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Dahlqvist, M., Thore, A. & Rosén, J. (2018). Electronic structure, bonding characteristics, and mechanical properties in (W2/3Sc1/3)(2)AIC and (W2/3Y1/3)(2)AIC i-MAX phases from first-principles calculations. Journal of Physics: Condensed Matter, 30(30), Article ID 305502.
Open this publication in new window or tab >>Electronic structure, bonding characteristics, and mechanical properties in (W2/3Sc1/3)(2)AIC and (W2/3Y1/3)(2)AIC i-MAX phases from first-principles calculations
2018 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 30, article id 305502Article in journal (Refereed) Published
Abstract [en]

With the recent discovery of in-plane chemically ordered MAX phases (i-MAX) of the general formula ((M2/3M1/32)-M-1)(2)AC comes addition of non-traditional MAX phase elements. In the present study, we use density functional theory calculations to investigate the electronic structure, bonding nature, and mechanical properties of the novel (W2/3Sc1/3)(2)AlC and (W2/3Y1/3)(2)AlC i-MAX phases. From analysis of the electronic structure and projected crystal orbital Hamilton populations, we show that the metallic i-MAX phases have significant hybridization between W and C, as well as Sc(Y) and C states, indicative of strong covalent bonding. Substitution of Sc for Y (M-2) leads to reduced bonding strength for W-C and Al-Al interactions while M-2-C and M-2-Al interactions are strengthened. We also compare the Voigt-Reuss-Hill bulk, shear, and Youngs moduli along the series of M-1 = Cr, Mo, and W, and relate these trends to the bonding interactions. Furthermore, we find overall larger moduli for Sc-based i-MAX phases.

Place, publisher, year, edition, pages
Institute of Physics and Engineering in Medicine, 2018
Keywords
atomic laminate; MAX phase; chemical order; electronic structure; first-principles calculations; bond analysis
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-149843 (URN)10.1088/1361-648X/aacc19 (DOI)000437420900001 ()29893717 (PubMedID)
Note

Funding Agencies|Knut and Alice Wallenberg (KAW) Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research (SSF) [EM16-0004]

Available from: 2018-08-02 Created: 2018-08-02 Last updated: 2018-08-20
Dahlqvist, M., Thore, A. & Rosén, J. (2018). Electronic structure, bonding characteristics, and mechanical properties in (W2/3Sc1/3)(2)AIC and (W2/3Y1/3)(2)AIC i-MAX phases from first-principles calculations. Journal of Physics: Condensed Matter, 30(30), Article ID 305502.
Open this publication in new window or tab >>Electronic structure, bonding characteristics, and mechanical properties in (W2/3Sc1/3)(2)AIC and (W2/3Y1/3)(2)AIC i-MAX phases from first-principles calculations
2018 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 30, no 30, article id 305502Article in journal (Refereed) Published
Abstract [en]

With the recent discovery of in-plane chemically ordered MAX phases (i-MAX) of the general formula ((M2/3M1/32)-M-1)(2)AC comes addition of non-traditional MAX phase elements. In the present study, we use density functional theory calculations to investigate the electronic structure, bonding nature, and mechanical properties of the novel (W2/3Sc1/3)(2)AlC and (W2/3Y1/3)(2)AlC i-MAX phases. From analysis of the electronic structure and projected crystal orbital Hamilton populations, we show that the metallic i-MAX phases have significant hybridization between W and C, as well as Sc(Y) and C states, indicative of strong covalent bonding. Substitution of Sc for Y (M-2) leads to reduced bonding strength for W-C and Al-Al interactions while M-2-C and M-2-Al interactions are strengthened. We also compare the Voigt-Reuss-Hill bulk, shear, and Youngs moduli along the series of M-1 = Cr, Mo, and W, and relate these trends to the bonding interactions. Furthermore, we find overall larger moduli for Sc-based i-MAX phases.

