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Ordering and phase separation in Gd-doped ceria: a combined DFT, cluster expansion and Monte Carlo study
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, S-10044 Stockholm, Sweden..
KTH Royal Inst Technol, Dept Mat Sci & Engn, S-10044 Stockholm, Sweden.;Mat Ctr Leoben Forsch GmbH, A-8700 Leoben, Austria..
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Mat Sci & Engn, S-10044 Stockholm, Sweden..
2017 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 39, p. 26606-26620Article in journal (Refereed) Published
Abstract [en]

Ordering of dopants and oxygen vacancies is studied for Gd-doped ceria (x(Gd) <= 0.25) by means of a combined density functional theory (DFT) and cluster expansion approach, where the cluster interactions derived from DFT calculations are further used in Monte Carlo simulations. The methodology is meticulously tested and the stability of the obtained solutions with respect to the volume change, applied exchange-correlation approximation and other modelling parameters is carefully analysed. We study Gd and vacancy ordering in the case of thermodynamic equilibrium and vacancy ordering for quenched Gd configurations. We find that at the thermodynamic equilibrium there exists a transition temperature (T-C) below which phase separation into C-type Gd2O3 and pure CeO2 occurs. The phase separation is observed in the whole studied concentration range and the transition temperature increases with concentration from ca. 600 (x(Gd) = 0.03) to 1000 K (x(Gd) = 0.25). Above T-C the distribution of Gd is random, oxygen vacancies tend to cluster in the coordination shells along < 1, 1/2, 0 > and < 1, 1, 1 >, and the nearest neighbour position is preferred for Gd-vacancy. In the quenched Gd case, where Gd atoms are immobilised below 1500 K, the vacancy ordering is significantly frustrated. In fact, we observe an oxygen freezing transition below temperature T-F approximate to T-C - 350 K, which is close to temperatures at which a change in the conductivity slope is observed experimentally.

Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2017. Vol. 19, no 39, p. 26606-26620
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-337327DOI: 10.1039/c7cp04106cISI: 000412763700014PubMedID: 28949350OAI: oai:DiVA.org:uu-337327DiVA, id: diva2:1170216
Funder
Swedish Research Council, VR 348-2012-6196Available from: 2018-01-02 Created: 2018-01-02 Last updated: 2018-09-03
In thesis
1. Configurational Thermodynamics of the CeO2-Gd2O3 System: A Combined DFT, Cluster Expansion and Monte Carlo Approach to Bulk and Interfaces
Open this publication in new window or tab >>Configurational Thermodynamics of the CeO2-Gd2O3 System: A Combined DFT, Cluster Expansion and Monte Carlo Approach to Bulk and Interfaces
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, we study the configurational thermodynamics of Ce1-xGdxO2-x/2 x ≤ 1 or CGO. We apply a combined Density Functional Theory (DFT), cluster expansion and Monte Carlo (MC) approach in which the configurational energy of CGO is described by means of the Ising-type Hamiltonian. The interactions are determined by the cluster expansion of total energies calculated with DFT for a set of various cation–anion configurations. This allows one to perform on- the-fly calculations of the configurational energies in MC simulations of cation and anion ordering.

The cluster interactions are essentially electrostatic and long-range, and describe the configurational energetics in the entire range of concentrations rather well.

The phase diagram obtained in the MC simulations allows one to rationalise existing experimental data and is largely in agreement with that. We observe the phase separation into pure oxides, in equilibrium, below ca. 1000 K for x ≤ 1 (however it is kinetically hindered as diffusion of cations below ca. 1500 K is slow). We also observe the C-type oxygen–vacancy ordering in the x = 0.3–1 range below ca. 1200–3300 K (C phase), and a largely disordered, fluorite (F) phase in the x ≤ 0.3 range and above the ordering temperature.

The DFT supercell calculations of the F and C phase configurations obtained in MC simulations allow us to study the effect of concentration and ordering on the lattice relaxations, lattice parameter and elastic moduli, providing insights into relation between preparation conditions, structure and properties.

The bulk cluster interactions appear to be applicable also to coherent CeO2/C- type Gd2O3 interfaces, hence we examine a configurational energy landscape of the oxygen vacancy migration therein.

This combined approach can be applied to study configurational thermodynamics of similar materials as well as the influence of configurational state on ionic conductivity and other properties.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 78
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1717
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-359578 (URN)978-91-513-0434-2 (ISBN)
Public defence
2018-10-30, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, 13:12 (English)
Opponent
Supervisors
Available from: 2018-10-05 Created: 2018-09-03 Last updated: 2018-10-16

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Žguns, Pjotrs A.Skorodumova, Natalia V.

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