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.

No abstract

A long-standing challenge in physics is to understand why cementite is the predominant carbide in steel. Here we show that the prevalent formation of cementite can be explained only by considering its stability at elevated temperature. A systematic highly accurate quantum mechanical study was conducted on the stability of binary iron carbides. The calculations show that all the iron carbides are unstable relative to the elemental solids, -Fe and graphite. Apart from a cubic Fe 23 C 6 phase, the energetically most favorable carbides exhibit hexagonal close-packed Fe sublattices. Finite-temperature analysis showed that contributions from lattice vibration and anomalous Curie-Weis magnetic ordering, rather than from the conventional lattice mismatch with the matrix, are the origin of the predominance of cementite during steel fabrication processes.

There is an urgent need to deposit uniform, high-quality, conformal SiN(x) thin films at a low-temperature. Conforming to these constraints, we recently developed a plasma enhanced atomic layer deposition (ALD) process with bis(tertiary-butyl-amino)silane (BTBAS) as the silicon precursor. However, deposition of high quality SiNx thin films at reasonable growth rates occurs only when N2 plasma is used as the coreactant; strongly reduced growth rates are observed when other coreactants like NH3 plasma, or N2-H2 plasma are used. Experiments reported in this Letter reveal that NH(x)- or H- containing plasmas suppress film deposition by terminating reactive surface sites with H and NH(x) groups and inhibiting precursor adsorption. To understand the role of these surface groups on precursor adsorption, we carried out first-principles calculations of precursor adsorption on the β-Si3N4(0001) surface with different surface terminations. They show that adsorption of the precursor is strong on surfaces with undercoordinated surface sites. In contrast, on surfaces with H, NH2 groups, or both, steric hindrance leads to weak precursor adsorption. Experimental and first-principles results together show that using an N2 plasma to generate reactive undercoordinated surface sites allows strong adsorption of the silicon precursor and, hence, is key to successful deposition of silicon nitride by ALD.

No abstract

The interaction of interstitial carbon with substitutional silicon and the effect of this interaction on the diffusion of carbon within body-centered-cubic iron, are computed using electronic density-functional theory. Both the activation energy for diffusion and the diffusion prefactor are predicted. Good agreement is found for those cases where a comparison with experimental data is possible.

We have studied all possible elementary reactions (including isomerization reactions) involved in the interaction of CH 4 (methane), CH 3 CH 3 (ethane), CH 2 CH 2 (ethene), and CHCH (ethyne) with the Ru(0001) surface using density functional theory based first-principles calculations. Site preference and adsorption energies for all the reaction intermediates and activation energies for the elementary reactions are calculated. From the calculated adsorption and activation energies, we find that dehydrogenation of the adsorbates is thermodynamically favored in agreement with experiments. Dehydrogenation of CH (methylidyne) is the most difficult in the dehydrogenation of CH 4 (methane). CH 3 CH 3 (ethane), CH 2 CH 2 (ethene), and CHCH (ethyne) dehydrogenate through the CH 3 C (ethylidyne) intermediate. Of the five possible pathways for the production of CH 3 C (ethylidyne), the CH 2 CH (ethenyl)−CH 2 C (ethenylidene) pathway is the most dominant. In the case of ethene, the ethynyl−ethenylidene pathway is also the dominant pathway on Pt(111). Comparison of α and β-C−H bond scission reactions, important for the Fischer−Tropsch process, shows that alkenes should be the major products compared to the formation of alkynes. Dehydrogenation becomes slightly favorable at lower coverages of the hydrocarbon fragments while hydrogenation becomes slightly unfavorable. In addition to resolving the dominant pathways during decomposition of the above hydrocarbons, the activation energies calculated in this paper can also be used in the modeling of processes that involve the considered elementary reactions at longer length and time scales.

No abstract