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Phase stability and physical properties of nanolaminated materials from first principles
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The MAX phase family is a set of nanolaminated, hexagonal materials typically comprised of three elements: a transition metal (M), an A-group element (A), and carbon and/or nitrogen (X). In this thesis, first-principles based methods have been used to investigate the phase stability and physical properties of a number of MAX and MAX-like phases.

Most theoretical work on MAX phase stability use the constraint of 0 K conditions, due to the very high computational cost of including temperature dependent effects such as lattice vibrations and electronic excitations for all relevant competing phases in the ternary or multinary chemical space. Despite this, previous predictions of the existence of new MAX phases have to a large extent been experimentally verified. In an attempt to provide a possible explanation for this consistency, and thus help strengthen the confidence in future predictions, we have calculated the temperature dependent phase stability of Tin+1AlCn, to date the most studied MAX phases. We show that both the electronic and vibrational contribution to the Gibbs free energies of the MAX phases are  cancelled by the corresponding contributions to the Gibbs free energies of the competing phases. We further show that this is the case even when thermal expansion is considered.

We have also investigated the stability of two hypothetical MAX-like phases, V2Ga2C and (Mo1-xVx)2Ga2C, motivated by a search for ways to attain new two-dimensional MAX phase derivatives, so-called MXenes. We predict that it is possible to synthesize both phases. For x≤0.25, stability of (Mo1-xVx)2Ga2C is indicated for both ordered and disordered solid solutions on the M sublattice. For x=0.5 and x≥0.75, stability is only indicated for disordered solutions. The ordered solutions are stable at temperatures below 1000 K, whereas stabilization of the disordered solutions requires temperatures of up to 2100 K, depending on the V concentration.

Finally, we have investigated the electronic, vibrational, and magnetic properties of the recently synthesized MAX phase Mn2GaC. We show that the electronic band structure is anisotropic, and determine the bulk, shear, and Young’s modulus to be 157, 93, and 233 GPa, respectively, and Poisson’s ratio to be 0.25. We further predict the magnetic critical order-disorder temperature of Mn2GaC to be 660 K. We base the predictions on Monte Carlo simulations of a bilinear Heisenberg Hamiltonian constructed from magnetic exchange interaction parameters derived using two different supercell methods: the novel magnetic direct cluster averaging method (MDCA), and the Connolly-Williams method (CW). We conclude that CW is less computationally expensive than MDCA for chemically and topologically ordered phases such as Mn2GaC.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. , 57 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1742
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-124564DOI: 10.3384/diss.diva-124564ISBN: 978-91-7685-835-6 (print)OAI: oai:DiVA.org:liu-124564DiVA: diva2:900124
Public defence
2016-03-04, Planck, Fysikhuset, Campus Valla, Linköping, 09:30 (English)
Opponent
Supervisors
Available from: 2016-02-03 Created: 2016-02-03 Last updated: 2017-11-03Bibliographically approved
List of papers
1. Temperature dependent phase stability of nanolaminated ternaries from first-principles calculations
Open this publication in new window or tab >>Temperature dependent phase stability of nanolaminated ternaries from first-principles calculations
2014 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 91, 251-257 p.Article in journal (Refereed) Published
Abstract [en]

Methods based on first-principles calculations have proven effective for predicting the thermodynamic stability of materials that have not previously been considered. However, the vast majority of these predictions are based on 0 K calculations, which means that little is known about the effects of temperature on their accuracy. This causes considerable uncertainty with respect to stability predictions of new hypothetical phases. In this work we combine first-principles calculations with an optimization procedure to calculate the phase stability as a function of temperature for Ti2AlC, Ti3AlC2 and Ti4AlC3 MAX phases with respect to their most competing phases in the Ti-Al-C phase diagram, in a temperature interval from 0 to 2000 K. To model nonzero temperatures, we include effects from the electronic and vibrational free energies to the Gibbs free energy for all relevant competing phases. We show that, due to a mutual cancellation of the temperature dependent energy terms, the results of neither the harmonic nor the quasiharmonic calculations differ significantly from the calculated 0 K formation energies. We thus provide a plausible explanation for the success of previous 0 K predictions, an explanation which also serves as evidence for the hypothesis that the phase stability in many materials systems is primarily governed by the 0 K energy terms.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
First-principles; Phase stability; Ternary carbides; Harmonic approximation; Quasiharmonic approximation; Density functional theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-109360 (URN)10.1016/j.commatsci.2014.04.055 (DOI)000339129100032 ()
Available from: 2014-08-15 Created: 2014-08-15 Last updated: 2017-12-05Bibliographically approved
2. 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
3. First-principles calculations of the electronic, vibrational, and elastic properties of the magnetic laminate Mn2GaC
Open this publication in new window or tab >>First-principles calculations of the electronic, vibrational, and elastic properties of the magnetic laminate Mn2GaC
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 116, no 10, 103511- p.Article in journal (Refereed) Published
Abstract [en]

In this paper, we report the by first-principles predicted properties of the recently discovered magnetic MAX phase Mn2GaC. The electronic band structure and vibrational dispersion relation, as well as the electronic and vibrational density of states, have been calculated. The band structure close to the Fermi level indicates anisotropy with respect to electrical conductivity, while the distribution of the electronic and vibrational states for both Mn and Ga depend on the chosen relative orientation of the Mn spins across the Ga sheets in the Mn–Ga–Mn trilayers. In addition, the elastic properties have been calculated, and from the five elastic constants, the Voigt bulk modulus is determined to be 157 GPa, the Voigt shear modulus 93 GPa, and the Young's modulus 233 GPa. Furthermore, Mn2GaC is found relatively elastically isotropic, with a compression anisotropy factor of 0.97, and shear anisotropy factors of 0.9 and 1, respectively. The Poisson's ratio is 0.25. Evaluated elastic properties are compared to theoretical and experimental results for M 2 AC phases where M = Ti, V, Cr, Zr, Nb, Ta, and A = Al, S, Ge, In, Sn

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-111954 (URN)10.1063/1.4894411 (DOI)000342833700023 ()
Available from: 2014-11-11 Created: 2014-11-11 Last updated: 2017-12-05Bibliographically approved
4. Magnetic exchange interactions and critical temperature of the nanolaminate Mn2GaC from first-principles supercell methods
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

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