Structural characterization of three-layer lanthanide titanate Aurivillius oxides Bi2A2Ti3O12 (A2 = La2, Pr2, Nd2, LaPr, LaNd, PrNd) by combined Rietveld refinements of X-ray diffraction and neutron powder data revealed that these materials reside in the orthorhombic space group B2cb. Quantification of the octahedral distortion features of archetype perovskites by screening data in the Inorganic Crystal Structure Database (ICSD) has been completed. However, these regions comprise only a small fraction of the energy contained in the solar spectrum, so the ability to modify the optical absorption characteristics to absorb longer wavelengths is critical.
Visible Light Sensitization
The first method is to insert transition metal ions with a valence band energy level into the band gap of the host material. This process has been shown to be a practical method of visible light sensitization of the host material, generally an oxide. An alternative method for introducing valence states into the band gap of the host oxide is the partial or complete substitution of cationic species.
Electron-Hole Recombination and Back-Reaction of Products
These e-h pairs can then be used in oxidation/reduction reactions with absorbed species on the surface of the photocatalysts.18-21 However, this concept of visible light sensitization of the host via doping does not extend to large-scale replacement (> 1%) of the cations which can lead to a shift of the entire UV-VIS adsorption edge. This method of changing the band gap can shift the entire UV-VIS absorption edge and provide more efficient utilization of the EM spectrum.22-25. The availability of separate reaction sites is particularly attractive as there is a physical barrier to both recombination of the photogenerated electrons and holes, and the back reaction of the photodegradation products.1,26.
Particle Size/Specific Surface Area Effects
Amorphous Materials
Structural Trends in Lanthanide Titanate Aurivillius Oxides
Abstract
Introduction
Experimental Procedure
- Powder Synthesis and Characterization
Conclusions
Acknowledgements
Buckling the Equatorial Anion Plane: Octahedral Anion
Abstract
Introduction
- General Composition ABX 3
- Historical Literature Review and Data Selection
Experimental Procedure
- Neutron Diffraction
Distortion of the BX 6 Octahedra
- Space Group Limitations
- Special Positions of the Equatorial Anions
- Definitions of Coplanar and Non-Coplanar (Buckled)
- Methodology and Mathematical Analysis
Results and Discussion
- Substitution Series Results
- Temperature Experiments
Conclusions
Buckling the Equatorial Anion Plane: Octahedral Anion
Abstract
Introduction
- Aurivillius Phase Background
- Neutron Diffraction Analysis
Experimental
- Aurivillius Phase Synthesis
Quantification of Equatorial Anion Distortions and Octahedral Tilts
- Equatorial Anion Distortion
- Octahedral Tilt Angle Determination
Results and Discussion
Conclusions
Acknowledgements
Structural and Electronegative Sensitization of Band Gaps in
Abstract
Twelve members of 3-layer lanthanide titanate Aurivillius oxides have been successfully designed as phase-pure compositions and their corresponding band gaps have been reported. Cation substitutions at perovskite-like A-site positions allowed systematic manipulation of band gaps and were shown to shift the optical absorption edge by as much as 0.3 eV for 3-layer lanthanide titanate Aurivillius oxides. A Vegard relationship between crystal chemical composition and band gap was observed over the range of compositions studied.
In addition to the structural manipulation of the optical absorption characteristics, an electronegative method has been proposed to increase the solar capture efficiency, based on the deliberate replacement of Bi3+ ions on the perovskite-like A-site positions.
Introduction
Experimental
- Powder Synthesis and Characterization
- Diffuse Reflectance Measurements and Band Gap Determination
- X-ray Phase Purity
- Rietveld Refinements
Ti-O bond length via A-site spectator ion substitutions has been investigated as a method for tailoring the optical absorption characteristics of a selected group of Aurivillius phase materials. The "band gap" for these materials was defined as the 50% absorption level of the normalized optical absorption data. A somewhat conservative error estimate of ± 0.02 eV is suggested, based on a resynthesis study and repeat measurement.
These calculated bandgap values were then used to track the optical absorption edge progression toward the visible as a function of A site chemistry. This method was found to be a good reflection of the optical absorption characteristics of the investigated phases and alleviated any slope dependence of the calculated band gaps that may have confounded the trends in the UV-Vis data. All compositions reported herein formed single-phase Aurivillius oxides with no detected impurities over the entire range of A-site cation substitutions tested.
A subset of the compositions studied in this investigation, Bi2A2Ti3O12 (A2 = La2, Pr2, Nd2, LaPr, LaNd, and PrNd) and their representative x-ray diffraction patterns demonstrating phase purity are included in Chapter 1, Figs. Combined Rietveld refinement of x-ray and neutron powder diffraction data for six members of the 3-layer Aurivillius titanate lanthanide oxides, Bi2A2Ti3O12 (A2 = La2, Pr2, Nd2, LaPr, LaNd, and PrNd), have been discussed previously in Chapter 1. The reader is referred to Chapters 1 and 3 for a further enumeration of the structural aspects of these phases, including: lattice parameters, static mixing behavior, and octahedral tilt and distortion for these phases.
