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Item 57Fe Mössbauer effect study of Al-substituted lepidocrocites.(1996) Grave, Eddy De; Costa, Geraldo Magela da; Bowen, L. H.; Schwertmann, U.; Vandenberghe, Robert EmileSeven A|-containing lepidocrocite samples, ~/-Fe,_xAlxOOH, prepared from FeC12/AI(NO3)3 solutions with initial A1/(A1 + Fe) mole ratios C~ of 0.0025, 0.01, 0.025, 0.05, 0.075, 0.10 and 0.15 tool/ mol, were examined by means of M6ssbauer spectroscopy at room temperature (RT) and at various temperatures in the range of 8 to 80 K. The spectra at RT and 80~ consist of broadened quadrupole doublets and were analyzed in terms of a single doublet and of a model-independent quadrupole-splitting distribution, the latter yielding the best fit. The observed variations of the quadrupole-splitting parameters with increasing C i are inconclusive as to whether the A1 cations are substituting into the structure. The temperature at which the onset of magnetic ordering is reflected in the spectra, was measured by the therrnoscan method with zero source velocity. A gradual shift from 50 K for Ci = 0.0025 mol/mol to 44 K for Ci = 0.10 mol/mol was observed for that temperature. As compared to earlier studies of Al-free ~-FeOOH samples with similar morphological characteristics, the fractional doublet area in the mixed sextet-doublet spectra at 35 K is significantly higher for the present lepidocrocites. This observation is ascribed to the substitution of A1 cations into the lepidocrocite structure. A similar conclusion is inferred from the variation with C~ of the maximum-probability hyperfine field derived from the spectra recorded at 8 K and fitted with a model-independent hyperfine-field distribution. The magnetic results suggest that for the sample corresponding to Ci = 0.15 mol/mol, not all of the initially present A1 has been incorporated into the structure.Item Characterization of soil-oxide analogues by applied-field Mossbauer spectroscopy.(1998) Grave, Eddy De; Costa, Geraldo Magela da; Bowen, L. H.; Barrero, C. A.; Vandenberghe, Robert EmileIn this contribution, the effect of high external magnetic fields upon the Mössbauer spectra of aluminous _- and -Fe2O3, of aluminous _- and -FeOOH and of aluminous Fe3O4 is reviewed. It is shown that the shapes of these spectra are characteristic of these materials and also to some extent of their crystallinities and Al-for-Fe substitutions. Based upon this evaluation, the potential for application of external-field Mössbauer spectroscopy to soil-related analytical purposes is demonstrated for two soil samples. Limitations of the technique are discussed. Finally, some suggestions for further research in this field are indicated.Item Mössbauer studies of nano-sized aluminum-substituted maghemites.(1994) Costa, Geraldo Magela da; Grave, Eddy De; Bryan, A. M.; Bowen, L. H.Several Al-maghemites prepared from Al-lepidocrocites have been studied by M6ssbauer spectroscopy from 6 to 300 K, and also at 4.2 K with an applied field of 6 T. A linear decrease in the A and B site fields at 4.2 K for samples containing up to 10% of aluminum suggests a limit for the substitution. The importance of applied-field measurements for a correct characterization of small-particle maghemite is also shown.Item Temperature dependence of the hyperfine parameters of maghemite and Al-substituted maghemites.(1995) Costa, Geraldo Magela da; Grave, Eddy De; Bowen, L. H.; Bakker, P. M. A. de; Vandenberghe, Robert EmileSynthetic aluminum-substituted maghemite samples, 7-(AlyFel_y)203, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by M6ssbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: 6A---- 6B-- 0.12 mm/s. From the temperature dependence of 6B it was possible to determine the characteristic M6ssbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing A1 content, at least up to 10 mole%. The temperature dependence of the Aand B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: JAB = --25 K; JAA = --18 K and JBB= --3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% A1. The gradual decrease of the saturation value of the A-site hyperfine field with increasing A1 substitution and the constancy of this quantity for the B sites, suggest that the A1 cations occupy the B sites.Item The center shift in the Mössbauer spectra of maghemite and aluminum maghemites.(1994) Costa, Geraldo Magela da; Grave, Eddy De; Bowen, L. H.; Vandenberghe, Robert Emile; Bakker, P. M. A. deSynthetic, relatively well-crystallized aluminum-substituted maghemite samples, y-(Aly. Fe~_y)203, with y = 0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by X-ray diffraction and zero-field Mrssbauer 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 (6) 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 6B and 6A, regardless of the A1 content. For the Al-free sample the center shifts are found as: 64 = 0.370 mm/s and 6a = 0.491 mm/s at 4.2 K and 6n = 0.233 mm/s and fib = 0.357 mm/s at 275 K (relative to metallic iron), with an estimated error of 0.005 mm/s. Both 6A and fib are observed to decrease with increasing AI concentration. The effective hyperfine fields for the non-substituted maghemite sample are: Hefr.A = 575 kOe and Hofr.B = 471 kOe at 4.2 K and He~,A = 562 kOe and H~er.B = 449 kOe at 275 K, with an error of 1 kOe. The B-site hyperfine field remains approximately constant with A1 substitution, while for the A site a slight decrease with increasing A1 content was observed.Item Variable-temperature Mössbauer spectroscopic study of nano-sized maghemite and Al-substituted maghemites.(1995) Costa, Geraldo Magela da; Grave, Eddy De; Bowen, L. H.; Bakker, P. M. A. de; Vandenberghe, Robert EmileSynthetic aluminum-substituted maghemite samples, "g-(Fe~ _xAlx)203, have been prepared by thermal decomposition of Al-lepidocrocite (3,-Fe~_xAlxOOH), with x = 0, 0.04, 0.06, 0.14 and 0.18. The particles are needle-shaped and the mean crystallite diameter along the [311] crystallographic direction was found to be between 2.0 and 5.0 nm. MSssbauer spectra were collected at 6 K and from 80 K up to 475 K at steps of 25 K. In a wide range of temperatures the spectra of the non-substituted sample consist of a superposition of a broad sextet and a superparamagnetic doublet, whereas for the Al-maghemites this range is much smaller. From the temperature variation of the fractional doublet area two different parameters were defined: the temperature corresponding to a 50/50 doublet-sextet spectrum (T1/2), and the temperature below which the doublet ceases to exist (To). These two parameters (T1/2 and To) decrease from 390 K and 92 K (Al-free sample), to 118 K and 64 K (4 mole % AI) and to 100 K and 48 K (18 mole % A1), respectively. The average hyperfine fields at 6 K undergo a steep drop in going from the AIfree sample (I:I~f = 506 kOe) to the sample with 4 mole % A1 (I:Ihr = 498 kOe), but for higher substitutions the effect is much smaller. The A- and B-site quadrupole splittings, obtained from the data between 220 K and 475 K, were found as: AEQ.A = 0.86 --+ 0.04 mm/s and AEo.B = 0.65 _+ 0.04 mm/s for the 4 mole % A1 sample. The characteristic MiJssbauer temperature, determined from the temperature dependence of the average isomer shift, was found to be in the range of 500-600 K.