liu.seSearch for publications in DiVA
Change search
Link to record
Permanent link

Direct link
BETA
Hellman, Olle
Publications (10 of 28) Show all publications
Xu, B., Hellman, O. & Bellaiche, L. (2019). Order-disorder transition in the prototypical antiferroelectric PbZrO3. Physical Review B, 100(2), Article ID 020102.
Open this publication in new window or tab >>Order-disorder transition in the prototypical antiferroelectric PbZrO3
2019 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 100, no 2, article id 020102Article in journal (Refereed) Published
Abstract [en]

The prototypical antiferroelectric PbZrO3 has several unsettled questions, such as the nature of the antiferroelectric transition, a possible intermediate phase, and the microscopic origin of the Pbam ground state. Using first-principles calculations, we show that no phonon becomes truly soft at the cubic-to-Pbam transition temperature, and the order-disorder character of this transition is clearly demonstrated based on molecular dynamics simulations and potential energy surfaces. The out-of-phase octahedral tilting is an important degree of freedom, which can collaborate with other phonon distortions and form a complex energy landscape with multiple minima Candidates of the possible intermediate phase are suggested based on the calculated kinetic barriers between energy minima, and the development of a first-principles-based effective Hamiltonian. The use of this latter scheme further reveals that specific bilinear interactions between local dipoles and octahedral tiltings play a major role in the formation of the Pbam ground state, which contrasts with most of the previous explanations.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:liu:diva-159247 (URN)10.1103/PhysRevB.100.020102 (DOI)000476685600001 ()
Note

Funding Agencies|ONR [N00014-17-1-2818]; DARPA under the MATRIX program [HR0011-15-2-0038]; Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions; Arkansas High Performance Computer Center at the University of Arkansas; Cloud Pilot project

Available from: 2019-08-07 Created: 2019-08-07 Last updated: 2019-08-07
Sangiovanni, D., Hellman, O., Alling, B. & Abrikosov, I. (2016). Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics. PHYSICAL REVIEW B, 93(9), 094305
Open this publication in new window or tab >>Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics
2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 9, p. 094305-Article in journal (Refereed) Published
Abstract [en]

We revisit the color-diffusion algorithm [Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in non equilibrium ab initio molecular dynamics (NE-AIMD) and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored atom) is accelerated toward the neighboring defect site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k(NE) increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k(NE)(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures and reaches a factor of 4 orders of magnitude at the lowest temperature considered in the present study.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-127268 (URN)10.1103/PhysRevB.93.094305 (DOI)000372711200003 ()
Note

Funding Agencies|Knut and Alice Wallenberg Foundation [2011.0094]; Swedish Research Council (VR) [621-2011-4417, 2015-04391, 637-2013-7296, 330-2014-336]; Linkoping Linnaeus Initiative LiLi-NFM [2008-6572]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials [MatLiU 2009-00971]; Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; Tomsk State University Academic D. I. Mendeleev Fund Program

Available from: 2016-04-20 Created: 2016-04-19 Last updated: 2019-06-28
Shulumba, N., Raza, Z., Hellman, O., Janzén, E., Abrikosov, I. & Odén, M. (2016). Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN. Physical Review B, 94(10), Article ID 104305.
Open this publication in new window or tab >>Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN
Show others...
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 10, article id 104305Article in journal (Refereed) Published
Abstract [en]

Wurtzite aluminium nitride is a technologically important wide band gap semiconductor with an unusually high thermal conductivity, used in optical applications and as a heatsink substrate. Many of its properties depend on an accurate description of its lattice dynamics, which have thus far only been captured in the quasiharmonic approximation. In this work, we demonstrate that anharmonicity has a considerable impact on its phase stability and transport properties, since anharmonicity is much stronger in the rocksalt phase. We compute a pressure-temperature phase diagram of AlN, demonstrating that the rocksalt phase is stabilised by increasing temperature, with respect to the wurtzite phase. We demonstrate that including anharmonicity, we can recover the thermal conductivity of the wurtzite phase (320 Wm−1K−1 under ambient conditions), and compute the hitherto unknown thermal conductivity of the rocksalt phase (96 Wm−1K−1). We also show that the electronic band gap decreases with temperature. These findings provide further evidence that anharmonic effects cannot be ignored in high temperature applications.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-122955 (URN)10.1103/PhysRevB.94.104305 (DOI)000384061100003 ()
Note

Funding agencies; Swedish Research Council (VR programs) [2015-04391, 621-2012-4401]; Swedish Foundation for Strategic Research (SSF program) [SRL10-0026]; VINNOVA [M-Era.net Project] [2013-02355 (MC2)]; Swedish Research Council VR program [637-2013-7296]; Swedish Foundati

