Long-range vs. short-range ordering in synthetic Cr-substituted goethites

A series of Cr-substituted goethites with (Cr:Fe molar ratio up to 0.12) were prepared. Thermal analysis of the solids indicates the formation of cation-deficient compounds that are more stable towards the transformation to hematite as the Cr content increases. Powder X-ray diffraction (PXRD) and ex...

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Detalles Bibliográficos
Publicado: 2004
Materias:
chromium
geochemistry
goethite
mineral synthesis
soil chemistry
Acceso en línea:https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167037_v68_n14_p3053_Sileo
http://hdl.handle.net/20.500.12110/paper_00167037_v68_n14_p3053_Sileo
Aporte de:
Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA) de Universidad de Buenos Aires
id paper:paper_00167037_v68_n14_p3053_Sileo
record_format dspace
spelling paper:paper_00167037_v68_n14_p3053_Sileo2023-06-08T14:38:45Z Long-range vs. short-range ordering in synthetic Cr-substituted goethites chromium geochemistry goethite mineral synthesis soil chemistry A series of Cr-substituted goethites with (Cr:Fe molar ratio up to 0.12) were prepared. Thermal analysis of the solids indicates the formation of cation-deficient compounds that are more stable towards the transformation to hematite as the Cr content increases. Powder X-ray diffraction (PXRD) and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the whole series of the substituted solids. XRD patterns demonstrate that the order around Fe remains typical of a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the incorporation of Cr causes a slight decrease in the cell volume with the c -cell parameter following the Vegard's law. This decrease is accompanied by changes in opposite directions of the various Me-Me distances. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom changes slightly upon Cr substitution: Measurements in the Cr K-edge show that the Cr environment remains unchanged in the whole series. All the observed trends in both average Rietveld and local EXAFS distances can be traced back to the differences in the coordination polyhedra around Cr and Fe. The polyhedron around Cr is more symmetric and can be described as Cr(OH0.5)6 as opposed to the polyhedron around Fe that contains two distinct sets of ligands, FeO3 (OH)3. The effects caused by substitution are governed by this difference, rather than by the smaller size of Cr(III) as compared to Fe(III). Simultaneous use of XAS and Rietveld refinement of XRD data permits tracing the trends in the average long range ordering (Me-Me distances) to local changes in distances and angles when Cr3+ substitutes Fe3+ in goethite. Complex changes in the various interatomic distances and angles may result in deceivingly simple long-range trends. These trends are therefore of limited value as probes for the atomic scale changes. On the other hand, XAS provide direct information on the fundamental, atomic-scale changes. © 2004 Elsevier Ltd. 2004 https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167037_v68_n14_p3053_Sileo http://hdl.handle.net/20.500.12110/paper_00167037_v68_n14_p3053_Sileo
institution Universidad de Buenos Aires
institution_str I-28
repository_str R-134
collection Biblioteca Digital - Facultad de Ciencias Exactas y Naturales (UBA)
topic chromium
geochemistry
goethite
mineral synthesis
soil chemistry
spellingShingle chromium
geochemistry
goethite
mineral synthesis
soil chemistry
Long-range vs. short-range ordering in synthetic Cr-substituted goethites
topic_facet chromium
geochemistry
goethite
mineral synthesis
soil chemistry
description A series of Cr-substituted goethites with (Cr:Fe molar ratio up to 0.12) were prepared. Thermal analysis of the solids indicates the formation of cation-deficient compounds that are more stable towards the transformation to hematite as the Cr content increases. Powder X-ray diffraction (PXRD) and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the whole series of the substituted solids. XRD patterns demonstrate that the order around Fe remains typical of a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the incorporation of Cr causes a slight decrease in the cell volume with the c -cell parameter following the Vegard's law. This decrease is accompanied by changes in opposite directions of the various Me-Me distances. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom changes slightly upon Cr substitution: Measurements in the Cr K-edge show that the Cr environment remains unchanged in the whole series. All the observed trends in both average Rietveld and local EXAFS distances can be traced back to the differences in the coordination polyhedra around Cr and Fe. The polyhedron around Cr is more symmetric and can be described as Cr(OH0.5)6 as opposed to the polyhedron around Fe that contains two distinct sets of ligands, FeO3 (OH)3. The effects caused by substitution are governed by this difference, rather than by the smaller size of Cr(III) as compared to Fe(III). Simultaneous use of XAS and Rietveld refinement of XRD data permits tracing the trends in the average long range ordering (Me-Me distances) to local changes in distances and angles when Cr3+ substitutes Fe3+ in goethite. Complex changes in the various interatomic distances and angles may result in deceivingly simple long-range trends. These trends are therefore of limited value as probes for the atomic scale changes. On the other hand, XAS provide direct information on the fundamental, atomic-scale changes. © 2004 Elsevier Ltd.
title Long-range vs. short-range ordering in synthetic Cr-substituted goethites
title_short Long-range vs. short-range ordering in synthetic Cr-substituted goethites
title_full Long-range vs. short-range ordering in synthetic Cr-substituted goethites
title_fullStr Long-range vs. short-range ordering in synthetic Cr-substituted goethites
title_full_unstemmed Long-range vs. short-range ordering in synthetic Cr-substituted goethites
title_sort long-range vs. short-range ordering in synthetic cr-substituted goethites
publishDate 2004
url https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_00167037_v68_n14_p3053_Sileo
http://hdl.handle.net/20.500.12110/paper_00167037_v68_n14_p3053_Sileo
_version_ 1768544579888349184

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