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|Title:||Variable-temperature Mössbauer spectroscopic study of nano-sized maghemite and Al-substituted maghemites.|
|Authors:||Costa, Geraldo Magela da|
Grave, Eddy De
Bowen, L. H.
Bakker, P. M. A. de
Vandenberghe, Robert Emile
|Citation:||COSTA, G. M. da et al. Variable-temperature Mössbauer spectroscopic study of nano-sized maghemite and Al-substituted maghemites. Clays & Clay Minerals, v. 43, p. 562-568, 1995. Disponível em: <http://www.clays.org/journal/archive/volume%2043/43-5-562.pdf>. Acesso em: 20 abr. 2017.|
|Abstract:||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  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.|
|Appears in Collections:||DEQUI - Artigos publicados em periódicos|
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