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Materials Design from First Principles: stability and magnetism of nanolaminates
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.ORCID iD: 0000-0001-5036-2833
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, first-principles calculations within density functional theory are presented, with a principal goal to investigate the phase stability of so called Mn+1AXn (MAX) phases. MAX phases are a group of nanolaminated materials comprised of a transition metal (M), a group 12-16 element (A), and carbon or nitrogen (X). They combine ceramic and metallic characteristics, and phase stability studies are motivated by a search for new phases with novel properties, such as magnetism, and for the results to be used as guidance in attempted materials synthesis in the lab.

To investigate phase stability of a hypothetical material, a theoretical approach has been developed, where the essential part is to identify the set of most competing phases relative to the material of interest. This approach advance beyond more traditional evaluation of stability, where the energy of formation of the material is generally calculated relative to its single elements, or to a set of ad hoc chosen competing phases. For phase stability predictions to be reliable, validation of previous experimental work is a requirement prior to investigations of new, still hypothetical, materials. It is found that the predictions from the developed theoretical approach are consistent with experimental observations for a large set of MAX phases. The predictive power is thereafter demonstrated for the new phases Nb2GeC and Mn2GaC, which subsequently have been synthesized as thin films. It should be noted that Mn is used for the first time as sole M-element in a MAX phase. Hence, the theory is successfully used to find new candidates, and to guide experimentalists in their work on novel promising materials. Phase stability is also evaluated for MAX phase alloys. Incorporation of oxygen in different M2AlC phases are studied, and the results show that oxygen prefer different sites depending on M-element, through the number of available non-bonding M d-electrons. The theory also predicts that oxygen substituting for carbon in Ti2AlC stabilizes the material, which explains the  experimentally observed 12.5 at% oxygen (x = 0.5) in Ti2Al(C1-xOx).

Magnetism is a recently attained property of MAX phase materials, and a direct result of this Thesis work. We have demonstrated the importance of choice of magnetic spin configuration and electron correlations approximations for theoretical evaluation of the magnetic ground state of Cr2AC (A = Al, Ga, Ge). Furthermore, alloying Cr2AlC with Mn to obtain the first magnetic MAX phase have been theoretically predicted and experimentally verified. Using Mn2GaC as model system, Heisenberg Monte Carlo simulations have been used to explore also noncollinear magnetism, suggesting a large set of possible spin configurations (spin waves and spin spirals) to be further investigated in future theoretical and experimental work.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 81 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1571
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-104764DOI: 10.3384/diss.diva-104764ISBN: 978-91-7519-411-0 (print)OAI: oai:DiVA.org:liu-104764DiVA: diva2:698864
Public defence
2014-03-14, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 09:15 (English)
Opponent
Supervisors
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2017-11-03Bibliographically approved
List of papers
1. Phase stability of Ti2AlC upon oxygen incorporation: A first-principles investigation
Open this publication in new window or tab >>Phase stability of Ti2AlC upon oxygen incorporation: A first-principles investigation
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 2, 024111-1-024111-8 p.Article in journal (Refereed) Published
Abstract [en]

The phase stability of Ti2AlC upon oxygen incorporation has been studied by means of first-principles calculations. Recent experimental observations of this so-called MAX phase (M = early transition metal, A = A-group element, and X = C or N) show that the characteristic nanolaminated structure is retained upon oxygen incorporation, with strong indications of O substituting for C. Therefore, a solid solution of C and O on the carbon sublattice has been simulated by the so-called special quasirandom structure method. Through a developed systematic approach, the enthalpy of formation of Ti2Al(C1−x,Ox) has been compared to all experimentally known competing phases, and has been found favorable for all C to O ratios at the composition of the MAX phase. A negative isostructural formation enthalpy has also been predicted for Ti2Al(C1−x,Ox). Altogether, the results indicate that a large amount of oxygen, at least up to x=0.75, might be present in the Ti2AlC MAX-phase structure without decomposition of the material into its competing phases. Furthermore, an effect of an increased oxygen content is a corresponding increase in the bulk modulus and a change in electronic properties. These results are of importance for further understanding and identification of possible composition range of the MAX-phase oxycarbide, and hence for the prospect of tuning the material properties by a varying incorporation of oxygen.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-53771 (URN)10.1103/PhysRevB.81.024111 (DOI)
Available from: 2010-02-09 Created: 2010-02-03 Last updated: 2017-12-12Bibliographically approved
2. Oxygen incorporation and defect formation in Ti2AlC, V2AlC and Cr2AlC from first-principles calculations
Open this publication in new window or tab >>Oxygen incorporation and defect formation in Ti2AlC, V2AlC and Cr2AlC from first-principles calculations
2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We have studied oxygen incorporation and defect formation in M2AlC (M = Ti, V, Cr) MAX phases using first principles calculations. Evaluating phase stability and electronic structure for different oxygen and/or vacancy configurations, we show that oxygen prefer different lattice sites depending on M-element, which can be correlated to the number of available non-bonding M d-electrons. The results show that oxygen substitutes for carbon in Ti2AlC, while forming an interstitial oxygen in the Al-layer for Cr2AlC. We also predict that oxygen incorporation in Ti2AlC stabilizes the material, which explains the experimentally observed 12.5 at% oxygen (x = 0.5) in Ti2Al(C1-xOx). Due to similar valence electron configuration of Ti2AlC and the hypothetical M2AlC (M =Zr, Hf), we also investigate if oxygen can be used to stabilize the latter MAX phases.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-104758 (URN)
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2017-11-03Bibliographically approved
3. Stability trends of MAX phases from first principles
Open this publication in new window or tab >>Stability trends of MAX phases from first principles
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 22, 220102- p.Article in journal (Refereed) Published
Abstract [en]

