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| 2. |
- Kullgren, Jolla, 1978-, et al.
(författare)
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DFT-based Monte Carlo Simulations of Impurity Clustering at CeO2(111)
- 2017
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Ingår i: The Journal of Physical Chemistry C. - : AMER CHEMICAL SOC. - 1932-7447 .- 1932-7455. ; 121:28, s. 15127-15134
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Tidskriftsartikel (refereegranskat)abstract
- The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+U and lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+U data for all symmetrically inequivalent (F-)(n)(Ce3+)(n) nearest-neighbor clusters with n = 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ ions. The distributions of surface hydroxyl groups are noticeably different.
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| 3. |
- Mitev, Pavlin, et al.
(författare)
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Calculation of anharmonic OH phonon dispersion curves for the Mg(OH)(2) crystal
- 2010
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:3, s. 034120-
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Tidskriftsartikel (refereegranskat)abstract
- Anharmonic OH phonon dispersion curves have been calculated for the Mg(OH)(2) crystal. A crystal Hamiltonian was set up for the vibrational problem, where the coordinates consists of the bond lengths of two hydroxide ions in the central unit cell. Its two-dimensional potential energy surface was constructed from first principle calculations within the density functional theory approximation. Dispersion curves were calculated by diagonalizing the Hamiltonian in a basis of singly excited crystal functions. The single particle functions used to construct the crystal states were taken from a Morse oscillator basis set. These well chosen functions made it possible to restrict calculations to include only very few functions, which greatly contributed to a transparent presentation of the underlying theory. All calculations could be done analytically except for the calculation of a few integrals. We have compared our results with those of a series of harmonic lattice dynamics calculations and have found that the anharmonicity shifts the IR and Raman dispersion curves downward appreciably and slightly changes the energy differences between both curves. From an analysis of the harmonic results we conclude that incorporating the coupling between OH stretching motion and the motion of their centers of mass will appreciably change the overall features of the dispersion curves. Extension of the anharmonic model along these lines will cause no problem to the theoretical approach presented in this paper.
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| 4. |
- Wolf, Matthew J., et al.
(författare)
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Fluorine clusters at CeO2(111) - A DFT+U and Monte Carlostudy
- 2017
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- STM experiments on CeO2(111) reveal depressions in the surface oxygen sub-lattice which are observed to form clusters of various shapes and sizes [1].While these depressions were assumed to be oxygen vacancies, subsequent DFTcalculations have indicated that clusters of oxygen vacancies are energeticallyunstable [2-4]. Recently, we showed theoretically that fluorine impurities shouldappear almost identical to oxygen vacancies in STM experiments, but that theirproperties are more in line with those of the defects observed in experiments [5].Here, I will present the results of a further investigation into the distribution ofF impurity clusters at CeO2(111), using a combination of DFT+U calculations,and Monte Carlo sampling based on a simple but accurate pair potential whichwas fitted to the DFT results. The distribution is characterised in terms of thenumber of clusters of a certain size, and also on their topology, i.e. whetherthey are compact or open/linea r. Our results compare favourably with theexperiments, and also exhibit some interesting physics in their own right.[1] F. Esch et al., Science 309, 752 (2005).[2] J. Conesa, Cat. Today 143, 315 (2009).[3] C. Zhang et al., Phys. Rev. B 79, 075433 (2009).[4] X.-P. Wu & X.-Q. Gong, Phys. Rev. Lett. 116, 086102 (2016).[5] J. Kullgren, M. J. Wolf et al., Phys. Rev. Lett. 112, 156102 (2014).
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