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Electron charge distribution of CaAl2−xZnx: Maximum Entropy Method combined with Rietveld analysis of High-resolution-synchrotron X-ray powder diffraction data
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
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2008 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 181, no 8, 1998-2005 p.Article in journal (Refereed) Published
Abstract [en]

Using short wavelength X-rays from synchrotron radiation (SPring-8), high-resolution powder diffraction patterns were collected. In order to study both the structural relationship and the mechanism of stability in the CaAl2−xZnx system, among the Laves phases (MgCu2 and MgNi2 type) and KHg2-type structures, the charge density distribution of CaAl2−xZnx as a function of x was obtained from the diffraction data by Rietveld analysis combined with the maximum entropy method (MEM). In the MEM charge density maps overlapping electron densities were clearly observed, especially in the Kagomé nets of the Laves phases. In order to clarify the charge redistribution in the system, the deformation charge densities from the densities formed by the constituent free atoms are discussed. In the ternary MgNi2-type phase, partial ordering of Al and Zn atoms is observed, a finding that is supported by ab-initio total energy calculations.

Place, publisher, year, edition, pages
2008. Vol. 181, no 8, 1998-2005 p.
Keyword [en]
Intermetallic compounds, Laves phases, Charge density, Maximum entropy method
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-24698DOI: 10.1016/j.jssc.2008.04.034ISI: 000259098000041OAI: oai:DiVA.org:su-24698DiVA: diva2:198132
Available from: 2008-02-25 Created: 2008-02-12 Last updated: 2011-03-10Bibliographically approved
In thesis
1. Studies of inorganic crystal structures and gas adsorption process in mesoporous crystals: New approach through analysis of electron charge distribution by synchrotron powder X-ray diffraction
Open this publication in new window or tab >>Studies of inorganic crystal structures and gas adsorption process in mesoporous crystals: New approach through analysis of electron charge distribution by synchrotron powder X-ray diffraction
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Using synchrotron powder XRD experiments, I studied (i) structures of silica mesoporous MCM-41 and MCM-48 crystals, (ii) gas adsorption processes of Ar and N2 in the mesopores of the mesoporous crystals, and (iii) bonding features of CaAl2-xZnx crystals through the analysis of electron charge density distributions.

In the case of mesoporous crystals, the following two different approaches were taken depending on the number of observed X-ray reflections

1. The analytical expression was chosen and further developed to study the size and shape of the mesopores with the plane group of p6mm by powder XRD experiments. For MCM-41, following the determination of the mesopores’ size and shape, and the thickness of the adsorbed gas layer as a function of the gas pressure were successfully observed. In addition, for carbon pipe mesoporous crystals CMK-5, the carbon pipe thickness was determined and the diffraction pattern was discussed quantitatively focusing on the “accidental extinction.”

2. Maximum entropy method (MEM) was used for the structural study of MCM-48 (3D bicontinuous Ia-3d ) and for the gas adsorption process within the mesopores.

By adopting the MEM approach, the study of “bonding electrons” and associated atomic displacements from the ideal Kagome net in the Laves phase CaAl2-xZnx were observed. In particular, the two Kagome nets in the C36 structure showed different feature in the electron density distributions between each other.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, 2008. 55 p.
Keyword
XRD, Mesoporous, Gas adsorption, Laves Phase, MEM
Research subject
Structural Chemistry
Identifiers
urn:nbn:se:su:diva-7367 (URN)978-91-7155-588-5 (ISBN)
Public defence
2008-03-17, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 13:30
Opponent
Supervisors
Available from: 2008-02-25 Created: 2008-02-12Bibliographically approved
2. Structure and Stability of Bi1-xSbx and CaAl2-xZnx
Open this publication in new window or tab >>Structure and Stability of Bi1-xSbx and CaAl2-xZnx
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on two intermetallic systems Bi1-xSbx and CaAl2-xZnx. Bi and Sb transform into a peculiar incommensurate composite structure under pressure and our intention was to examine the structure and how alloying influences this incommensurate structure. Our investigation was obscured by the occurrence of a phase separation, which accompanied the transition A7 → Bi III. Most remarkable is the finding that structural parameters of phases with the Bi III composite structure were observed to be almost unaffected by pressure and composition effects.

Secondly, phase and structural stability relations within the AB2 pseudo-binary system CaAl2-xZnx was studied. In CaAl2-xZnx, the C36-type Laves phase was observed for the first time, but not the C14-type, and a VEC induced structural transition C15-type → C36-type takes place with increasing x. The exchange of Al by Zn decreases the size of the B-type atom network surrounding Ca and at concentrations x > 0.95, the Laves phase structure is succeeded by the CeCu2 type structure, which tolerates a larger size ratio A/B. The C15-type → C36-type transformation in the CaAl2-xZnx system is induced by the valence electron concentration VEC.

Through the analysis of MEM/Rietveld from synchrotron powder X-ray diffraction data, the total charge distributions and deformation charge densities were obtained for cubic and 4H hexagonal Laves phases in CaAl2-xZnx and also for orthorhombic CaZn2. The overlap electrons, which take an important role in stabilising the crystal structures, are clearly observed in the Kagome-net between B-net atoms. The Ca atoms are located in the space formed by the B-net atoms.

From the high-pressure experiments it was concluded that at elevated temperatures, 1000-1500 °C, the hexagonal C36 structure type transforms to the C15 structure type between 6.2 and 9.4 GPa, and the orthorhombic CeCu2 phase to a hexagonal Laves phase between 7.5 and 15.5 GPa.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi, 2005. 86 p.
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-624 (URN)91-7155-109-3 (ISBN)
Public defence
2005-09-19, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 13:00
Opponent
Supervisors
Available from: 2005-08-27 Created: 2005-08-27 Last updated: 2011-03-10Bibliographically approved

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