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Syntheses optimization, structural and thermoelectric properties of 1/1 Tsai-type quasicrystal approximants in RE-Au-SM systems (RE = Yb, Gd and SM = Si, Ge)
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
2013 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 13, 135402- p.Article in journal (Refereed) Published
Abstract [en]

Yb-Cd (Tsai-type) quasicrystals constitute the largest icosahedral quasicrystal family where Yb can be replaced by other rare earth elements (RE) and Cd by pairs of p- and d-block elements. YbCd6 is a prototype 1/1 Tsai-type approximant phase which has a similar local structure to the Yb-Cd quasicrystal. In this study, the syntheses of Yb15.78Au65.22Ge19.00, Gd14.34Au67.16Ge18.5 and Gd14.19Au69.87Si15.94 Tsai-type 1/1 quasicrystal approximants are optimized using the self-flux technique. The crystal structures of the compounds are refined by collecting single crystal x-ray diffraction data. The structural refinements indicated that the compounds are essentially isostructural with some differences at their cluster centers. The basic polyhedral cluster unit in all the three compounds can be described by concentric shells of icosahedra symmetry and of disordered tetrahedra and/or a rare earth atom at the cluster center. Furthermore, the thermoelectric properties of the compounds are probed and their dimensionless figures of merit are calculated at different temperatures. A significant difference is observed in their thermoelectric properties, which could arise due to the slight difference in their crystal structure and chemical composition, as we move from Ge to Si and/or Gd to Yb. Therefore, this study shows the systematic effect of the chemical substitution of structurally similar materials on their thermoelectric properties.

Place, publisher, year, edition, pages
2013. Vol. 25, no 13, 135402- p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:uu:diva-197952DOI: 10.1088/0953-8984/25/13/135402ISI: 000315992900009OAI: oai:DiVA.org:uu-197952DiVA: diva2:615413
Available from: 2013-04-10 Created: 2013-04-08 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Syntheses, structure determination, magnetic and thermoelectric properties of quasicrystal approximants in RE-Au-SM systems (RE = Gd, Tb and Yb and SM = Si, Ge)
Open this publication in new window or tab >>Syntheses, structure determination, magnetic and thermoelectric properties of quasicrystal approximants in RE-Au-SM systems (RE = Gd, Tb and Yb and SM = Si, Ge)
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this study, new compositions of Tsai-type 1/1 quasicrystal approximants Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19), Tb(14)Au(70)Si(16) and Yb(16)Au(65)Ge(19)are synthesized using both self-flux and arc-melting-annealing techniques. Both syntheses routes resulted single phase samples. The crystal structures of the compounds are determined by collecting single crystal X-ray and/or powder X-ray and powder neutron diffraction intensities.  The atomic structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. However, there are subtle structural variations at their cluster centers and in the so-called cubic interstices which affects some of their physical properties.

Thermoelectric and magnetic properties of the compounds are investigated. Significant differences are observed in the thermoelectric properties of Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19) and Yb(16)Au(65)Ge(19) compounds which are explained on the bases of their crystal structures and chemical compositions. Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd(14)Au(70)Si(16) and Tc ≈ 13.1 K for Gd(14)Au(67)Ge(19), whereas, for the Tb(14)Au(70)Si(16) compound a ferrimagnetic-like transition is observed at Tc ≈ 9 K. Moreover, a re-entrant spin-glass transition is observed at TRSG ≈ 3.3 K for Gd(14)Au(67)Ge(19) compound. Finally, the magnetic structure of the Tb(14)Au(70)Si(16) compound was determined from powder neutron diffraction data which is to our knowledge the first magnetic structure refinement report in the family of quasicrystals and approximants.

Place, publisher, year, edition, pages
Uppsala, Sweden: Acta Universitatis Upsaliensis, 2014. 82 p.
Keyword
Quasicrystals, quasicrystal approximants, thermoelectric properties, magnetic properties
National Category
Condensed Matter Physics
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-230004 (URN)
Presentation
2014-09-08, Å2001, Ångstöm laboratory, Uppsala, 15:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Available from: 2014-09-01 Created: 2014-08-18 Last updated: 2014-09-01Bibliographically approved
2. Quasicrystal Approximants in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge): Syntheses, structures and properties
Open this publication in new window or tab >>Quasicrystal Approximants in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge): Syntheses, structures and properties
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this study, new Tsai-type 1/1 quasicrystal approximants (ACs) in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge) were synthesized using high temperature synthesis techniques such as self-flux, arc-melting-annealing and novel arc-melting-self-flux methods. The syntheses not only provided appropriate samples for the intended structural and physical property measurements but could also be adapted to other systems, especially where crystal growth is a challenge. The newly developed arc-melting-self-flux method uses a temperature program that oscillates near the nucleation and melting points of the intended phase in order to obtain large single crystals. Self-flux methods employed to synthesize Ho-Au-Si and Tb-Au-Si ACs using a precursor alloy ≈Au79Si21 resulted in 100 mm3 and 8 mm3 single crystals, respectively.

The crystal structures of the compounds are determined by either one or combinations of the following diffraction techniques; single crystal x-ray, powder x-ray, powder neutron and single crystal neutron diffraction methods.  The crystal structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. In the present compounds there are some structural variations at the cluster center and in the so-called cubic interstices located at (¼, ¼, ¼).

For the current ACs; either thermoelectric, magnetic or both properties were investigated. The measured properties were understood further by correlating the properties with the atomic structures of the ACs. Significant differences are observed in the thermoelectric properties, particularly on the lattice thermal conductivities (Kphonon) of Gd-Au-Si, Gd-Au-Ge and Yb-Au-Ge ACs. The difference is attributed to the presence of chemical and positional disorder.

Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd-Au-Si and Tc ≈ 13.1 K for Gd-Au-Ge. For a Tb-Au-Si AC with 14 % central-Tb occupancy, a ferrimagnetic-like transition was observed at Tc ≈ 9 K. Later, it was noted that the Tc and other magnetic properties depend on the occupancy of the central-RE site. Consistent decrease of Tc with increasing central-Tb occupancy is observed. The dependency of magnetic behavior with central-RE occupancy was clarified by solving the magnetic structure of the Tb-Au-Si AC.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1400
Keyword
Quasicrystal approximant, structure, physical property
National Category
Inorganic Chemistry Condensed Matter Physics
Research subject
Chemistry with specialization in Inorganic Chemistry; Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-300683 (URN)978-91-554-9646-3 (ISBN)
External cooperation:
Public defence
2016-09-30, Å2005, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2016-09-07 Created: 2016-08-10 Last updated: 2016-09-13

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Gebresenbut, Girma HailuGomez, Cesar Pay

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