Results and Discussion
- Structural Sensitization of the 3-Layer Lanthanide Titanates
- Electronegative Sensitization of the 3-Layer Lanthanide Titanates
- Correlation of Band Gaps with Structure
Structural sensitization concerns compounds in which the A-site cation positions are populated only by members of the lanthanides, subject to static mixing considerations that have not been explicitly explored, as noted previously. However, an almost perfect Vegard-type relationship exists between the band gap and the composition within the three-layer lanthanide titanates, and a redshift of the optical absorption edge of ~0.3 eV is possible by manipulation of the lanthanide ions located on the A place. positions only; Table 4.III and Fig. Bi3+ has an ionic radius similar to that of the first few members of the lanthanide series, albeit slightly larger, and a Pauling electronegativity nearly double that of these rare earth elements.
In addition, Bi3+ exhibits stereochemically active lone pair electrons, similar to lead, which may have an influence on the first coordination sphere of the titanium cations. Band gaps in the bismuth substitution series were observed to decrease with increasing Bi3+ content of the A-site positions. Increased solar capture efficiency (a decrease in band gaps) was observed to correlate with increased bismuth content of the site.
The Vegard-like relationship between crystal chemical composition and band gap observed during this investigation applies to all members of the 3-layer lanthanide titanates studied. We hypothesized that the relaxation behavior of the Ti–O bond lengths as a function of the spectator ions residing on the perovskite A site may influence the optical absorption properties of this select material set. Since the difference in optical absorption properties of these materials is influenced by the structure in these materials, an accurate description of the ligand environments of the titanium cation becomes important in the development of photocatalysts based on transition metals in octahedral anion coordination, such as the Aurivillius phase material.
We previously demonstrated that changing the crystal field environments in layered perovskites similar to anatase is possible by using spectator ions on the perovskite A-site position, see Chapters 2 and 3.
Conclusions
Acknowledgements
2.1, has been investigated to determine whether distortion of the equatorial anion environments can be resolved from fractional coordinates of refined neutron diffraction data. Of the thirty-two crystal classes, only thirteen allow non-coplanar distortion of the equatorial anions in traditional perovskites. In the octahedral environments of the perovskite family of materials, three types of distortions exist associated with the equatorial oxygens within the first coordination sphere of the B site.
Determination of the coplanarity numbers, φAB & φCD, allows for the full quantification of the distortion of the equatorial anions in perovskites. In addition, it allows resolution of the angle of intersection between the planes (φAB & φCD) with relative ease, albeit indirectly. The distortion, or non-coplanarity number, or φAB, is equal to the magnitude of the angle between the two planes defined by the four equatorial anions in the BX6 octahedra.
The spatial arrangement of the equatorial anions in the BX6 octahedra meets the necessary minimum qualifications for a strain measurement of the type φAB & φCD. The fractional coordinates of each of the equatorial oxygen positions are given as u', v' and w', each corresponding to the refined natural crystallographic basis. In fact, in the case of full zirconium substitution, the deformation of the equatorial oxygen reaches ∼7° deviation from planarity; see Table 2.I and Fig.
This reduction in distortion was expected because crystal structures tend to become more symmetric with increasing temperature; therefore, an associated decrease in the degree of distortion of the octahedra was expected. Temperature effects on the structural configuration assumed by the octahedra were investigated and it was observed that with increasing temperature, relaxation of the equatorial anions occurs. Whatmore, “A reinvestigation of the crystal structure of the perovskite PbZrO3 by X-ray and neutron diffraction,” Acta Crystallogr., Sect.
This method of analysis is equivalent to determining the direction cosines of a vector in Cartesian space. The following set of equations allows the calculation of the three corresponding pitch angles as described. An increase in the absolute magnitude of the slopes is associated with a decrease in the average ionic radius of the A-site.
Over the range of cation substitutions in the 3-layer Aurivillius phases, the tilt of the internal octahedra varies linearly from ∼ 1.7° to ∼ 9.1°. Not surprisingly, most of the compounds found within ICSD were based on the various substitution series of alkaline earth sites A. Similar to the case of traditional perovskites, the equatorial anion distortion is created in the Aurivillius family of perovskite layers.
As in traditional perovskites, the octahedral tilt in these phases has been attributed to the optimization of the first coordination sphere of A. Similar to the case of traditional perovskites, the equatorial anion distortion is created in the Aurivillius family of layered perovskites. As in traditional perovskites, the octahedral tilt in these phases has been attributed to the first coordination sphere of the A-site cation and is shown to vary inversely with the average ionic radius of the A-site.
A critical average A-site ionic radius of 1.4 was established, including distortion of the octahedral environments within the. As in the traditional perovskites, octahedral tilting in these phases has been attributed to site cation and shown that the site's ionic radius of 1.4 Å varies, including distortion of the octahedral environments within the. The initial x-ray and neutron powder diffraction studies of the 3-layer lanthanide titanate Aurivillius oxides were undertaken to understand the subtle structural changes due to cation substitutions at the A-site positions within the perovskite-like blocks.
