Browsing by Author "Naicker, Vishnu Visvanathan."

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Profile analysis of X-ray powder diffraction data.
(1988) Naicker, Vishnu Visvanathan.; Engel, D. W.
Various strategies have been tested for obtaining integrated intensities from x-ray powder diffractometer data. An asymmetric pseudo-Voigt profile function was used to fit the pattern in the region above 2θ = 35̊ (Cu-Kα radiation). At lower angles where the asymmetry was strongest and the profile function not suitable the peaks were integrated numerically. A smooth background function was estimated from the regions of minimum intensity of the pattern. The profile parameters were initially refined in small ranges of about 10̊ 2θ in order to determine their 2θ-dependence. Thereafter final refinements of peak intensities were undertaken using the profile parameters thus determined. Analysis of data from the mineral Fe-akermanite, Ca₂Mg₀. ₄Fe₀. δSi₂O₇, generated 173 integrated intensities with 1 > 2δ(1) out of 187 positions separated in 2θ by more than 0,02̊. Of the total of 213 reflections in the range, those overlapping exactly or separated by < 0,02̊ were treated as single peaks. The structure was refined using an overall isotropic temperature parameter and a parameter to compensate for preferred orientation, giving an unweighted residual of 10,4% for 14 parameters.
A study of the vanadium oxide bronze 0-VOB, and vanadium oxides V2O5 and VO2, using hyperfine interaction techniques.
(1999) Naicker, Vishnu Visvanathan.; Bharuth-Ram, Krishanlal.; Lieb, Klaus Peter
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.