We have developed a systematic method to investigate the phase stability of M(n+1)AX(n) phases, here applied for M=Sc, Ti, V, Cr, or Mn, A=Al, and X=C or N. Through a linear optimization procedure including all known competing phases, we identify the set of most competitive phases for n=1-3 in each system. Our calculations completely reproduce experimental occurrences of stable MAX phases. We also identify and suggest an explanation for the trend in stability as the transition metal is changed across the 3d series for both carbon- and nitrogen-based systems. Based on our results, the method can be used to predict stability of potentially existing undiscovered phases.

Place, publisher, year, edition, pages
American Physical Society, 2010
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-58288 (URN)10.1103/PhysRevB.81.220102 (DOI)000279147000001 ()
Note
Original Publication: Martin Dahlqvist, Björn Alling and Johanna Rosén, Stability trends of MAX phases from first principles, 2010, Physical Review B. Condensed Matter and Materials Physics, (81), 22, 220102. http://dx.doi.org/10.1103/PhysRevB.81.220102 Copyright: American Physical Society http://www.aps.org/ Available from: 2010-08-10 Created: 2010-08-09 Last updated: 2017-12-12
4. Discovery of the Ternary Nanolaminated Compound Nb2GeC by a Systematic Theoretical-Experimental Approach
Open this publication in new window or tab >>Discovery of the Ternary Nanolaminated Compound Nb2GeC by a Systematic Theoretical-Experimental Approach
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2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 3, 035502- p.Article in journal (Refereed) Published
Abstract [en]

Since the advent of theoretical materials science some 60 years ago, there has been a drive to predict and design new materials in silicio. Mathematical optimization procedures to determine phase stability can be generally applicable to complex ternary or higher-order materials systems where the phase diagrams of the binary constituents are sufficiently known. Here, we employ a simplex-optimization procedure to predict new compounds in the ternary Nb-Ge-C system. Our theoretical results show that the hypothetical Nb2GeC is stable, and excludes all reasonably conceivable competing hypothetical phases. We verify the existence of the Nb2GeC phase by thin film synthesis using magnetron sputtering. This hexagonal nanolaminated phase has a and c lattice parameters of similar to 3.24 angstrom and 12.82 angstrom.

Place, publisher, year, edition, pages
American Physical Society, 2012
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-79981 (URN)10.1103/PhysRevLett.109.035502 (DOI)000306466900014 ()
Note

Funding Agencies|European Research Council under the European Community|258509|Swedish Research Council (V.R.)||Swedish Foundation for Strategic Research||Swedish Agency for Innovation Systems (VINNOVA) Excellence Center FunMat||

Available from: 2012-08-17 Created: 2012-08-17 Last updated: 2017-12-07
5. Correlation between magnetic state and bulk modulus of Cr2AlC
Open this publication in new window or tab >>Correlation between magnetic state and bulk modulus of Cr2AlC
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 21Article in journal (Refereed) Published
Abstract [en]

The effect of magnetism on the bulk modulus (B0) of M2AlC (M  = Ti, V, and Cr) has been studied using first principles calculations. We find that it is possible to identify an energetically favorable magnetic Cr2AlC phase without using any adjustable parameter, such as the Hubbard U. Furthermore, we show that an in-plane spin polarized configuration has substantially lower B0 as compared to the non-magnetic model. The existences of local magnetic moments on Cr atoms considerably improve agreement between theory and experiment regarding trends in B0 for M2AlC phases.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2013
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-96430 (URN)10.1063/1.4808239 (DOI)000320674500104 ()
Note

Funding Agencies|European Research Council under the European Community|258509|Swedish Research Council (VR)|621-2012-4425621-2011-4417|

