S. Cottenier

Crystal structure prediction for iron as inner core material in heavy terrestrial planets

S. Cottenier, M.I.J. Probert, T. Van Hoolst, V. Van Speybroeck, M. Waroquier
Earth and Planetary Science Letters
312, 237–242
2011
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Abstract 

The relative stability of different crystal structures for pure Fe under applied pressure is calculated from quantum mechanics, using the highly accurate APW+lo method. In the pressure range of 0–100 TPa, we corroborate the prediction that iron adopts subsequently the bcc, hcp, fcc, hcp and bcc structures. In contrast to previous studies, we identify a family of stacking fault structures that are competing with the ground state structure at all pressures. Implications for the properties of the inner core of the Earth and heavy terrestrial exoplanets are discussed.

Spin-density wave in Cr: Nesting versus low-lying thermal excitations

V. Vanhoof, M. Rots, S. Cottenier
Physical Review B
80 (18), 184420
2009
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Abstract 

It is well known that present versions of density functional theory do not predict the experimentally observed spin-density wave state to be the ground state of Cr. Recently, a so-called “nodon model” has been proposed as an alternative way to reconcile theory and experiment: the ground state of Cr is truly antiferromagnetic, and the spin-density wave appears due to low-lying thermal excitations (“nodons”). We examine in this paper whether the postulated properties of these nodons are reproduced by ab initio calculations.

Open Access version available at UGent repository

The magnetization of γ′-Fe4N: theory vs. experiment

E.L. Peltzery y Blanca, J. Desimoni, N.E. Christensen, H. Emmerich, S. Cottenier
Physica Status Solidi (b) - Basic Solid State Physics
246 (5), 909-928
2009
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Abstract 

By reviewing the experimental and theoretical literature on γ′-Fe4N, and by a systematic survey of predictions by the LDA, PBE, WC, LDA + U (2×), PBE + U (2×) and B3PW91 exchange-correlation functionals, the structural, magnetic and hyperfine properties of this material as well as their pressure dependencies are interpreted. The hypothesis is put forward that γ′-Fe4N as found in Nature is exactly at a steep transition between low-spin and high-spin behaviour. PBE + U (U = 0.4 eV) is identified as the most accurate exchange-correlation functional for this material, although it is needed to fix the magnetization at the experimental value to obtain a satisfying description. Remaining disagreement between theory and experiment is pointed out. A recent experimental claim for a giant magnetic moment in γ′-Fe4N is discussed, and is not reproduced by our calculations. We expect that the new insight obtained in the present work can lead to a consistent ab initio modeling of other materials in the iron-nitrogen binary system. In an accompanying didactic section, the physics behind some common exchange-correlation functionals is outlined. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transition Metal Impurities on the Bond-Centered Site in Germanium

S. Decoster, S. Cottenier, B. De Vries, H. Emmerich, U. Wahl, J.G. Correia, A. Vantomme
Physical Review Letters
102 (6), 065502
2009
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Abstract 

We report on the lattice location of ion implanted Fe, Cu, and Ag impurities in germanium from a combined approach of emission channeling experiments and ab initio total energy calculations. Following common expectation, a fraction of these transition metals (TMs) was found on the substitutional Ge position. Less expected is the observation of a second fraction on the sixfold coordinated bond-centered site. Ab initio calculated heats of formation suggest this is the result of the trapping of a vacancy by a substitutional TM impurity, spontaneously forming an impurity-vacancy complex in the split-vacancy configuration. We also present an approach to displace the TM impurities from the electrically active substitutional site to the bond-centered site.

Lattice location study of ion implanted Sn and Sn-related defects in Ge

S. Decoster, S. Cottenier, U. Wahl, J.G. Correia, A. Vantomme
Physical Review B
81, 155204
2010
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Abstract 

In this work, we present a lattice location study of Sn in Ge. From emission channeling experiments, we determined the exact lattice location of ion implanted 121Sn atoms and compared the results to predictions from density-functional calculations. The majority of the Sn atoms are positioned on the substitutional site, as can be expected for an isovalent impurity, while a second significant fraction occupies the sixfold coordinated bond-centered site, which is stable up to at least 400 °C. Corroborated by ab initio calculations, we attribute this fraction of bond-centered Sn atoms to the Sn-vacancy defect complex in the split-vacancy configuration. Furthermore, we are able to assign specific defect complex geometries to resonances from earlier Mössbauer spectroscopy studies of Sn in Ge.

