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Non-Standard Crystallography: Examples in 3- and 3+1 Dimensional Space
Stockholm University, Faculty of Science, Department of Physical, Inorganic and Structural Chemistry.
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Crystallography today is by many seen as merely a tool for determining the structure of a material on the atomic level. It is expected that whatever comes out of the tool is the indisputable truth, a fact. This thesis is based on five publications illustrating that this is very far from the real world of materials research. The experiences drawn from the structural work in the papers are put together to illustrate when to be alert, and how to proceed with a structural investigation using non-standard crystallography. The focus is on interpreting the signs of additional order being present in a structure. The signs may be weak, such as extreme thermal vibration, or unit cell deformation. Or the signs can be strong, with superstructure reflections indicating the presence of either commensurate or incommensurate superstructures.

Place, publisher, year, edition, pages
Stockholm: Institutionen för fysikalisk kemi, oorganisk kemi och strukturkemi , 2007. , 83 p.
Keyword [en]
Modulation, Crystallography, intermetallic, materials
National Category
Chemical Sciences
Research subject
Structural Chemistry
Identifiers
URN: urn:nbn:se:su:diva-7194ISBN: 978-91-7155-524-3 (print)OAI: oai:DiVA.org:su-7194DiVA: diva2:197825
Public defence
2007-12-17, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 10:00
Opponent
Supervisors
Available from: 2007-11-26 Created: 2007-11-26Bibliographically approved
List of papers
1. Solving Approximant Structures Using a 'Strong Reflections' Approach
Open this publication in new window or tab >>Solving Approximant Structures Using a 'Strong Reflections' Approach
2004 In: Ferroelectrics, ISSN 0015-0193, Vol. 305, no 1, 273-277 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24572 (URN)
Note
Part of urn:nbn:se:su:diva-7194Available from: 2007-11-26 Created: 2007-11-26Bibliographically approved
2. The Sn1-xSb1+x, x ~ 0.5, Solid Solution: the Relationship Between α and β
Open this publication in new window or tab >>The Sn1-xSb1+x, x ~ 0.5, Solid Solution: the Relationship Between α and β
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2006 In: 30th annual condensed matter and materials meeting. Wagga Wagga, Australia, 2006, 2006Chapter in book (Other academic) Published
Identifiers
urn:nbn:se:su:diva-24573 (URN)1-920791-09-4 (ISBN)
Note
Part of urn:nbn:se:su:diva-7194Available from: 2007-11-26 Created: 2007-11-26Bibliographically approved
3. The Samson phase, β-Mg2Al3, Revisited
Open this publication in new window or tab >>The Samson phase, β-Mg2Al3, Revisited
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2007 In: Zeitschrift für Kristallographie, ISSN 0044-2968, Vol. 222, no 6, 259-288 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24574 (URN)000247610300001 ()
Note
Part of urn:nbn:se:su:diva-7194Available from: 2007-11-26 Created: 2007-11-26Bibliographically approved
4. Vacancy Ordering Effects in AlB2-type ErGe2-x(0.4 < x < or = 0.5)
Open this publication in new window or tab >>Vacancy Ordering Effects in AlB2-type ErGe2-x(0.4 < x < or = 0.5)
2008 (English)In: Acta Crystallographica Section B: Structural Science, ISSN 0108-7681, E-ISSN 1600-5740, Vol. 64, no 3, 272-280 p.Article in journal (Refereed) Published
Abstract [en]

In the Er-Ge system, the compostion range ErGe2 to Er2Ge3 has been investigated. Eight samples were produced by arc melting of the elements, and analyzed using X-ray powder diffraction. Nine crystal structures were found to be present in the samples. The structures are described as a homologous series and presented within the superspace formalism using the superspace group X2/m(0)0s, X representing the centring vector (½, ½, 0, ½). In this description the modulation vector q = (a* + c*) is shown to be a direct measure of the Ge content as ErGe2 -  ( falls in the range to ½). The large composition range is achieved by extended vacancy ordering in the planar 63 net of Ge with subsequent relaxation.

Keyword
modulation, rare-earth germanides, vacancy ordering.
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-24575 (URN)10.1107/S0108768108005119 (DOI)000255902100002 ()
Available from: 2007-11-26 Created: 2007-11-26 Last updated: 2011-02-25Bibliographically approved
5. The Origins of Superstructure Ordering and Incommensurability in Stuffed CoSn-type Phases
Open this publication in new window or tab >>The Origins of Superstructure Ordering and Incommensurability in Stuffed CoSn-type Phases
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2008 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 130, no 26, 8195-8214 p.Article in journal (Refereed) Published
Abstract [en]

The CoSn structure type contains large interstitial void spaces that frequently host electropositive guest atoms, such as rare earth elements. In this stuffing process, an intriguing ordering occurs between the neighboring void spaces leading to a family of long-period superstructures comprising intergrowths of the ScFe6Ge6 and ScFe6Ga6 structure types. This superstucture ordering culminates in incommensurability in the REFe6Ge6–δGaδ systems with RE = Sc, Tb, and Lu. In this work, we derive a 3 + 1D superspace model encompassing this series of structures and investigate the origins of the structural trends in this family with electronic structure calculations, at both the LDA-DFT and extended Hckel levels. Using our 3 + 1D model, we refine the structures of four new ErFe6Ge6−δGaδ (0 ≤ δ ≤ 6) phases, two commensurate and two incommensurate, from powder X-ray diffraction data. The refinement results confirm trends observed in the Sc-, Tb-, and Lu-based series: a gradual lengthening and, eventually, turning of the q-vector as Ge is progressively exchanged for Ga. These trends, and the incommensurate ordering as a whole, are traced to a tension between two modes by which the host lattice responds to stuffing atom insertion: (1) an atomic charge modulation enhancing the anionic character of the cavity walls around the guest atoms, and (2) a positional modulation expanding the cavities occupied by guest atoms. These two modes direct the stuffing atom ordering pattern toward opposite ends of the ScFe6Ge6−ScFe6Ga6 intergrowth series. The full series of structures, complex and incommensurate, reflects various degrees of balance between these two factors.

Place, publisher, year, edition, pages
American Chemical Society, 2008
National Category
Chemical Sciences
Identifiers
urn:nbn:se:su:diva-24576 (URN)10.1021/ja077380+ (DOI)000257152800035 ()
Note
Part of urn:nbn:se:su:diva-7194Available from: 2007-11-26 Created: 2007-11-26 Last updated: 2010-09-14Bibliographically approved

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