L. H. Bowen
North Carolina State University
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Featured researches published by L. H. Bowen.
Hyperfine Interactions | 1990
R. E. Vandenberghe; E. De Grave; C. Landuydt; L. H. Bowen
A review of the systematic Mössbauer studies on the most encountered iron oxides and hydroxides is given in which the qualitative and quantitative aspects, helpful in the characterization of natural smaples, are emphasized. The present possibilities of Mössbauer spectroscopy in soil characterization are further illustrated from some examples of natural soils.
Journal of Magnetism and Magnetic Materials | 1982
E. De Grave; L. H. Bowen; S.B. Weed
Abstract A Mossbauer spectroscopic study is reported on a series of aluminum-substituted hematites, α(Fe1-cAlc)2O3, with c up to 0.32. These samples were prepared by heating aluminum-substituted goethites, αFeOOH, at 500°C. X-ray line broadening gives particle dimensions of ≃200 A to >1000 A. Heating the samples to 900° improves crystallinity but reduces the maximum obtainable c to ≃0.15. At 77 K for c⩽0.04 the magnetic structure is antiferromagnetic with the spins aligning close to the (111) axis. For c⩾0.08 and for all compositions at 298 K the spins are perpendicular to (111) and for this weakly ferromagnetic phase the supertransferred hyperfine field is (5±1) kOe per magnetic neighbor. Samples with 0.04⩽c
Journal of Magnetism and Magnetic Materials | 1983
E. De Grave; D. Chambaere; L. H. Bowen
Abstract The Mossbauer spectra of α(Fe1-cAlc)2O3 with c=0.048 indicate the presence of two distinct spin phases below 245 K, one of which gradually disappears with increasing temperature. A careful study of the hyperfine parameters as a function of temperature reveals complex spin reorientation phenomena in both phases.
Journal of Magnetism and Magnetic Materials | 1988
E. De Grave; L. H. Bowen; R. Vochten; R. E. Vandenberghe
Abstract An aluminous hematite with 6.5 at% substitution and obtained by thermal decomposition at 500°C of an aluminous goethite, has been annealed at many different temperatures between 550 and 900°C. The resulting compounds have been characterized by X-ray diffraction and electron microscopy. XPS and zeta-potential measurements indicate an enrichment in Al of the surface of the particles at high sintering temperatures. Mossbauer spectra were recorded as a function of temperature and the results compared with those obtained for as-prepared aluminous hematites as a function of the Al substitution. This shows that the amount of Al leaving the hematite lattice at high temperatures is insignificant. A careful evaluation of the Mossbauer data of thermally treated, substituted and non-substituted hematite has allowed to distinguish the effects of crystallinity from those of the Al substitution. It is further suggested that the two low-temperature phases observed in the Mossbauer spectra, coexist within the particles and that the magnetic exchange interaction is on the basis of the canted spin directions.
Clays and Clay Minerals | 1986
D. D. Amarasiriwardena; E. DeGrave; L. H. Bowen; S. B. Weed
Mixtures of synthetic Al-substituted goethite (α-Fe0.814Al0.186OOH) and hematite (α-Fe1.658Al0.342O3) (75, 50, 25, 3% hematite by weight) were studied by Mössbauer spectroscopy to evaluate the use of that technique for quantitative analysis. Mössbauer spectra for these mixtures, obtained in the temperature range 12–130 K, were better fitted by a distribution of magnetic fields than by two magnetic sextets. Spectra at 80 K were equally as good as those from lower temperature to determine the hematitegoethite ratio. The recoil-free fractions of the individual components were about equal at any fixed temperature, but thickness effects caused as much as 30% error in the determination of the ratio of components in mixtures.
