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Thermodynamic properties of paramagnetic α- and β−Mn from first principles: The effect of transverse spin fluctuations
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology. (Materials Technology, Unit of Properties)ORCID iD: 0000-0001-5518-3308
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.ORCID iD: 0000-0002-9920-5393
2017 (English)In: Physical Review Materials, Vol. 1, no 073803Article in journal (Refereed) Published
Abstract [en]

First-principles-based thermodynamic modeling of cubic alpha and beta phases of Mn represent a challenge due to their structural complexity and the necessity of simultaneous treatment of several types of disorder (electronic, magnetic, and vibrational) that have very different characteristic time scales. Here we employ mean-field theoretical models to describe the different types of disorder and then we connect each layer of theory to the others using the adiabatic principle of separating faster and slower degrees of freedom. The slowest (vibrational) degrees of freedom are treated using the Moruzzi, Janak, and Schwarz formalism [Phys. Rev. B 37, 790 (1988)] of the Debye-Gruneisen model parametrized based on the first-principles calculated equation of state which includes the free-energy contributions due to the fast (electronic and magnetic) degrees of freedom via the Fermi-Dirac distribution function and a mean-field theory of transverse spin fluctuations. The magnetic contribution due to transverse spin fluctuations has been computed self-consistently within the disordered local moment picture of the paramagnetic state. The obtained results for thermodynamic properties such as lattice parameter, linear thermal expansion coefficient, and heat capacity of both phases show a good agreement with available experimental data. We also tested the assumption about the nature (localized versus delocalized) of magnetic moment on site IV in alpha-Mn and site I in beta-Mn on the thermodynamic properties of these two phases. Similar to the findings of experimental studies, we conclude that magnetic moment on site IV in alpha-Mn is not of a localized character. However, a similar analysis suggests that the magnetic moment of site I in beta-Mn should be treated as localized.

Place, publisher, year, edition, pages
2017. Vol. 1, no 073803
National Category
Condensed Matter Physics
Research subject
Physics; Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-223663DOI: 10.1103/PhysRevMaterials.1.073803ISI: 000419032600001OAI: oai:DiVA.org:kth-223663DiVA, id: diva2:1186234
Funder
VINNOVA, 2012-02892
Note

QC 20180228

Available from: 2018-02-27 Created: 2018-02-27 Last updated: 2018-02-28Bibliographically approved
In thesis
1. Finite temperature properties of elements and alloy phases from first principles
Open this publication in new window or tab >>Finite temperature properties of elements and alloy phases from first principles
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

First principles calculations are usually concerned with properties calculated at temperature 0 K. However, the industrially important materials are functioning at finite temperatures. To fill such a gap a first-principles based modeling of free energy has been developed in this thesis and finite temperature properties of different phases of Fe and Mn have been calculated and contrasted with available experimental data.

In particular, using partitioning of the Helmholtz free energy, thermophysical properties of paramagnetic Fe have been reported. The heat capacity, lattice constant, thermal expansion and elastic moduli of γ- and δ-Fe show a good agreement with available experimental data. In the case of α-Fe, we observe a good agreement for elastic moduli and thermal expansion with experiments but the heat capacity is not well-reproduced in the calculations because of the large contribution of magnetic short-range which our models are not capable of capturing.

α- and β-Mn theoretically pose a challenge for direct simulations of thermodynamic properties because of the complexity of magnetic and crystal structure. The partitioning of free energy has been used and thermodynamics of these phases have been derived. The obtained results show a good agreement with experimental data suggesting that, despite the complexities of these phases, a rather simple approach can well describe their finite temperature properties. High temperature phases of Mn, γ and δ, are also theoretically challenging problems. Employing a similar approach to Fe, thermophysical properties of these high symmetry phases of Mn have been reported which also show good agreement with available experimental data.

The point defect and metal-self diffusion in titanium carbide (TiC), a refractory material, have been investigated in the present work. The common picture of metal-vacancy exchange mechanism for metal self-diffusion was shown to be unable to explain the experimentally observed values of activation energy. Several new clusters of point defects such as vacancies and interstitials have been found and reported which are energetically lower that a single metal vacancy. In a subsequent study, we showed that some of these clusters can be considered as mediators of metal self-diffusion in TiC.

Evaluation of structural properties of Ti(O,C), a solid solution of TiC and β-TiO, from supercell approach is an extremely difficult task. For a dilute concentration of O, we show the complexity of describing an impurity of O in TiC using supercell approach. A single-site method such as the exact muffin-tin orbital method in the coherent potential approximation (EMTO-CPA) is a good alternative to supercell modeling of Ti(O,C). However, a study of Ti(O,C) using EMTO-CPA requires a further development of the technique regarding the partitioning of space. The shape module of EMTO has been modified for this purpose. With the help of the modified module, Ti(O,C) have been studied using EMTO-CPA. The results for the divacancy concentration and corresponding lattice parameter variations show good agreement with experimental data.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 78
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-223668 (URN)978-91-7729-687-4 (ISBN)
Public defence
2018-03-26, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
VINNOVA
Note

QC 20180228

Available from: 2018-02-28 Created: 2018-02-27 Last updated: 2018-03-08Bibliographically approved

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Korzhavyi, Pavel A.

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