Available from: 2013-08-20 Created: 2013-08-19 Last updated: 2017-12-06
6. Magnetic ground state of Cr2AlC, Cr2GaC, and Cr2GeC from first-principles interplay of spin configurations and strong electrons correlation
Open this publication in new window or tab >>Magnetic ground state of Cr2AlC, Cr2GaC, and Cr2GeC from first-principles interplay of spin configurations and strong electrons correlation
2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We have studied the interplay between spin configuration and electron correlations approximations as well as their influence on calculated lattice parameters, magnetic moments, and bulk modulus of the nanolaminated MAX phase materials Cr2AlC, Cr2GaC, and Cr2GeC. By considering non-, ferro- and, and five different antiferromagnetic configurations, we show the importance of including a broad range of magnetic states in search for the ground state. Our calculations show that when electron correlation is treated on the level of the generalized gradient approximation or with an additional Hubbard U interaction term up to a value of 1 eV, the magnetic ground state of Cr2AC (A=Al, Ga, Ge) is in-plane antiferromagnetic with finite Cr local moments and calculated lattice parameters and bulk modulus close to experimentally reported values.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-104759 (URN)
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2017-11-03Bibliographically approved
7. Magnetic nanoscale laminates with tunable exchange coupling from first principles
Open this publication in new window or tab >>Magnetic nanoscale laminates with tunable exchange coupling from first principles
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 22, 220403- p.Article in journal (Refereed) Published
Abstract [en]

The M(n+1)AX(n) (MAX) phases are nanolaminated compounds with a unique combination of metallic and ceramic properties, not yet including magnetism. We carry out a systematic theoretical study of potential magnetic MAX phases and predict the existence of stable magnetic (Cr(1-x)Mn(x))(2)AlC alloys. We show that in this system ferromagnetically ordered Mn layers are exchange coupled via nearly nonmagnetic Cr layers, forming an inherent structure of atomic-thin magnetic multilayers, and that the degree of disorder between Cr and Mn in the alloy can be used to tune the sign and magnitude of the coupling.

Place, publisher, year, edition, pages
American Physical Society, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-73313 (URN)10.1103/PhysRevB.84.220403 (DOI)297763300001 ()
Available from: 2012-01-03 Created: 2012-01-02 Last updated: 2017-12-08
8. A Nanolaminated Magnetic Phase: Mn2GaC
Open this publication in new window or tab >>A Nanolaminated Magnetic Phase: Mn2GaC
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2014 (English)In: Materials Research Letters, ISSN 2166-3831, Vol. 2, no 2, 89-93 p.Article in journal (Refereed) Published
Abstract [en]

Layered magnetic materials are fascinating from the point of view of fundamental science as well as applications. Discoveries such as giant magnetoresistance (GMR) in magnetic multilayers have revolutionized data storage and magnetic recording, and concurrently initiated the search for new layered magnetic materials. One group of inherently nanolaminated compounds are the so called Mn+1AXn (MAX) phases. Due to the large number of isostructural compositions, researchers are exploring the wide range of interesting properties, and not primarily functionalization through optimization of structural quality. Magnetic MAX phases have been discussed in the literature, though this is hitherto an unreported phenomenon. However, such materials would be highly interesting, based on the attractive and useful properties attained with layered magnetic materials to date. Here we present a new MAX phase, (Cr1–xMnx)2GeC, synthesized as thin film in heteroepitaxial form, showing single crystal material with unprecedented structural MAX phase quality. The material was identified using first-principles calculations to study stability of hypothetical MAX phases, in an eort to identify a potentially magnetic material. The theory predicts a variety of magnetic behavior depending on the Mn concentration and Cr/Mn atomic conguration within the sublattice. The analyzed thin films display a magnetic signal well above room temperature and with partly ferromagnetic ordering. These very promising results open up a field of new layered magnetic materials, with high potential for electronics and spintronics applications.

Place, publisher, year, edition, pages
Taylor & Francis, 2014
Keyword
MAX phases, sputtering, transmission electron microscopy (TEM), ab initio calculation
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-77774 (URN)10.1080/21663831.2013.865105 (DOI)
Note

On the day of the defence date the status of this article was previous Manuscript. The original title of the Manuscript was Magnetic nanoscale laminates from first principles and thin film synthesis.

Available from: 2012-05-29 Created: 2012-05-29 Last updated: 2017-11-03Bibliographically approved
9. Complex magnetism in nanolaminated Mn2GaC
Open this publication in new window or tab >>Complex magnetism in nanolaminated Mn2GaC
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2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We have used first-principles calculations and Heisenberg Monte Carlo simulations to search for the magnetic ground state of Mn2GaC, a recently synthesized magnetic nanolaminate. We have, independent on method, identified a range of low energy collinear as well as non-collinear magnetic configurations, indicating a highly frustrated magnetic material with several nearly degenerate magnetic states. An experimentally obtained magnetization of only 0.29 per Mn atom in Mn2GaC may be explained by canted spins in an antiferromagnetic configuration of ferromagnetically ordered sub-layers with alternating spin orientation, denoted AFM[0001]. Furthermore, low temperature X-ray diffraction show a new basal plane peak appearing upon a magnetic transition, which is consistent with the here predicted change in inter-layer spacing for the AFM[0001] configuration.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-104760 (URN)
Available from: 2014-02-25 Created: 2014-02-25 Last updated: 2017-11-03Bibliographically approved

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