Navegando por Autor "San, E. Van"

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    About the Morin transition in hematite in relation with particle size and aluminum substitution.
    (2001) Vandenberghe, Robert Emile; San, E. Van; Grave, Eddy De; Costa, Geraldo Magela da
    The results of a MSssbauer study of the Morin transition behaviour in three series of hematite and Al-hematite samples are reviewed and discussed. The first two series comprise small-particle hematites and Al-hematites prepared from decomposition of lepidocrocite, whereas in the third series Al-hematites up to the #m range are obtained from co-precipitated oxinates. It is demonstrated that the Morin transition temperature follows quite well the overall properties of the samples such as particle size and Al substitution, while the transition region is rather determined by all kind of distributive effects. A model involving intermediate states is suggested for the Morin transition behaviour in non-ideal hematite.
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    Al hematites prepared by homogeneous precipitation of oxinates : material characterization and determination of the Morin transition.
    (2002) Costa, Geraldo Magela da; San, E. Van; Grave, Eddy De; Vandenberghe, Robert Emile; Barrón, Vidal; Datas, L.
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    Mossbauer characterization of iron oxides and (oxy)hydroxides : the present state of the art.
    (2000) Vandenberghe, Robert Emile; Barrero, C. A.; Costa, Geraldo Magela da; San, E. Van; Grave, Eddy De
    M¨ossbauer spectroscopy is a powerful direct technique for the identification and quantification of iron oxides and (oxy)hydroxides in soils and sediments. However, further characterization with respect to structural properties such as crystallinity, Al substitution, stoichiometry, water content, etc. is rather limited. With some examples of synthetic and natural goethite and hematite sample series it is illustrated that the hyperfine parameters depend on much more structural features than the Al content and crystallinity alone. Neither the Morin transition in hematite nor the Verwey transition in magnetite is directly applicable for analytical purposes in natural samples.