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2017-12-01Bibliographically approved
Nilsson, J. O., Yu Vekilova, O., Hellman, O., Klarbring, J., Simak, S. & Skorodumova, N. V. (2016). Ionic conductivity in Gd-doped CeO2: Ab initio color-diffusion nonequilibrium molecular dynamics study. Physical Review B, 93(2), 024102
Open this publication in new window or tab >>Ionic conductivity in Gd-doped CeO2: Ab initio color-diffusion nonequilibrium molecular dynamics study
Show others...
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 2, p. 024102-Article in journal (Refereed) Published
Abstract [en]

A first-principles nonequilibrium molecular dynamics (NEMD) study employing the color-diffusion algorithm has been conducted to obtain the bulk ionic conductivity and the diffusion constant of gadolinium-doped cerium oxide (GDC) in the 850-1150 K temperature range. Being a slow process, ionic diffusion in solids usually requires simulation times that are prohibitively long for ab initio equilibrium molecular dynamics. The use of the color-diffusion algorithm allowed us to substantially speed up the oxygen-ion diffusion. The key parameters of the method, such as field direction and strength as well as color-charge distribution, have been investigated and their optimized values for the considered system have been determined. The calculated ionic conductivity and diffusion constants are in good agreement with available experimental data.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-124465 (URN)10.1103/PhysRevB.93.024102 (DOI)000367662100002 ()
Note

Funding Agencies|Swedish Energy Agency (STEM) [355151]; Carl Tryggers Foundation [CTS 14:433]; Swedish Research Council (VR) [2014-5993, 2014-4750, 637-2013-7296]; LiLi-NFM; Swedish Government Strategic Research Area Grant in Materials Science

Available from: 2016-02-02 Created: 2016-02-01 Last updated: 2017-11-30
Shulumba, N., Hellman, O., Raza, Z., Alling, B., Barrirero, J., Mücklich, F., . . . Odén, M. (2015). Anharmonicity changes the solid solubility of an alloy at high temperatures.
Open this publication in new window or tab >>Anharmonicity changes the solid solubility of an alloy at high temperatures
Show others...
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti1−xAlxN alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy, corresponding to the true equilibrium state of the system. We demonstrate that the anharmonic contribution and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti1−xAlxN alloy, lowering the maximum temperature for the miscibility gap from 6560 K to 2860 K. Our local chemical composition measurements on thermally aged Ti0.5Al0.5N alloys agree with the calculated phase diagram.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-122956 (URN)
Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2015-11-30Bibliographically approved
Mei, A. B., Hellman, O., Wireklint, N., Schlepuetz, C. M., Sangiovanni, D., Alling, B., . . . Greene, J. E. (2015). Dynamic and structural stability of cubic vanadium nitride. Physical Review B. Condensed Matter and Materials Physics, 91(5), 054101
Open this publication in new window or tab >>Dynamic and structural stability of cubic vanadium nitride
Show others...
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 5, p. 054101-Article in journal (Refereed) Published
Abstract [en]

Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-114566 (URN)10.1103/PhysRevB.91.054101 (DOI)000348872600001 ()
Note

Funding Agencies|Swedish Research Council (VR) program [637-2013-7296, 2014-5790, 2009-00971, 2013-4018]; Swedish Government Strategic Research Area Grant in Materials Science [SFO Mat-LiU 2009-00971]; DOE Office of Science by Argonne National Laboratory [DE-AC02-06CH11357]

Available from: 2015-03-02 Created: 2015-02-26 Last updated: 2019-06-28
Isaeva, L., Hellman, O., Lashley, J. C., Abrikosov, I. & Eriksson, O. (2015). Dynamic stabilization of cubic AuZn. In: MATERIALS TODAY-PROCEEDINGS: . Paper presented at 14th International Conference on Martensitic Transformations (ICOMAT) (pp. 569-572). ELSEVIER SCIENCE BV, 2
Open this publication in new window or tab >>Dynamic stabilization of cubic AuZn
Show others...
2015 (English)In: MATERIALS TODAY-PROCEEDINGS, ELSEVIER SCIENCE BV , 2015, Vol. 2, p. 569-572Conference paper, Published paper (Refereed)
Abstract [en]

