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artículo con referato
"Comment on “Oxygen Vacancy Ordering and Electron Localization in CeO2: Hybrid Functional Study”"
M. Verónica Ganduglia-Pirovano, Gustavo E. Murgida, Valeria Ferrari and Ana María Llois
J. Phys. Chem. C 121(38) (2017) 21080-21083
Han et al. have recently published an article on the ordering of oxygen vacancies and electron localization in bulk CeO2 using density functional theory (DFT) with the Heyd-Scuseria-Ernzerhof (HSE06) functional. On the basis of their results, they concluded that oxygen vacancies tend to linearly order in the [111] CeO2 direction with a weakened excess charge localization compared with the case of a single vacancy. Moreover, distinct vacancy-induced lattice relaxations were found to be crucial for the interpretation of their results. This Comment is written to prevent misconceptions regarding the localization of the excess charge and the associated lattice relaxations discussed in the work of Han et al. that misses any citation to previous recent work on related subjects using the DFT+U approach with the Perdew-Becke-Ernzerhof (PBE) functional. As an example, Han et al. describe the case of an isolated bulk vacancy and claim that the two excess electrons left behind upon vacancy formation localize on two nearest-neighbor Ce4+ cations that are reduced to Ce3+. Moreover, for the cases of a third-neighbor vacancy pair (VV3) and vacancy line along the [111] direction, their results indicate a homogeneous distribution of the Ce3+ ions on nearest-neighbor sites to the vacancies; namely, each vacancy has two first cationic Ce3+ neighbors. It is astonishing that they get such results because with DFT(PBE)+U the second-neighbor cationic sites to an isolated vacancy are preferred, and the distribution of first-neighboring Ce3+ to vacancy pairs is predicted to be inhomogeneous. The consistency between both the DFT(PBE)+U and DFT(HSE06) approaches for describing the electronic structure of partially reduced ceria and for predicting energy differences between different Ce3+ distributions is well documented. Here we used exactly the same DFT(HSE06)-based methodology and computational code (VASP) as Han et al. and reconsidered the cases of isolated vacancies, VV3 vacancy pairs, and lines along the [111] direction in bulk CeO2. From our point of view, their study suffers from a number of critical flaws. The main issues are (I) not having investigated in detail many possible configurations of the Ce3+ ions and (II) the reproducibility and correctness of their calculated defect structures. These aspects are vital for the interpretation of their results, as outlined in the following sections.
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