Physics and Chemistry of Minerals | 1991
P. M. A. de Bakker; E. De Grave; R. E. Vandenberghe; L. H. Bowen; R.J. Pollard; R. M. Persoons
The lepidocrocite (γ-FeOOH) to maghemite (γ-Fe2O3), and the maghemite to hematite (α-Fe2O3) transition temperatures have been monitored by TGA and DSC measurements for four initial γ-FeOOH samples with different particle sizes. The transition temperature of γ-FeOOH to γ-Fe2O3 and the size of the resulting particles were not affected by the particle size of the parent lepidocrocite. In contrast, the γ-Fe2O3 to γ-Fe2O3 transition temperature seems to depend on the amount of excess water molecules present in the parent lepidocrocite. Thirteen products obtained by heating for one hour at selected temperatures, were considered. Powder X-ray diffraction was used to qualify their composition and to determine their mean crystallite diameters. Transmission electron micrographs revealed the particle morphology. The Mössbauer spectra at 80 K and room temperature of the mixed and pure decomposition products generally had to be analyzed with a distribution of hyperfine fields and, where appropriate, with an additional quadrupole-splitting distribution. The Mössbauer spectra at variable temperature between 4.2 and 400 K of two single-phase γ-Fe2O3 samples with extremely small particles show the effect of superparamagnetism over a very broad temperature range. Only at the lowest temperatures (T⩽55 K), two distributed components were resolved from the magnetically split spectra. In the external-field spectra the ΔmI=0 transitions have not vanished. This effect is an intrinsic property of the maghemite particles, indicating a strong spin canting with respect to the applied-field direction. The spectra are successfully reproduced using a bidimensional-distribution approach in which both the canting angle and the magnetic hyperfine field vary within certain intervals. The observed distributions are ascribed to the defect structure of the maghemites (unordered vacancy distribution on B-sites, large surface-to-bulk ratio, presence of OH- groups). An important new finding is the correlation between the magnitude of the hyperfine field and the average canting angle for A-site ferric ions, whereas the B-site spins show a more uniform canting. The Mössbauer parameters of the two hematite samples with MCD104 values of respectively 61.0 and 26.5 nm display a temperature variation which is very similar to that of small-particle hematites obtained from thermal decomposition of goethite. However, for a given MCD the Morin transition temperature for the latter samples is about 30 K lower. This has tentatively been ascribed to the different mechanisms of formation, presumably resulting in slightly larger lattice parameters for the hematite particles formed from goethite, thus shifting the Morin transition to lower temperatures.
Clays and Clay Minerals | 1994
Gm Dacosta; Eddy De Grave; L. H. Bowen; Pma Debakker; R. E. Vandenberghe
Synthetic, relatively well-crystallized aluminum-substituted maghemite samples, γ-(Aly·Fe1−y)2O3, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mössbauer spectroscopy in the range 8 K to 475 K, and also with an external field of 60 kOe at 4.2 K and 275 K. It was found that there are two different converging models for fitting the zero-field spectra of the maghemites with a superposition of two Lorentzian-shaped sextets, both resulting in inconsistent values for the hyperfine fields (Hhf) and/or the center shifts (δ) of the tetrahedral (A) and octahedral (B) ferric ions. From the applied-field measurements it is concluded that there is a constant difference of 0.12 ± 0.01 mm/s between δB and δA, regardless of the Al content. For the Al-free sample the center shifts are found as: δA = 0.370 mm/s and δB = 0.491 mm/s at 4.2 K and δA = 0.233 mm/s and δB = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both δA and δB are observed to decrease with increasing Al concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Heff,A = 575 kOe and Heff,B = 471 kOe at 4.2 K and Heff,A = 562 kOe and Heff,B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with Al substitution, while for the A site a slight decrease with increasing Al content was observed.
