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Quasicrystal Approximants in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge): Syntheses, structures and properties
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström.ORCID iD: 55626173300
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 [en]
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: urn:nbn:se:uu:diva-300683ISBN: 978-91-554-9646-3OAI: oai:DiVA.org:uu-300683DiVA: diva2:951905
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
List of papers
1. 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)
Open this publication in new window or tab >>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)
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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-197952 (URN)10.1088/0953-8984/25/13/135402 (DOI)000315992900009 ()
Available from: 2013-04-10 Created: 2013-04-08 Last updated: 2016-09-06Bibliographically approved
2. Ferromagnetism and re-entrant spin-glass transition in quasicrystal approximants Au-SM-Gd (SM = Si, Ge)
Open this publication in new window or tab >>Ferromagnetism and re-entrant spin-glass transition in quasicrystal approximants Au-SM-Gd (SM = Si, Ge)
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2013 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 42, 426004- p.Article in journal (Refereed) Published
Abstract [en]

Magnetic susceptibility and specific heat measurements on quasicrystalline approximants Au–Si–Gd and Au–Ge–Gd reveal that a ferromagnetic (FM) transition occurs at Tc = 22.5(5) °K for Au–Si–Gd and at Tc = 13(1) °K for Au–Ge–Gd, which are the first examples of ferromagnetism in crystalline approximants. In addition, a re-entrant spin-glass (RSG) transition is observed at TRSG = 3.3 °K for Au–Ge–Gd in contrast to Au–Si–Gd. The different behaviors are understood based on the recent structural models reported by Gebresenbut et al (2013 J. Phys.: Condens. Matter 25 135402). The RSG transition in Au–Ge–Gd is attributed to a random occupation of the center of the Gd12 icosahedron by Gd atoms; a central Gd spin hinders the long-range FM order.

Place, publisher, year, edition, pages
IOP Publishing in Bristol, UK: , 2013
Keyword
Quasicrystal approximants, magnetic property, ferromagnetism
National Category
Condensed Matter Physics
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-209234 (URN)10.1088/0953-8984/25/42/426004 (DOI)000325337700023 ()
Available from: 2013-10-15 Created: 2013-10-15 Last updated: 2016-09-06Bibliographically approved
3. Long range ordered magnetic and atomic structures of the quasicrystal approximant in the Tb-Au-Si system
Open this publication in new window or tab >>Long range ordered magnetic and atomic structures of the quasicrystal approximant in the Tb-Au-Si system
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2014 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 26, no 32, 322202- p.Article in journal (Refereed) Published
Abstract [en]

The atomic and magnetic structure of the 1/1 Tb(14)Au(70)Si(16) quasicrystal approximant has been solved by combining x-ray and neutron diffraction data. The atomic structure is classified as a Tsai-type 1/1 approximant with certain structural deviations from the prototype structures; there are additional atomic positions in the so-called cubic interstices as well as in the cluster centers. The magnetic property and neutron diffraction measurements indicate the magnetic structure to be ferrimagnetic-like below 9 K in contrast to the related Gd(14)Au(70)Si(16) structure that is reported to be purely ferromagnetic.

Keyword
Long range magnetic order, quasicrystal approximant
National Category
Condensed Matter Physics Engineering and Technology
Research subject
Materials Science; Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-229039 (URN)10.1088/0953-8984/26/32/322202 (DOI)000340656800002 ()
Available from: 2014-07-28 Created: 2014-07-28 Last updated: 2016-09-06Bibliographically approved
4. Tailoring Magnetic Behavior in the Tb-Au-Si Quasicrystal Approximant System
Open this publication in new window or tab >>Tailoring Magnetic Behavior in the Tb-Au-Si Quasicrystal Approximant System
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2016 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 55, no 5, 2001-2008 p.Article in journal (Refereed) Published
Abstract [en]

A novel synthesis method, "arc-melting-self-flux", has been developed and a series of five Tsai-type 1/1 approximant crystals in the Tb-Au-Si system have been synthesized. The synthesis method, by employing a temperature program which oscillates near the melting and nucleation points of the approximants, has provided high-quality and large single crystals in comparison to those obtained from the standard arc-melting-annealing and self-flux methods. The atomic structures of the approximants have been determined from single-crystal X-ray diffraction data and described using concentric atomic clusters with icosahedral symmetry. The compounds are nearly isostructural with subtle variations; two types of atomic clusters which mainly vary at their cluster centers are observed. One type contains a Tb site at the center, and the other contains a disordered tetrahedron decorated with Au/Si mixed sites. Both cluster types can be found coexisting in the approximants. The compounds have different average weighted ratios of central Tb to disordered tetrahedron in the bulk material. Furthermore, a strategy for chemically tuning magnetic behavior is presented. Magnetic property measurements on the approximants revealed that the magnetic transition temperature (T-c) decreases as the occupancy of the central Tb site increases. T-c decreased from 11.5 K for 0% occupancy of the central Tb to 8 K for 100% occupancy. Enhanced magneto crystalline anisotropy is observed for the approximants with higher central Tb occupancy in comparison to their low central Tb occupancy counterparts. Hence, the previously reported "ferrimagnetic-like" magnetic structure model remains valid.

National Category
Inorganic Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-282805 (URN)10.1021/acs.inorgchem.5b02286 (DOI)000371753500008 ()26914797 (PubMedID)
Funder
Swedish Research Council, 2009-3609, 349-2014-3946
Available from: 2016-04-13 Created: 2016-04-07 Last updated: 2016-09-06Bibliographically approved
5. Single crystal growth, structure determination and magnetic behavior of RE-Au-Si quasicrystal approximants (RE = Ho and Tb)
Open this publication in new window or tab >>Single crystal growth, structure determination and magnetic behavior of RE-Au-Si quasicrystal approximants (RE = Ho and Tb)
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(English)Manuscript (preprint) (Other academic)
Keyword
Approximants of quasicrystals, single crystal growth, magnetic properties
National Category
Natural Sciences
Research subject
Chemistry with specialization in Inorganic Chemistry; Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-300678 (URN)
Funder
Swedish Research Council, 2009-3609Swedish Research Council, 349-2014-3946
Available from: 2016-08-10 Created: 2016-08-10 Last updated: 2016-09-06Bibliographically approved

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