A study of the vanadium oxide bronze 0-VOB, and vanadium oxides V2O5 and VO2, using hyperfine interaction techniques.
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Date
1999
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Abstract
One of the main interests in the vanadium oxides V2O5 and VO2 is that, when doped with a
metal such as Fe, these oxides display semiconductor-to-metal transitions at certain critical
temperatures. These transitions are also accompanied with changes in the crystallographic
phases of the oxides. This thesis describes the use of hyperfine interactions at dopant sites
in the vanadium oxides V2O5 and VO2 to infer information on the phase transitions that
take place in these oxides.
The hyperfine interaction techniques of Mossbauer Spectroscopy and Time Differential
Perturbed Angular Correlation (TDPAC) are used to study the hyperfine parameters in the
Fe - V2O5 system and Cd - V2O5 system, respectively. X-ray powder diffraction
spectroscopy were also conducted on the samples to establish the phases created.
A large part of this project was spent in the design of apparatus. The apparatus constructed
were (i) a furnace to perform a solid state reaction in order to introduce Fe into V2O5, the
maximum operating temperature of the furnace being 1473 K, (ii) a Mossbauer sample
chamber and sample holder which enabled the sample to be heated up to a temperature of
873 K, and (iii) a device constructed to determine the electrical conductivities of powder
samples at temperatures ranging from 773 K to room temperature.
For the Mossbauer studies, the Fe-V2O5 system was studied as a function of the Fe
concentration. Six symmetric doublets, with intensities changing as the Fe concentration
changed, were observed. Correlating the Mossbauer components of the individual spectra
with the phases identified using powder x-ray diffraction patterns in terms of the reflection
intensities, allowed two of the doublets to be assigned to lattice sites in the vanadium oxide
bronze system, θ-YOB, a further two doublets to substitutional and interstitial sites in the
Fe doped V2O5 system, respectively, and the fifth doublet to the super-paramagnetic Fe2O3
phase. The sixth doublet observed was attributed to an unresolved crystallographic phase
observed in the x-ray diffraction spectra at large Fe concentrations.
The magnitude of the quadrupole splittings of the doublets assigned to the vanadium oxide
bronze and the Fe-V2O5 systems indicate that the electronic environment of the Fe atoms in
the bronze phase displays a greater symmetry than those in the V2O5 phase.
In order to gain insight on the semiconducting nature of the Fe doped V2O5 and the θ-VOB
phases, temperature dependent Mossbauer measurements ranging from 300 K to 573 K,
together with electrical conductivity measurements, were performed on a few samples. The
temperature dependent Mossbauer spectra displayed the usual second order Doppler shift
of the isomer shifts for the various components as a function of temperature, but no
significant change in the magnitude of the quadrupole splittings. From this result, on the
basis of the Duncan-Golding correlation diagram, the valence state of the Fe ions was
inferred to be 3+. No components were observed (with increasing temperature) that could
be correlated with the population of Fe2+ states. This therefore suggests that the
semiconducting properties of the Fe doped V2O5 phase and the θ-VOB phase are associated
with electron hopping between V4+ - V5+ valence sites rather than Fe3+ - Fe2+ valence sites.
111In-TDPAC measurements were made on V2Os and VO2. For V2O5, the measurements
yielded one distinct substitutional cation site for the 1llCd ions, with quadrupole coupling
constant vQ =88,1(3) MHz, and asymmetry η =0,619(3)
In VO2, temperature dependent TDPAC measurements yielded two well defined
quadrupole coupling frequencies for the 1llCd probe nuclei, the first, vQ =43,0(7) MHz,
observed at room temperature, corresponding to a monoclinic or triclinic phase of VO2, and
the second, vQ =89,1(1) MHz, observed at 423 K and above, corresponding to the rutile
phase of VO2.
Description
Thesis (Ph.D.)-University of Durban-Westville, 1999.
Keywords
Vanadium oxide., Theses--Physics.