Place, publisher, year, edition, pages
Institute of Physics and Engineering in Medicine, 2018
Keywords
atomic laminate; MAX phase; chemical order; electronic structure; first-principles calculations; bond analysis
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-149843 (URN)10.1088/1361-648X/aacc19 (DOI)000437420900001 ()29893717 (PubMedID)
Note

Funding Agencies|Knut and Alice Wallenberg (KAW) Foundation [KAW 2015.0043]; Swedish Foundation for Strategic Research (SSF) [EM16-0004]

Available from: 2018-08-02 Created: 2018-12-20 Last updated: 2018-08-20
Ingason, A. S., Pålsson, G. K., Dahlqvist, M. & Rosén, J. (2016). Long-range antiferromagnetic order in epitaxial Mn2GaC thin films from neutron reflectometry. PHYSICAL REVIEW B, 94(2), 024416
Open this publication in new window or tab >>Long-range antiferromagnetic order in epitaxial Mn2GaC thin films from neutron reflectometry
2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 2, p. 024416-Article in journal (Refereed) Published
Abstract [en]

The nature of the magnetic structure in magnetic so-called MAX phases is a topic of some controversy. Here we present unpolarized neutron-diffraction data between 3.4 and 290.0 K and momentum transfer between Q = 0.0 and 1.1 angstrom(-1), as well as complementary x-ray-diffraction data on epitaxial thin films of the MAX phase material Mn2GaC. This inherently layered material exhibits neutron-diffraction peaks consistent with long-ranged antiferromagnetic order with a periodicity of two structural unit cells. The magnetic structure is present throughout the measured temperature range. The results are in agreement with first-principles calculations of antiferromagnetic structures for this material where the Mn-C-Mn atomic trilayers are found to be ferromagnetically coupled internally but spin flipped or rotated across the Ga layers. The present findings have significant bearing on the discussion regarding the nature of the magnetic structure in magnetic MAX phases.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-130382 (URN)10.1103/PhysRevB.94.024416 (DOI)000379501200005 ()
Note

Funding Agencies|European Research Council (ERC) under the European Communitys Seventh Framework Programme FP7 (ERC Grant) [258509]; Swedish Research Council [621-2012-4425]; Knut and Alice Wallenberg Foundation program

Available from: 2016-08-15 Created: 2016-08-05 Last updated: 2017-11-03
Thore, A., Dahlqvist, M., Alling, B. & Rosén, J. (2016). Magnetic exchange interactions and critical temperature of the nanolaminate Mn2GaC from first-principles supercell methods. Physical Review B, 93(5)
Open this publication in new window or tab >>Magnetic exchange interactions and critical temperature of the nanolaminate Mn2GaC from first-principles supercell methods
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 5Article in journal (Refereed) Published
Abstract [en]

In this work, we employ and critically evaluate a first-principles approach based on supercell calculations for predicting the magnetic critical order-disorder temperature 𝑇𝑐 . As a model material we use the recently discovered nanolaminate Mn2GaC.

First, we derive the exchange interaction parameters 𝐽𝑖𝑗 between pairs of Mn atoms on sites 𝑖 and 𝑗 of the bilinear Heisenberg Hamiltonian using the novel magnetic direct cluster averaging method (MDCA), and then compare the 𝐽’s from the MDCA calculations to the same parameters calculated using the Connolly-Williams method. We show that the two methods yield closely matching results, but observe that the MDCA method is computationally less effective when applied to highly ordered phases such as Mn2GaC.

