Energy band structure and electronic transport properties of niobium monoxide

Walter Edward Wahnsiedler, Purdue University

Abstract

Niobium Monoxide demonstrates unusual electronic transport properties for a transition metal oxide. It is metallic in nature, being silvery and quite electrically conductive. A detailed study of its electronic quantum mechanical energy band structure and prediction of electronic transport properties based on a priori calculations would be expected to shed light on these unusual properties. Such work has not been previously performed. The one-electron energy band structure of niobium monoxide was calculated in the Augmented Plane Wave approximation. Relativistic self consistent field ionic wave functions were used to provide the spherically symmetric part of the muffin-tin potential, and sphere radii were chosen so that the cation and anion spheres were tangent, and the spherically symmetric potentials equal at the point of tangency. A charge transfer of one electron between cation and anion was taken as the result of comparison with self consistent augmented plane wave calculations on similar oxides. A direct computation of the potential between the spheres was carried out to fourteenth nearest neighbors, with the Madelung approximation used to treat neighbors beyond the fourteenth shell. One-electron energy eigenvalues were then determined for the pseudopotential so derived. It was found that the energy dispersion curves exhibited numerous crossings of the Fermi level in an energy region populated by states primarily derived from the niobium 4d electronic state. This explains the highly conductive character of the compound and identifies its electronic conduction as being primarily due to niobium 4d electrons. From these results, a density-of-states curve was constructed, which, with the application of a slit function to incorporate resolution limitations of the experimental equipment, compared well with the spectrum obtained from X-ray photoelectron spectroscopy. Detailed calculations of electronic transport properties were also made from the energy dispersion curves, in the relaxation time approximation. The magnetic field free electrical conductivity of niobium monoxide was used to calculate relaxation times. Comparisons were made with measured Hall effect and transverse magnetoresistance values. Predicted Hall coefficients were within observed experimental ranges, and denoted predominantly hole-type conduction. Predicted magnetoresistance values were 10000 times smaller than observed. This difference is postulated to be due to one or more of several effects which give rise to anomalously large, linearly magnetic field dependent, magnetoresistances in other metallic materials. Possibilities include nonstoichiometry of the measured samples, geometric flaws in sample preparation, crystalline imperfections, impurity inclusions and quantum mechanical effects.

Degree

Ph.D.

Advisors

Honig, Purdue University.

Subject Area

Chemistry

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