Physics and Chemistry of Minerals | 1986
E. De Grave; R. M. Persoons; D. Chambaere; R. E. Vandenberghe; L. H. Bowen
Four synthetic lepidocrocite samples with a significant difference in crystallinity have been studied by the 57Fe Mössbauer effect technique at temperatures ranging between 12 K and 360 K. In the magnetically ordered region, the spectra display broad hyperfine field distributions, the profile of which could be derived numerically. In a broad transition region of approximately 20 K, an additional doublet must be included in the fitting model. From Mössbauer thermoscanning measurements at zero-velocity and with and without the application of an external magnetic field, it is found that the doublet is due to Fe3+ species in a paramagnetic state. Neither the hyperfine field nor the Néel temperature distributions are markedly affected by the crystallinity. In the pure paramagnetic state, the broadened doublets are best described by a distribution of quadrupole splittings in the range 0.3–1.9 mm/s. By numerical manipulations of the derived probability profiles it was possible to distinguish between the iron species in the outer surface layers of the γ-FeOOH particles and those in the innermost layers, the relative amount of each being correlated with the measured surface areas. The centre shift is quite uniform and its temperature dependence, determined in detail for one of the samples, is perfectly described by the Debije approximation for the second-order Doppler shift. From the obtained Debije temperature, a Mössbauer fraction f of 0.79 at room temperature was calculated, which is in good agreement with the results obtained for a reference lepidocrocite/hematite mixture. Finally, line shape simulations aimed to explain the external field spectrum at 180 K provided strong indications that both the sign and the asymmetry parameter of the electric field gradient are non-uniform as well.
Hyperfine Interactions | 1990
P. M. A. de Bakker; E. De Grave; R. E. Vandenberghe; L. H. Bowen
Two small-particle maghemite (λ-Fe2O3) samples have been investigated with the Mössbauer effect. From the results of the model-independent hyperfine-field distribution fits some characteristic temperature-dependent parameters have been obtained. The spectra at the lowest temperatures could be fitted with two strongly overlapping hyperfine-field distributions with different isomer shifts. Spectra in applied magnetic fields ranging from 40 to 60 kOe, and at 4.2 K showed non vanishing Δm1=0 absorption lines. The hyperfine-field distribution and canting-angle distribution method for fitting these spectra did not yield reasonable results. Therefore, a bi-dimensional hyperfine-field-canting-angle distribution has been applied, and was found to reproduce the experimental line shapes with remarkable adequacy. The resulting distribution profiles revealed a linear correlation between Hhf and the angle between the magnetic moments and the external field.
Clays and Clay Minerals | 1989
Robert C. Graham; S. B. Weed; L. H. Bowen; D. D. Amarasiriwardena; S. W. Buol
The mineralogy of the clay fraction was studied for soils and saprolite on two Blue Ridge Front mountain slopes. The clay fraction contained the weathering products of primary minerals in the mica gneiss and schist parent rocks. Gibbsite is most abundant in the saprolite and residual soil horizons, where only chemical weathering has been operable. In colluvial soil horizons, where both physical and chemical weathering have occurred, the clay fraction consists largely of comminuted primary phyllosilicates —muscovite, chlorite, and possibly biotite—and their weathering products: vermiculite, interstratified biotite/vermiculite (B/V), and kaolinite. The clay-size chlorite contains Fe2+ as indicated by Mössbauer spectroscopy, and is more resistant to weathering than biotite. The vermiculite and B/V, both weathering products of biotite, contain Fe3+. Vermiculite in colluvial soils and, especially, surface horizons is weakly hydroxy-interlayered. The kaolinite in the clay fraction resulted at least partly from the comminution of kaolinized biotite in coarser fractions.The hematite content ranged from 0 to 8% of the clay fraction and strongly correlates (r =.95) with dry clay redness, as measured by the redness rating: RR = (10 - YR hue) × (chroma) ÷ (value). The hematite is largely a product of the weathering of almandine; thus, the soil redness is dependent on the amount of almandine in the parent materials and its degree of weathering in the soils. Goethite (13–22% of the clay fraction) imparts a yellow-brown hue to soils derived from almandine-free parent rocks. The release of Fe in relatively low concentrations during the weathering of Fe-bearing primary minerals, particularly biotite, appears to have promoted the formation of goethite.