The crystal and electronic structures of oxides containing d0 transition metals in octahedral coordination

 
 
 
 

The crystal and electronic structures of oxides containing d0 transition metals in octahedral coordination

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Title: The crystal and electronic structures of oxides containing d0 transition metals in octahedral coordination
Author: Eng, Hank W
Description: Photocatalytic properties of a material are highly dependent upon the energy levels of the conduction and valence bands relative to the reactantfs HOMO and LUMO. Band edge positions control the wavelength of light that can be absorbed and the permissible surface redox reactions. These band edges are affected by (a) the charge transfer energy from the oxygen to metal, (b) local symmetry about the metal, (c) the connectivity of the MOn polyhedra, and (d) inductive effects from highly electropositive spectator cations. Ternary perovskite, ordered double perovskite and perovskite-related oxides with d0 transition metals (Ti4+, Nb5+, Ta5+, Mo6+, and W6+) in octahedral coordination have been systematically investigated to quantitatively understand the effects of cation substitution and structural features, such as symmetry about the transition metal. Measurements of the materialfs energetic band gap were made using UV/Vis diffuse reflectance spectroscopy, and linear muffin tin orbital calculations were utilized to help discern trends in the measured band gaps. The cubic perovskite oxides give the minimal band gap energies for the transition metal of interest in octahedral coordination. Band gaps of elpasolite compounds are shown to be a measure of the oxygen-to-metal charge transfer, and thus, is a measure of that transition metalfs effective electronegativity. Distortions from the ideal, cubic structure lead to increases in the band gap. Other than going from ternary to quarternary perovskites, out-of-center distortions of the octahedra have the greatest affect on the band gap. Changes in the M-O-M bond angle have the next most significant affect on the band gap. Changes in dimensionality have almost no affect on the band gap; however, structural distortions arising from those changes may increase the band gap. Finally, inductive effects of spectator ions were negligible in the double perovskite compounds, although slight variations in band gap energies occurred with the less electronegative transition metals. Synthesis and structural refinement of seventeen Mo and W ordered double perovskite oxides (A2MŒMO6; A = Ca, Sr, Ba; MŒ = Mg, Ca, Zn, Sr, Ba; M = Mo, W) are reported. Comparisons to the crystal structures predicted by the structural prediction software SPuDS are also made.
Permanent Link: http://rave.ohiolink.edu/etdc/view?acc_num=osu1070570079
http://hdl.handle.net/2374.OX/7159
Date: 2003

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