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Energy Relevant Materials: Investigations Based on First Principles
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. (Tillämpad materialfysik, Applied Material Physics)
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Energy production, storage and efficient usage are all crucial factors for environmentally sound and sustainable future technologies. One important question concerns the refrigeration industry, where the energy efficiency of the presently used technologies is at best 40% of the theoretical Carnot limit. Magnetic refrigerators offer a modern low-energy demand and environmentally friendly alternative. Iron phosphide based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of the central elements on the most promising sources of renewable energy. Considerable international research focuses on finding good solid state materials for hydrogen storage. On the other hand, hydrogen gas is obtained from hydrogen containing chemical compounds, which after breaking the chemical bounds usually yield to a mixture of different gases. Palladium-silver alloys are frequently used for hydrogen separation membranes for producing purified hydrogen gas. All these applications need a fundamental understanding of the structural, magnetic, chemical and thermophysical properties of the involved solid state materials. In the present thesis ab initio electronic structure methods are used to study the crystallographic and magnetic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys. Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are presented. In contrast to the observation, for the ferromagnetic state the body centered orthorhombic structure (bco, space group Imm2) is predicted to have lower energy than the hexagonal structure (hex, space group P62m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The magneto-structural effects and the mechanisms responsible for the structural phase transition are discussed in details. The magnetic properties of Fe2P can be subtly tailored by Mn doping. It has been shown experimentally that Mn atoms preferentially occupy one of the two different Fe sites of Fe2P. Theoretical results for the Mn site occupancy in MnFeP1-xSix are presented. The single crystal and polycrystalline elastic constants and the Debye temperature of Pd1-xAgx binary alloys are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation of the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from the simple expected trend. The complex electronic origin of these anomalies is demonstrated. Within the present thesis, all relaxed crystal structures are obtained using the Projector AugmentedWave full-potential method. The chemical and magnetic disorder is treated using the Exact Muffin-Tin Orbitals method in combination with the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.

Place, publisher, year, edition, pages
US-AB , 2010. , 83 p.
Keyword [en]
magneto-caloric, DFT, elastic constants
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-25050ISBN: 978-91-7415-752-9 (print)OAI: oai:DiVA.org:kth-25050DiVA: diva2:358351
Presentation
2010-11-04, N111, KTH, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101101Available from: 2010-11-01 Created: 2010-10-06 Last updated: 2010-11-01Bibliographically approved
List of papers
1. Ab initio study of structural and magnetic properties of Si-doped Fe(2)P
Open this publication in new window or tab >>Ab initio study of structural and magnetic properties of Si-doped Fe(2)P
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2010 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 82, no 8, 085103- p.Article in journal (Refereed) Published
Abstract [en]

Ab initio electronic-structure methods are used to study the properties of Fe(2)P(1-x)Si(x) in ferromagnetic and paramagnetic states. The site preference and lattice relaxation are calculated with the projector augmented wave method as implemented in the Vienna ab initio simulation package. The paramagnetic state is modeled by the disordered local magnetic moment scheme, and the chemical and magnetic disorder is treated using the coherent potential approximation in combination with the exact muffin-tin orbital formalism. The calculated lattice parameters, atomic positions, and magnetic properties are in good agreement with the experimental and other theoretical results. In contrast to the observation, for the ferromagnetic state the body centered ortho-rhombic structure (bco, space group I (mm2) under bar) is predicted to have lower energy than the hexagonal structure (hex, space group P (6) over bar 2m). The zero-point spin fluctuation energy difference is found to be large enough to stabilize the hex phase. For the paramagnetic state, the hex structure is calculated to be the stable phase and the computed total energy versus composition indicates a hex to bco crystallographic phase transition with increasing Si content. The phonon vibrational free energy, estimated from the theoretical equation of state, turns out to stabilize the hexagonal phase, whereas the electronic and magnetic entropies favor the low symmetry orthorhombic structure.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-25812 (URN)10.1103/PhysRevB.82.085103 (DOI)000280606500006 ()2-s2.0-77957575441 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20101101

Available from: 2010-11-01 Created: 2010-11-01 Last updated: 2016-12-19Bibliographically approved
2. Order-disorder induced magnetic structures of FeMnP0.75Si0.25
Open this publication in new window or tab >>Order-disorder induced magnetic structures of FeMnP0.75Si0.25
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2011 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 83, no 13, 134420- p.Article in journal (Refereed) Published
Abstract [en]

We report on the synthesis and structural characterization of the magnetocaloric FeMnP0.75Si0.25 compound. Two types of samples (quenched and slowly cooled) were synthesized and characterized structurally and magnetically. We have found that minor changes in the degree of crystallographic order causes large changes in the magnetic properties. The slow-cooled sample, with a higher degree of order, is antiferromagnetic. The quenched sample has a net moment of 1.26 mu(B) per formula unit and ferrimagnetic behavior. Theoretical calculations give rather large values for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and the pyramidal lattice sites.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-25814 (URN)10.1103/PhysRevB.83.134420 (DOI)000291973200006 ()2-s2.0-79961081105 (Scopus ID)
Note

QC 20101101. Updated from manuscript to article in journal.

Available from: 2010-11-01 Created: 2010-11-01 Last updated: 2014-09-22Bibliographically approved
3. Ab initio study of the elastic anomalies in Pd-Ag alloys
Open this publication in new window or tab >>Ab initio study of the elastic anomalies in Pd-Ag alloys
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2009 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 79Article in journal (Refereed) Published
Abstract [en]

Ab initio total-energy calculations, based on the exact muffin-tin orbital method, are used to determine the elastic properties of Pd1−xAgx random alloys in the face-centered-cubic crystallographic phase. The compositional disorder is treated within the coherent-potential approximation. The single crystal and polycrystalline elastic constants and the Debye temperature are calculated for the whole range of concentration, 0≤x≤1. It is shown that the variation in the elastic parameters of Pd-Ag alloys with chemical composition strongly deviates from a simple linear or parabolic trend. The complex electronic origin of these anomalies is demonstrated.

 

Keyword
ab initio calculations, chemical analysis, CPA calculations, crystal structure, Debye temperature, elastic constants, linear muffin-tin orbital method, palladium alloys, silver alloys, total energy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-25818 (URN)10.1103/PhysRevB.79.085107 (DOI)000263816000020 ()2-s2.0-62549149978 (Scopus ID)
Note
QC 20101101Available from: 2010-11-01 Created: 2010-11-01 Last updated: 2012-03-20Bibliographically approved

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