Center for Molecular Modeling - SBO In Silico Materials B/10784/*

Crystal structure prediction for supersaturated AZO : the case of Zn3Al2O6

wim — Mon, 12 Aug 2013 11:45:28 +0000

K. Rijpstra, S. Cottenier, M. Waroquier, V. Van Speybroeck

CrystEngComm

2013 (15), 10440-10444

2013

Abstract

Increasing the Al concentration in Al-doped ZnO (AZO) is one way of improving the conductivity of this transparent conductive oxide (TCO). Beyond a certain concentration, an unwanted secondary phase develops with a low conductivity. Its stoichiometry is Zn3Al2O6, and its crystal structure has not yet been convincingly determined. By applying unbiased ab initio structure prediction tools, we predict the crystal structure of Zn3Al2O6 to be monoclinic with space group Pm. It can be described as a nanofabric, with one-dimensional Al2O3 wires penetrating a ZnO matrix. This crystal has a formation energy that is lower than any structure proposed before, and is consistent with all available experimental information. Knowledge of the nature of this phase can help to avoid its formation and therefore to engineer AZO crystals with an increased level of Al-doping and associated increased conductivity.

DOI

http://dx.doi.org/10.1039/C3CE41009A

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13_CrystEngComm_xxxx_Rijpstra.pdf

First-principles study of possible shallow donors in ZnAl2 O4 spinel

wim — Mon, 01 Jul 2013 09:18:37 +0000

H. Dixit, N. Tandon, S. Cottenier, R. Saniz, D. Lamoen, B. Partoens

Physical Review B

87 (17), 174101

2013

Abstract

ZnAl2O4 (gahnite) is a ceramic which is considered a possible transparent conducting oxide (TCO) due to its wide band gap and transparency for UV. Defects play an important role in controlling the conductivity of a TCO material along with the dopant, which is the main source of conductivity in an otherwise insulating oxide. A comprehensive first-principles density functional theory study for point defects in ZnAl2O4 spinel is presented using the Heyd, Scuseria, and Ernzerhof hybrid functional (HSE06) to overcome the band gap problem. We have investigated the formation energies of intrinsic defects which include the Zn, Al, and O vacancy and the antisite defects: Zn at the Al site (Zn-Al) and Al at the Zn site (Al-Zn). The antisite defect Al-Zn has the lowest formation energy and acts as a shallow donor, indicating possible n-type conductivity in ZnAl2O4 spinel by Al doping.

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1103/PhysRevB.87.174101

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13_PhysRevB_87_174101_Dixit.pdf

Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure

wim — Fri, 01 Feb 2013 09:46:43 +0000

S. Decoster, B. Johannessen, C.J. Glover, S. Cottenier, T. Bierschenk, H. Salama, F. Kremer, K. Temst, A. Vantomme, M.C. Ridgway

Applied Physics Letters

101 (26), 261904

2012

Abstract

We present an experimental lattice location study of Ga atoms in Ge after ion implantation at elevated temperature (250 degrees C). Using extended x-ray absorption fine structure (EXAFS) experiments and a dedicated sample preparation method, we have studied the lattice location of Ga atoms in Ge with a concentration ranging from 0.5 at. % down to 0.005 at. %. At Ga concentrations http://dx.doi.org/10.1063/1.4773185]

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1063/1.4773185

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12_ApplPhysLett_101_261904_Decoster.pdf

Tuning of CeO2 buffer layers for coated superconductors through doping

wim — Thu, 18 Oct 2012 07:05:20 +0000

D.E.P. Vanpoucke, S. Cottenier, V. Van Speybroeck, P. Bultinck, I. Van Driessche

Applied Surface Science

260, 32-35

2012

Abstract

The appearance of microcracks in cerium oxide (CeO 2) buffer layers, as used in buffer layer architectures for coated superconductors, indicates the presence of stress between this buffer layer and the substrate. This stress can originate from the differences in thermal expansion or differences in lattice parameters between the CeO 2 buffer layer and the substrate. In this article, we study, by means of ab initio density functional theory calculations, the influence of group IV doping elements on the lattice parameter and bulk modulus of CeO 2. Vegard's law behavior is found for the lattice parameter in systems without oxygen vacancies, and the Shannon crystal radii for the doping elements are retrieved from the lattice expansions. We show that the lattice parameter of the doped CeO 2 can be matched to that of the La 2Zr 2O 7 coated NiW substrate substrate for dopant concentrations of about 5%, and that bulk modulus matching is either not possible or would require extreme doping concentrations. [All rights reserved Elsevier].