Electron penetration into the nucleus and its effect on the quadrupole interaction

K. Koch, K. Koepernik, D. Van Neck, H. Rosner, S. Cottenier
Physical Review A
81, 032507
2010
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Abstract 

series expansion of the interaction between a nucleus and its surrounding electron distribution provides terms that are well-known in the study of hyperfine interactions: the familiar quadrupole interaction and the less familiar hexadecapole interaction. If the penetration of electrons into the nucleus is taken into account, various corrections to these multipole interactions appear. The best known correction is a scalar term related to the isotope shift and the isomer shift. This paper discusses a related tensor correction, which modifies the quadrupole interaction if electrons penetrate the nucleus: the quadrupole shift. We describe the mathematical formalism and provide first-principles calculations of the quadrupole shift for a large set of solids. Fully relativistic calculations that explicitly take a finite nucleus into account turn out to be mandatory. Our analysis shows that the quadrupole shift becomes appreciably large for heavy elements. Implications for experimental high-precision studies of quadrupole interactions and quadrupole moment ratios are discussed. A literature review of other small quadrupole-like effects is presented as well (pseudoquadrupole effect, isotopologue anomaly, etc.).

Open Access version available at UGent repository

Dynamic lattice distortions in Sr2RuO4: microscopic studies by perturbed angular correlation spectroscopy and ab initio calculations

S.N. Mishra, M. Rots, S. Cottenier
Journal of Physics: Condensed Matter
22, 385602
2010
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Abstract 

Applying time differential perturbed angular correlation (TDPAC) spectroscopy and ab initio calculations, we have investigated possible lattice instabilities in Sr2RuO4 by studying the electric quadrupole interaction of a 111Cd probe at the Ru site. We find evidence for a dynamic lattice distortion, revealed from the observations of: (i) a rapidly fluctuating electric-field gradient (EFG) tensor showing non-Arrhenius relaxation, (ii) an anomalous temperature dependence of the quadrupole interaction frequency, and (iii) a monotonic increase of the EFG asymmetry (η) below 300 K. We argue that the observed dynamic lattice distortion is caused by strong spin fluctuations associated with the inherent magnetic instability in Sr2RuO4.

Diluted manganese on the bond-centered site in germanium

S. Decoster, S. Cottenier, U. Wahl, J.G. Correia, L.M.C. Pereira, C. Lacasta, M.R. Da Silva, A. Vantomme
Applied Physics Letters
97, 151914
2010
A1

Abstract 

The functional properties of Mn-doped Ge depend to large extent on the lattice location of the Mn impurities. Here, we present a lattice location study of implanted diluted Mn by means of electron emission channeling. Surprisingly, in addition to the expected substitutional lattice position, a large fraction of the Mn impurities occupies the bond-centered site. Corroborated by ab initio calculations, the bond-centered Mn is related to Mn-vacancy complexes. These unexpected results call for a reassessment of the theoretical studies on the electrical and magnetic behavior of Mn-doped Ge, hereby including the possible role of Mn-vacancy complexes.

Open Access version available at UGent repository

Assessment of a low-cost protocol for an ab initio based prediction of the mixing enthalpy at elevated temperatures: The Fe-Mo system

K. Lejaeghere, S. Cottenier, S. Claessens, M. Waroquier, V. Van Speybroeck
Physical Review B
83, 184201
2011
A1

Abstract 

We demonstrate how a limited number of ab initio calculations in combination with a simple Debye model can predict a concentration- and temperature-dependent mixing enthalpy for a binary system. Fe-Mo is taken as a test case, and our predictions are compared with phase diagram information and a recently measured heat of solution for Mo in Fe. Crystallographic and magnetic information is calculated for the λ and μ intermetallic phases in the Fe-Mo phase diagram as well. The present methodology can be useful for making a quick survey of mixing enthalpies in a large set of binary systems, in particular in the dilute concentration ranges where tabulated data are often lacking and where calphad-style modeling is less reliable.

Open Access version available at UGent repository

Hyperfine field and hyperfine anomalies of copper impurities in iron

V.V. Golovko, F. Wauters, S. Cottenier, M. Breitenfeldt, V. De Leebeeck, S. Roccia, G. Soti, M. Tandecki, E. Traykov, S. Van Gorp, D. Zakoucky, N. Severijns
Physical Review C
84, 014323
2011
A1

Abstract 

A new value for the hyperfine magnetic field of copper impurities in iron is obtained by combining resonance frequencies from experiments involving β-NMR on oriented nuclei on 59Cu, 69Cu, and 71Cu with magnetic moment values from collinear laser spectroscopy measurements on these isotopes. The resulting value, i.e., Bhf(CuFe) = -21.794(10) T, is in agreement with the value adopted until now but is an order of magnitude more precise. It is consistent with predictions from ab initio calculations. Comparing the hyperfine field values obtained for the individual isotopes, the hyperfine anomalies in Fe were determined to be 59Δ69=0.15(9)% and 71Δ69=0.07(11)%.

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