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Energy relavant materials: Investigations based on first principles
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
2012 (English)Doctoral 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. The diiron phosphide-based materials have been proposed to be amongst the most promising candidates for working body of magnetic refrigerators. Hydrogen is one of 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 bonds usually yield to a mixture of different gases. Palladiumsilver 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 this thesis ab initio electronic structure methods are used to study the magnetic and crystallographic properties of Fe2P based magneto-caloric compounds and the thermophysical properties of Pd-Ag binary alloys.

The nature of magnetism and the strong sensitivity of the Curie temperature of the Fe2P1−xTx (T = boron, silicon and arsenic) are investigated. Using first principles theory, the change in the average magnetic exchange interactions upon doping is decomposed into chemical and structural contributions, the latter including the c/a and vol-ume effects. It is demonstrated that for the investigated alloys the structural effect can´be ascribed mainly to the c/a ratio that strengthens the magnetic exchange interactions between the two Fe sublattices. On the other hand, it is shown that the two types of Fe atoms have a very complicated co-dependency, which manifests in a metamagnetic behavior of the FeI sublattice. This behavior is strongly influenced by doping the P sites.

Lattice stability of pure Fe2P and the effect of Si doping on the phase stability are pre-sented. In contrast to the observation, for the ferromagnetic state the hexagonal structure (hex, space group P62m) has no the lowest energy. For the paramagnetic state, the hex structure is shown to be the stable phase and the computed total energy versuscomposition indicates a hex to bco (body centered orthorhombic, space group Imm2)crystallographic phase transition with increasing Si content. The mechanisms responsi-ble for the structural phase transition are discussed in details.

The magnetic properties of Fe2P can be subtly tailored by Mn doping. It was 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 elastic constants, the polycrystalline elastic moduli and the Debye temperature of disordered 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 linear trend. The complex electronic origin of these anomalies is demonstrated. The effect of long range order on the single crystal elastic constants of Pd0.5Ag0.5 alloy is also investigated.

Within this thesis most of the calculations were performed using the Exact Muffin-Tin Orbitals method. The chemical and magnetic disorder are treated via the Coherent Potential Approximation. The paramagnetic phase is modeled by the Disordered Local Magnetic Moments approach.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xi, 59 p.
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-90738ISBN: 978-91-7501-250-6 (print)OAI: oai:DiVA.org:kth-90738DiVA: diva2:506407
Public defence
2012-03-23, F3, Lindstedtvagen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120229Available from: 2012-02-29 Created: 2012-02-28 Last updated: 2012-02-29Bibliographically 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: 2017-12-12Bibliographically 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: 2017-12-12Bibliographically approved
3. Strongly enhanced magnetic moments in ferromagnetic FeMnP(0.5)Si(0.5)
Open this publication in new window or tab >>Strongly enhanced magnetic moments in ferromagnetic FeMnP(0.5)Si(0.5)
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 15, 152502- p.Article in journal (Refereed) Published
Abstract [en]

The compound FeMnP(0.5)Si(0.5) has been studied by magnetic measurements, Mossbauer spectroscopy, and electronic structure and total energy calculations. An unexpectedly high magnetic hyperfine field for Fe atoms located at the tetrahedral Me(1) site in the Fe(2)P structure is found, The saturation moment derived from magnetic measurements corresponds to 4.4 mu(B)/f.u. at low temperatures, a value substantially higher than previously reported, but in accordance with the results from our electron structure calculations, This high saturation moment and the tunable first order ferromagnetic transition make the Fe(2-x)Mn(x)P(1-y)Si(y), system promising for magnetocaloric applications.

Keyword
electronic structure, iron alloys, magnetic moments, magnetic transitions, magnetocaloric effects, manganese alloys, Mossbauer effect, phosphorus alloys, silicon alloys, total energy
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-48515 (URN)10.1063/1.3651272 (DOI)000295883800051 ()2-s2.0-80054970826 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20111207Available from: 2011-12-07 Created: 2011-11-21 Last updated: 2017-12-08Bibliographically approved
4. 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: 2017-12-12Bibliographically approved
5. Effect of long-range order on elastic properties of Pd(0.5)Ag(0.5) alloy from first principles
Open this publication in new window or tab >>Effect of long-range order on elastic properties of Pd(0.5)Ag(0.5) alloy from first principles
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 9, 094205- p.Article in journal (Refereed) Published
Abstract [en]

The effect of long-range order on single-crystal elastic constants of Pd(0.5)Ag(0.5) alloy has been investigated using first-principles electronic structure calculations. The lowest energy among the considered ordered, partially ordered, and disordered structures is found to be the L1(1) layered structure, which is formed by alternate (111) Pd and Ag layers. The ordering effect is found to follow a clear trend: in contrast to the disordered phase, for which the K(a) and K(c) compressibilities are equal, the L1(1) structure becomes less compressible along the c axis than along the a axis.

Keyword
electronic topological transitions, coherent-potential approximation, metallic random alloys, fermi surfaces, constants, energy, model
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-43428 (URN)10.1103/PhysRevB.84.094205 (DOI)000295083700003 ()2-s2.0-80053539850 (Scopus ID)
Note
QC 20111017Available from: 2011-10-17 Created: 2011-10-17 Last updated: 2017-12-08Bibliographically approved

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