Secondly, Monte Carlo simulations are used to derive the magnetic energy, specific heat, and 𝑇𝑐 . For Mn2GaC, we find 𝑇𝑐 = 660 K. The uncertainty in the calculated 𝑇𝑐 caused by possible uncertainties in the 𝐽’s is discussed and exemplified in our case by an analysis of the impact of the statistical uncertainties of the MDCA-derived 𝐽’s, resulting in a 𝑇𝑐 distribution with a standard deviation of 133 K.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-124563 (URN)10.1103/PhysRevB.93.054432 (DOI)000371391800004 ()
Note

Funding agencies: European Research Council under the European Community Seventh Framework Program (FP7)/ERC Grant [258509]; Swedish Research Council (VR) [621-2011-4417, 330-2014-6336]; Knut and Alice Wallenberg (KAW) Fellowship program; SSF synergy grant FUNCASE

Available from: 2016-02-03 Created: 2016-02-03 Last updated: 2017-11-30Bibliographically approved
Dahlqvist, M., Ingason, A. S., Alling, B., Magnus, F., Thore, A., Petruhins, A., . . . Rosén, J. (2016). Magnetically driven anisotropic structural changes in the atomic laminate Mn2GaC. Physical Review B, 93(1), 014410
Open this publication in new window or tab >>Magnetically driven anisotropic structural changes in the atomic laminate Mn2GaC
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2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 1, p. 014410-Article in journal (Refereed) Published
Abstract [en]

Inherently layered magnetic materials, such as magnetic M(n+1)AX(n) (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition M(n+1)AX(n) consists of M(n+1)AX(n) blocks separated by atomically thin A-layers where M is a transition metal, A an A-group element, X refers to carbon and/or nitrogen, and n is typically 1, 2, or 3. Here, we show that the recently discovered magnetic Mn2GaC MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of Mn2GaC MAX phases in future magnetoelectric and magnetocaloric applications.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-124463 (URN)10.1103/PhysRevB.93.014410 (DOI)000367779000005 ()
Note

Funding Agencies|European Research Council under the European Communities Seventh Framework Programme (FP7)/ERC Grant [258509]; Swedish Research Council (VR) [642-2013-8020, 621-2011-4417]; KAW Fellowship program; SSF synergy grant FUNCASE; VR Grant [621-2011-4426]; Russian Federation Ministry for Science and Education [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program

Available from: 2016-02-02 Created: 2016-02-01 Last updated: 2017-11-30
Thore, A., Dahlqvist, M., Alling, B. & Rosén, J. (2016). Phase stability of the nanolaminates V2Ga2C and (Mo1-xVx)(2)Ga2C from first-principles calculations. Physical Chemistry, Chemical Physics - PCCP, 18(18), 12682-12688
Open this publication in new window or tab >>Phase stability of the nanolaminates V2Ga2C and (Mo1-xVx)(2)Ga2C from first-principles calculations
2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 18, p. 12682-12688Article in journal (Refereed) Published
Abstract [en]

We here use first-principles calculations to investigate the phase stability of the hypothetical laminated material V2Ga2C and the related alloy (Mo1-xVx)(2)Ga2C, the latter for a potential parent material for synthesis of (Mo1-xVx)(2)C, a new two-dimensional material in the family of so called MXenes. We predict that V2Ga2C is thermodynamically stable with respect to all identified competing phases in the ternary V-Ga-C phase diagram. We further calculate the stability of ordered and disordered configurations of Mo and V in (Mo1-xVx)(2)Ga2C and predict that ordered (Mo1-xVx)(2)Ga2C for x <= 0.25 is stable, with an order-disorder transition temperature of similar to 1000 K. Furthermore, (Mo1-xVx)(2)Ga2C for x = 0.5 and x >= 0.75 is suggested to be stable, but only for disordered Mo-V configurations, and only at elevated temperatures. We have also investigated the electronic and elastic properties of V2Ga2C; the calculated bulk, shear, and Youngs modulus are 141, 94, and 230 GPa, respectively.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-128935 (URN)10.1039/c6cp00802j (DOI)000375689200034 ()27094754 (PubMedID)
Note

Funding Agencies|European Research Council under the European Community [258509]; Swedish Research Council (VR) [621-2011-4417, 330-2014-6336]; Knut and Alice Wallenberg (KAW) Fellowship program; SSF synergy grant FUNCASE; Marie Sklodowska Curie Actions, Cofund [INCA 600398]

