 8. It consists of equal proportions of bioctahedral (Mo2O10) and square prismatic (Mo2O8) dimers, both of which contain direct Mo−Mo bonds and are arranged in 1D chains. The Mo−Mo bond length in the Mo2O10 dimers is 2.684(8) Å, while there are two types of Mo2O8 dimers with Mo−Mo bonds lengths of 2.22(2) and 2.28(2) Å. Although the average Mo oxidation state in La2MoO5 is 4+, the very different Mo−Mo distances reflect the fact that the Mo2O10 dimers contain only Mo5+ (d1), while the prismatic Mo2O8 dimers only contain Mo3+ (d3), a result directly confirmed by density function theory calculations. This is due to the complete disproportionation of Mo4+, a phenomenon which has not previously been observed in solid-state compounds. La2MoO5 is diamagnetic, behavior which is not expected for a nonmetallic transition-metal oxide whose cation sites have an odd number of d electrons. The resistivity displays the Arrhenius-type activated behavior expected for a semiconductor with a band gap of 0.5 eV, exhibiting an unusually small transport gap relative to other diamagnetic oxides. Diffuse reflectance studies indicate that La2MoO5 is a rare example of a stable oxide semiconductor with strong infrared absorbance. It is shown that the d orbital splitting associated with the Mo2O8 and Mo2O10 dimeric units can be rationalized using simple molecular orbital bonding concepts. Among oxide compounds with direct metal-metal bonding, the A5B2O12 family of compounds has a particularly intriguing low-dimensional structure due to the arrangement of bioctahedral dimers into one-dimensional edge-sharing chains along the direction of the metal-metal bonds. Furthermore, these compounds can have a local magnetic moment due to the non-integer oxidation state (+4.5) of the transition metal, in contrast to the conspicuous lack of a local moment that is commonly observed when compounds with direct metal-metal bonding have integer oxidation states (due to the lifting of orbital degeneracy typically induced by the metal-metal bonding). Although a monoclinic C2/m structure has been previously proposed for Ln5Mo2O12 (Ln = La-Lu, Y) members of this family based on prior single crystal diffraction data, it is found that the average structural model misses many important structural features. Based on synchrotron powder diffraction data, it is shown that the C2/m monoclinic unit cell represents a superstructure relative to the base orthorhombic Immm subcell, and that the superstructure derives from the ordering of nearly-interchangeable Mo2O10 and LaO6 building blocks. The superstructure for this reason is typically highly faulted, indicated by the breadth of superstructure diffraction peaks. Finally, it is shown that oxygen vacancies can occur when Ln = La, resulting in an oxygen deficient stoichiometry of La5Mo2O11.55 and an approximately 10-fold reduction in the number of unpaired electrons due to the reduction of the average Mo valence from +4.5 to +4.05, as confirmed by magnetic susceptibility measurements. These oxygens are removed from the bioctahedral Mo2O10 dimers, resulting in an atypical local coordination environment for the Mo cations involved in Mo-Mo bonding. Based on the results of DFT calculations, the lifting of the d orbital degeneracy of Ln5Mo2O12 compounds is explained within a simple molecular orbital picture. It is found that this absorption gives rise to strong visible light absorption and infrared absorption that is not typically seen for 4d transition metal oxides. | |
| dcterms.available | 2017-09-20T16:51:51Z | |
| dcterms.contributor | Parise, John | en_US |
| dcterms.contributor | Khalifah, Peter G | en_US |
| dcterms.contributor | Grubbs, Robert | en_US |
| dcterms.contributor | McQueen, Tyrel. | en_US |
| dcterms.creator | Colabello, Diane Marie | |
| dcterms.dateAccepted | 2017-09-20T16:51:51Z | |
| dcterms.dateSubmitted | 2017-09-20T16:51:51Z | |
| dcterms.description | Department of Chemistry. | en_US |
| dcterms.extent | 230 pg. | en_US |
| dcterms.format | Application/PDF | en_US |
| dcterms.format | Monograph | |
| dcterms.identifier | http://hdl.handle.net/11401/77071 | |
| dcterms.issued | 2015-05-01 | |
| dcterms.language | en_US | |
| dcterms.provenance | Made available in DSpace on 2017-09-20T16:51:51Z (GMT). No. of bitstreams: 1
Colabello_grad.sunysb_0771E_12484.pdf: 7230622 bytes, checksum: 6a8616bf60cbe37f67286997e389f52d (MD5)
Previous issue date: 2015 | en |
| dcterms.publisher | The Graduate School, Stony Brook University: Stony Brook, NY. | |
| dcterms.subject | Chemistry | |
| dcterms.title | Understanding Direct Metal-Metal Bonding in Rare Earth Molybdates | |
| dcterms.type | Dissertation | |