Open Access version available at UGent repository

DOI

http://dx.doi.org/10.1016/j.apsusc.2012.01.032

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12_app. surface sc_260_32_Vanpoucke.pdf

Electronic structure of transparent oxides with the Tran–Blaha modified Becke–Johnson potential

wim — Mon, 30 Apr 2012 06:13:09 +0000

H. Dixit, R. Saniz, S. Cottenier, D. Lamoen, B. Partoens

Journal of Physics: Condensed Matter

24 (20), 205503

2012

Abstract

We present electronic band structures of transparent oxides calculated using the Tran–Blaha modified Becke–Johnson (TB-mBJ) potential. We studied the basic n-type conducting binary oxides In2O3, ZnO, CdO and SnO2 along with the p-type conducting ternary oxides delafossite CuXO2 (X=Al, Ga, In) and spinel ZnX2O4 (X=Co, Rh, Ir). The results are presented for calculated band gaps and effective electron masses. We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation (GGA) in density functional theory (DFT) and also compare the electronic band structure with available results from the quasiparticle GW method. It is shown that the calculated band gaps compare well with the experimental and GW results, although the electron effective mass is generally overestimated.

DOI

http://dx.doi.org/10.1088/0953-8984/24/20/205503

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12 j. phys. condens. matter 24, 205503 Dixit.pdf

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

wim — Wed, 16 Nov 2011 09:54:42 +0000

S. Cottenier, M.I.J. Probert, T. Van Hoolst, V. Van Speybroeck, M. Waroquier

Earth and Planetary Science Letters

312, 237–242

2011

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.

DOI

http://dx.doi.org/10.1016/j.epsl.2011.09.045

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11 epsl 312, 237–242 cottenier.pdf

Electronic structure and band gap of zinc spinel oxides beyond LDA: ZnAl2O4, ZnGa2O4 and ZnIn2O4

wim — Mon, 22 Aug 2011 13:35:09 +0000

H. Dixit, N. Tandon, R. Saniz, S. Cottenier, D. Lamoen, B. Partoens, V. Van Speybroeck, M. Waroquier

New Journal of Physics

13, 063002

2011

Abstract

We examine the electronic structure of the family of ternary zinc spinel oxides ZnX2O4 (X=Al, Ga and In). The band gap of ZnAl2O4 calculated using density functional theory (DFT) is 4.25 eV and is overestimated compared with the experimental value of 3.8–3.9 eV. The DFT band gap of ZnGa2O4 is 2.82 eV and is underestimated compared with the experimental value of 4.4–5.0 eV. Since DFT typically underestimates the band gap in the oxide system, the experimental measurements for ZnAl2O4 probably require a correction. We use two first-principles techniques capable of describing accurately the excited states of semiconductors, namely the GW approximation and the modified Becke–Johnson (MBJ) potential approximation, to calculate the band gap of ZnX2O4. The GW and MBJ band gaps are in good agreement with each other. In the case of ZnAl2O4, the predicted band gap values are >6 eV, i.e. ~2 eV larger than the only reported experimental value. We expect future experimental work to confirm our results. Our calculations of the electron effective masses and the second band gap indicate that these compounds are very good candidates to act as transparent conducting host materials.

DOI

http://dx.doi.org/10.1088/1367-2630/13/6/063002

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11 new j. phys. 13, 063002 dixit.pdf

SBO In Silico Materials

wim — Tue, 23 Apr 2013 13:06:03 +0000

Project promotor

V. Van Speybroeck, M. Waroquier

Financierende instantie

SBO-IWT

Project data

01/01/2009 to 31/12/2012

Bedrag

€250 000,00

Project titel

In Silico Materials Design and experimental validation for novel optical coatings

Personeel

S. Cottenier, K. Rijpstra, D.M. Tomecka

Extra info

Partner CMM: V. Van Speybroeck - M. Waroquier
[J.Paul (Flamac), D.Lamoen (UA), B.Partoens (UA), X.Gonze (UCL)]
(*) bedrag: partim CMM

Project ref.

SBO In Silico Materials B/10784/*

Ab initio Description of Iron and Steel: Thermodynamics and Kinetics (ADIS 2012)

wim — Tue, 26 Feb 2013 14:44:42 +0000

Tegernsee, Germany

Sunday, 29 April, 2012 to Friday, 4 May, 2012

Participant(s)

S. Cottenier

Project ref.

SBO In Silico Materials B/10784/*, FWO mandaat

Conference reference

ADIS 2012

School on "Functional Properties at the Atomic Scale"

wim — Wed, 13 Feb 2013 08:20:45 +0000

Vaalbeek, Belgium

Monday, 21 May, 2012 to Friday, 25 May, 2012

Participant(s)

S. Cottenier

Project ref.

SBO In Silico Materials B/10784/*

Conference reference

School on Functional Properties at the Atomic Scale