Available from: 2016-06-09 Created: 2016-06-07 Last updated: 2017-11-30
Thore, A., Dahlqvist, M., Alling, B. & Rosén, J. (2016). Phase stability of the nanonlaminates V2Ga2C and (Mo1-xVx)2Ga2C from first-principles calculations.
Open this publication in new window or tab >>Phase stability of the nanonlaminates V2Ga2C and (Mo1-xVx)2Ga2C from first-principles calculations
2016 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We here use first-principles calculations to investigate the phase stability of the hypothetical laminated materials V2Ga2C and the related alloy (Mo1-xVx)2Ga2C, the latter for a potential parent material for synthesis of (Mo1-xVx)2C, a new two-dimensional material in the family of so called MXenes. We predict that V2Ga2C is thermodynamically stable with respect to all identified competing phases in the ternary VGa-C phase diagram. We further predict the stability for ordered and disordered configurations of Mo and V in (Mo1-xVx)2Ga2C and predict that ordered (Mo1-xVx)2Ga2C for 𝑥 ≤ 0.25 is stable, with an orderdisorder transition temperature of ~1000 K. Furthermore, (Mo1-xVx)2Ga2C for 𝑥 = 0.5 and 𝑥 ≥ 0.75 is suggested to potentially be stable, but only for disordered Mo-V configurations, and only at elevated temperatures. We have also investigated the electronic and elastic properties of V2Ga2C; the calculated bulk, shear, and Young’s modulus are 141, 95, and 232 GPa, respectively.

National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-124561 (URN)
Available from: 2016-02-03 Created: 2016-02-03 Last updated: 2017-11-03Bibliographically approved
Dahlqvist, M., Alling, B. & Rosén, J. (2015). A critical evaluation of GGA plus U modeling for atomic, electronic and magnetic structure of Cr2AlC, Cr2GaC and Cr2GeC. Journal of Physics: Condensed Matter, 27(9), 095601
Open this publication in new window or tab >>A critical evaluation of GGA plus U modeling for atomic, electronic and magnetic structure of Cr2AlC, Cr2GaC and Cr2GeC
2015 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 9, p. 095601-Article in journal (Refereed) Published
Abstract [en]

In this work we critically evaluate methods for treating electron correlation effects in multicomponent carbides using a GGA + U framework, addressing doubts from previous works on the usability of density functional theory in the design of magnetic MAX phases. We have studied the influence of the Hubbard U-parameter, applied to Cr 3d orbitals, on the calculated lattice parameters, magnetic moments, magnetic order, bulk modulus and electronic density of states of Cr2AlC, Cr2GaC and Cr2GeC. By considering non-, ferro-, and five different antiferromagnetic spin configurations, we show the importance of including a broad range of magnetic orders in the search for MAX phases with finite magnetic moments in the ground state. We show that when electron correlation is treated on the level of the generalized gradient approximation (U = 0 eV), the magnetic ground state of Cr(2)AC (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. By comparing GGA and GGA + U results with experimental data we find that using a U-value larger than 1 eV results in structural parameters deviating strongly from experimentally observed values. Comparisons are also done with hybrid functional calculations (HSE06) resulting in an exchange splitting larger than what is obtained for a U-value of 2 eV. Our results suggest caution and that investigations need to involve several different magnetic orders before lack of magnetism in calculations are blamed on the exchange-correlation approximations in this class of magnetic MAX phases.

Place, publisher, year, edition, pages
IOP Publishing: Hybrid Open Access, 2015
Keywords
MAX phase; electron correlation; magnetic; Cr2AlC; Cr2GaC; Cr2GeC; GGA plus U
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-116505 (URN)10.1088/0953-8984/27/9/095601 (DOI)000349895500007 ()25671459 (PubMedID)
Note

Funding Agencies|European Research Council [258509]; Swedish Research Council (VR) [642-2013-8020, 621-2011-4417]; KAW Fellowship program

