Jose J. Plata

Charting the complete elastic properties of inorganic crystalline compounds

The elastic constant tensor of an inorganic compound provides a complete description of the response of the material to external stresses in the elastic limit. It thus provides fundamental insight into the nature of the bonding in the material, and it is known to correlate with many mechanical properties. Despite the importance of the elastic constant tensor, it has been measured for a very small fraction of all known inorganic compounds, a situation that limits the ability of materials scientists to develop new materials with targeted mechanical responses. To address this deficiency, we present here the largest database of calculated elastic properties for inorganic compounds to date. The database currently contains full elastic information for 1,181 inorganic compounds, and this number is growing steadily. The methods used to develop the database are described, as are results of tests that establish the accuracy of the data. In addition, we document the database format and describe the different ways it can be accessed and analyzed in efforts related to materials discovery and design.

High-throughput computational screening of thermal conductivity, Debye temperature, and Grüneisen parameter using a quasiharmonic Debye model

The quasiharmonic Debye approximation has been implemented within the aflow and Materials Project frameworks for high-throughput computational materials science (Automatic Gibbs Library, agl), in order to calculate thermal properties such as the Debye temperature and the thermal conductivity of materials. We demonstrate that the agl method, which is significantly cheaper computationally compared to the fully ab initio approach, can reliably predict the ordinal ranking of the thermal conductivity for several different classes of semiconductor materials. In particular, a high Pearson (i.e., linear) correlation is obtained between the experimental and agl computed values of the lattice thermal conductivity for a set of 75 compounds including materials with cubic, hexagonal, rhombohedral, and tetragonal symmetry.

Comparative Study on the Performance of Hybrid DFT Functionals in Highly Correlated Oxides: The Case of CeO2 and Ce2O3

The outstanding catalytic properties of cerium oxides rely on the easy Ce(3+) ↔ Ce(4+) redox conversion, which however constitutes a challenge in density functional based theoretical chemistry due to the strongly correlated nature of the 4f electrons present in the reduced materials. In this work, we report an analysis of the performance of five exchange-correlation functionals (HH, HHLYP, PBE0, B3LYP, and B1-WC) implemented in the CRYSTAL06 code to describe three properties of ceria: crystal structure, band gaps, and reaction energies of the CeO2 → Ce2O3 process. All five functionals give values for cell parameters that are in fairly good agreement with experiment, although the PBE0 hybrid functional is found to be the most accurate. Band gaps, 2p-4f-5d in the case of CeO2 and 4f-5d in the case of Ce2O3, are found to be, in general, overestimated and drop off when the amount of Hartree-Fock exchange in the exchange-correlation functional decreases. In contrast, the reaction energies are found to be underestimated, and increase when the amount of HF exchange lowers. Overall, at its standard formulation, the B1-WC functional seems to be the best choice as it provides good band gaps and reaction energies, and very reasonable crystal parameters.

A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: The case of the highly active metal/CeOx/TiO2(110) catalysts

The structural and electronic properties of CeO(x) species supported on the rutile TiO(2)(110) surface have been examined by means of periodic density-functional calculations that use a generalized gradient approximation functional including a Hubbard-like type correction. Deposition of Ce atoms leads in a first step to Ce(3+) ions bound to the surface through bridge and in-plane oxygen atoms, the released electrons occupying the Ti 3d empty orbitals. Further addition of Ce and molecular oxygen gives place to Ce(2)O(3) dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeO(x) nanoparticles (NPs) on the TiO(2) surface is highly exothermic and our calculations show that the redox properties of the Ce(III)-Ce(IV) couple are significantly altered when it is supported on TiO(2). In particular the reactivity against CO/O(2) indicates that on the surface the presence of Ce(III) is favored over Ce(IV) species. Our results also indicate that the CeO(x)/TiO(2) interface should be seen like a real mixed-metal oxide rather than a supported NP of ceria. Finally, in the context of the high catalytic activity of the M/CeO(x)/TiO(2) (M=Au,Cu,Pt) systems in the water-gas shift reaction, we have examined the dissociation of water on the CeO(x)/TiO(2) surface and estimated a barrier as small as 0.04 eV, i.e. approximately 8 times smaller than that computed for a TiO(2) oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M/CeO(x)/TiO(2) systems over M/TiO(2).

Communication: Improving the density functional theory+U description of CeO2 by including the contribution of the O 2p electrons

Density functional theory (DFT) based approaches within the local-density approximation or generalized gradient approximation frameworks fail to predict the correct electron localization in strongly correlated systems due to the lack of cancellation of the Coulomb self-interaction. This problem might be circumvented either by using hybrid functionals or by introducing a Hubbard-like term to account for the on site interactions. This latter DFT+U approach is less expensive and therefore more practical for extensive calculations in solid-state computational simulations. By and large, the U term only affects the metal electrons, in our case the Ce 4f ones. In the present work, we report a systematic analysis of the effect of adding such a U term also to the oxygen 2p electrons. We find that using a set of U(f) = 5 eV and U(p) = 5eV effective terms leads to improved description of the lattice parameters, band gaps, and formation and reduction energies of CeO(2).

Surface oxygen vacancies in gold based catalysts for CO oxidation

Experimental catalytic activity measurements, diffuse reflectance infrared Fourier spectroscopy, and density functional theory calculations are used to investigate the role and dynamics of surface oxygen vacancies in CO oxidation with O2 catalyzed by Au nanoparticles supported on a Y-doped TiO2 catalyst. Catalytic activity measurements show that the CO conversion is improved in a second cycle of reaction if the reactive flow is composed by CO and O2 (and inert) while if water is present in the flow, the catalyst shows a similar behaviour in two successive cycles. DRIFTS-MS studies indicate the occurrence of two simultaneous phenomena during the first cycle in dry conditions: the surface is dehydroxylated and a band at 2194 cm−1 increases (proportionally to the number of surface oxygen vacancies). Theoretical calculations were conducted in order to explain these observations. On one hand, the calculations show that there is a competition between gold nanoparticles and OH to occupy the surface oxygen vacancies and that the adsorption energy of gold on these sites increases as the surface is being dehydroxylated. On the other hand, these results evidence that a strong electronic transfer from the surface to the O2 molecule is produced after its adsorption on the Au/TiO2 perimeter interface (activation step), leaving the gold particle in a high oxidation state. This explains the appearance of a band at a wavenumber unusually high for the CO adsorbed on oxidized gold particles (2194 cm−1) when O2 is present in the reactive flow. These simultaneous phenomena indicate that a gold redispersion on the surface occurs under reactive flow in dry conditions generating small gold particles which are very active at low temperature. This fact is notably favoured by the presence of surface oxygen vacancies that improve the surface dynamics. The obtained results suggest that the reaction mechanism proceeds through the formation of a peroxo-like complex formed after the electronic transfer from the surface to the gas molecule.

An efficient and accurate framework for calculating lattice thermal conductivity of solids: AFLOW—AAPL Automatic Anharmonic Phonon Library

One of the most accurate approaches for calculating lattice thermal conductivity,

Analysis of the origin of lateral interactions in the adsorption of small organic molecules on oxide surfaces

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