Navegando por Autor "Bakker, P. M. A. de"

Agora exibindo 1 - 5 de 5

Influence of nonstoichiometry and the presence of maghemite on the Mössbauer spectrum of magnetite.
(1995) Costa, Geraldo Magela da; Grave, Eddy De; Bakker, P. M. A. de; Vandenberghe, Robert Emile
Several samples of large- and small-particle magnetite (Fe304), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mrssbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly- or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400*(2, transforms to a mixture of wellcrystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300"C in a controlled 02 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mrssbauer spectra of the decomposition products at T < 200"C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.
Synthesis and characterisation of some iron oxides by Sol-gel method.
(1994) Costa, Geraldo Magela da; Grave, Eddy De; Bakker, P. M. A. de; Vandenberghe, Robert Emile
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 Emile
Synthetic 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.
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. de
Synthetic, 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.
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 Emile
Synthetic 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.