Available from: 2015-03-27 Created: 2015-03-27 Last updated: 2017-12-04
Anasori, B., Dahlqvist, M., Halim, J., Ju Moon, E., Lu, J., Hosler, B. C., . . . Barsoum, M. W. (2015). Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3. Journal of Applied Physics, 118(9), 094304
Open this publication in new window or tab >>Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3
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2015 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 9, p. 094304-Article in journal (Refereed) Published
Abstract [en]

Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases-Mo2TiAlC2 and Mo2Ti2AlC3-synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m) Ti:1.1Al:2C with 1.5 less than= m less than= 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 degrees C for 4 h under Ar flow. In general, for m greater than= 2 an ordered 312 phase, (Mo2Ti) AlC2, was the majority phase; for mless than 2, an ordered 413 phase (Mo2Ti2)AlC3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo2TiAlC1.7 and Mo2Ti1.9Al0.9C2.5, respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo2TiAlC2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo2Ti2AlC3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the Mn+1Xn blocks. At 331 GPa and 367 GPa, respectively, the Youngs moduli of the ordered Mo2TiAlC2 and Mo2Ti2AlC3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10K for both phases showed metallic behavior. (C) 2015 AIP Publishing LLC.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-121746 (URN)10.1063/1.4929640 (DOI)000360926500020 ()
Note

Funding Agencies|U.S. Army Research Office [W911NF-12-1-0132, W911NF-11-1-0283]; Swedish Research Council [621-2011-4420, 642-2013-8020, 621-2014-4890]; Swedish Foundation for Strategic Research through the Synergy Grant FUNCASE Functional Carbides for Advanced Surface Engineering; Future Research Leaders 5 Program; ERC [258509]; Knut and Alice Wallenberg Foundation

Available from: 2015-10-07 Created: 2015-10-05 Last updated: 2017-12-01
Dahlqvist, M., Jansson, U. & Rosén, J. (2015). Influence of boron vacancies on phase stability, bonding and structure of MB2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) with AlB2 type structure. Journal of Physics: Condensed Matter, 27(43), 435702
Open this publication in new window or tab >>Influence of boron vacancies on phase stability, bonding and structure of MB2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W) with AlB2 type structure
2015 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 27, no 43, p. 435702-Article in journal (Refereed) Published
Abstract [en]

Transition metal diborides in hexagonal AlB2 type structure typically form stable MB2 phases for group IV elements (M = Ti, Zr, Hf). For group V (M = V, Nb, Ta) and group VI (M = Cr, Mo, W) the stability is reduced and an alternative hexagonal rhombohedral MB2 structure becomes more stable. In this work we investigate the effect of vacancies on the B-site in hexagonal MB2 and its influence on the phase stability and the structure for TiB2, ZrB2, HfB2, VB2, NbB2, TaB2, CrB2, MoB2, and WB2 using first-principles calculations. Selected phases are also analyzed with respect to electronic and bonding properties. We identify trends showing that MB2 with M from group V and IV are stabilized when introducing B-vacancies, consistent with a decrease in the number of states at the Fermi level and by strengthening of the B-M interaction. The stabilization upon vacancy formation also increases when going from M in period 4 to period 6. For TiB2, ZrB2, and HfB2, introduction of B-vacancies have a destabilizing effect due to occupation of B-B antibonding orbitals close to the Fermi level and an increase in states at the Fermi level.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2015
Keywords
boride; transition metal; chemical bonding; electronic structure; TiB2; NbB2; MoB2
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-122410 (URN)10.1088/0953-8984/27/43/435702 (DOI)000362574000015 ()26445165 (PubMedID)
Note

Funding Agencies|European Research Council under the European Community Seventh Framework Program (FP7)/ERC [258509]; KAW Fellowship program; Swedish Research Council (VR) [642-2013-8020, 621-2012-4425, 2014-5841]; Swedish Foundation of Strategic Research (SSF) Synergy Grant FUNCASE

Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2017-12-01
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ORCID iD: ORCID iD iconorcid.org/0000-0001-5036-2833

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