A recently developed temperature-dependent effective potential method is employed to study the martensitic phase transformation in AuZn. This method is based on ab initio molecular dynamics and allows to obtain finite-temperature lattice vibrational properties. We show that the transversal acoustic TA(2)[110] mode associated with the phase transition is stabilized at 300 K. Temperature evolution of single-phonon dynamic structure factor at the wave vector q= 1/3[1,1,0], associated with phonon softening and Fermi surface nesting, was also studied. (C) 2015 The Authors. Published by Elsevier Ltd.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2015
Keywords
Lattice dynamics; anharmonicity; shape-memory alloys
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-126282 (URN)10.1016/j.matpr.2015.07.349 (DOI)000371032100019 ()
Conference
14th International Conference on Martensitic Transformations (ICOMAT)
Available from: 2016-03-21 Created: 2016-03-21 Last updated: 2016-04-05
Shulumba, N., Hellman, O., Raza, Z., Abrikosov, I. & Odén, M. (2015). Influence of vibrational free energy on the phase stability of alloys from first principles.
Open this publication in new window or tab >>Influence of vibrational free energy on the phase stability of alloys from first principles
Show others...
2015 (English)Manuscript (preprint) (Other academic)
Abstract [en]

We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and pressure for substitutional alloys from first principles. Taking the example of the technologically important hard coating alloy Ti1-xAlxN as an example, we investigate the effect on the vibrational free energy of substituting Ti for other group IV elements. By constructing the phase diagrams for these three alloys, we show why Zr1-xAlxN and Hf1-xAlxN are so difficult to experimentally synthesise in a metastable solid solution: both have solubility regions that span only a small low-AlN concentration range at temperatures above 1500 K. Moreover, Hf1-xAlxN is dynamically unstable at low temperatures and across most of the concentration range. We also show the chemical and thermal expansion effects dominate mass disorder in the Gibbs free energy of mixing.

National Category
Condensed Matter Physics Physical Sciences
Identifiers
urn:nbn:se:liu:diva-122958 (URN)
Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2015-11-30Bibliographically approved
Shulumba, N., Hellman, O., Rogström, L., Raza, Z., Tasnádi, F., Abrikosov, I. & Odén, M. (2015). Temperature-dependent elastic properties of Ti1−xAlxN alloys. Applied Physics Letters, 107(23)
Open this publication in new window or tab >>Temperature-dependent elastic properties of Ti1−xAlxN alloys
Show others...
2015 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 107, no 23Article in journal (Refereed) Published
Abstract [en]

Ti1−xAlxN is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the newly developed symmetry imposed force constant temperature dependent effective potential method, that include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C11 decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy.

National Category
Condensed Matter Physics Physical Sciences
Identifiers
urn:nbn:se:liu:diva-122957 (URN)10.1063/1.4936896 (DOI)000367010800017 ()
Note

Funding agencies: Swedish Research Council (VR) [621-2011-4426, 621-2012-4401, 637-2013-7296]; Swedish Foundation for Strategic Research (SSF) [RMA08-0069, SRL10-0026]; VINNOVA [2013-02355(MC2)]; Erasmus Mundus Joint European Doctoral Program DocMASE; Ministry of Education

Vid tidpunkten för disputation förelåg publikation som manuskript

Available from: 2015-11-30 Created: 2015-11-30 Last updated: 2017-12-01Bibliographically approved
Abrikosov, I., Ponomareva, A. V., Nikonov, A. Y., Zharmukhambetova, A. M., Mosyagin, I., Lugovskoy, A. V., . . . Barannikova, S. A. (2015). Theoretical description of pressure-induced phase transitions: a case study of Ti-V alloys. High Pressure Research, 35(1), 42-48
Open this publication in new window or tab >>Theoretical description of pressure-induced phase transitions: a case study of Ti-V alloys
Show others...
2015 (English)In: High Pressure Research, ISSN 0895-7959, E-ISSN 1477-2299, Vol. 35, no 1, p. 42-48Article in journal (Refereed) Published
Abstract [en]

We discuss theoretical description of pressure-induced phase transitions by means of first-principles calculations in the framework of density functional theory. We illustrate applications of theoretical tools that allow one to take into account configurational and vibrational disorders, considering Ti-V alloys as a model system. The universality of the first-principles theory allows us to apply it in studies of different phenomena that occur in the Ti-V system upon compression. Besides the transitions between different crystal structures, we discuss isostructural transitions in bcc Ti-V alloys. Moreover, we present arguments for possible electronic transitions in this system, which may explain peculiar behaviour of elastic properties of V upon compression.

Place, publisher, year, edition, pages
Taylor andamp; Francis: STM, Behavioural Science and Public Health Titles, 2015
Keywords
phase stability; first-principles calculations; Ti-V alloys; high pressure
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-114586 (URN)10.1080/08957959.2014.992896 (DOI)000348672100007 ()
Note

Funding Agencies|Ministry of Education and Science of the Russian Federation [14.Y26.31.0005]; SSF [SRL 10-0026]; Russian Foundation for Basic Researches [13-02-00606a]; Tomsk State University Academic D.I. Mendeleev Fund Program; Program of Fundamental Research of State Academies of Sciences

Available from: 2015-02-27 Created: 2015-02-26 Last updated: 2017-12-04
Organisations

Search in DiVA

Show all publications