A detailed analysis of the formation energies for alkali, earth-alkali, and transition-metal hydrides is presented. The hydriding energies are computed for various crystal structures using density functional theory. The early transition metals are found to have a strong tendency for hydride formation which decreases as one goes to the right in the transition-metal series. A detailed analysis of the changes in band structure and electron density upon hydride formation has allowed us to understand the hydriding energy on the basis of three contributions. The first is the energy to convert the crystal structure of the metal to the structure formed by the metal ions in the hydride ͑fcc in most cases͒. In particular, for metals with a strong bcc preference such as V and Cr, this significantly lowers the driving force for hydride formation. A second contribution, which for some materials is dominant, is the loss of cohesive energy when the metal structure is expanded to form the hydride. This expansion lowers the cohesive energy of the metal and is a significant impediment to form stable hydrides for the middle to late transition metals, as they have high cohesive energies. The final contribution to the hydride formation energy is the chemical bonding between the hydrogen and metal in which it is inserted. This is the only contribution that is negative and hence favorable to hydride formation.
A coupled cluster expansion was used to direct an ab initio search for stable ordered structures on the cubic fluorite lattice across the ZrO 2 -Y 2 O 3 composition range. The energies of 453 arrangements of ͑Zr, Y͒ cations and ͑O, vacancy͒ anions on the fluorite lattice were calculated by using density functional theory ͑DFT͒ in the generalized gradient approximation. These DFT energies were used to construct a coupled cluster expansion that allowed the search for possible T = 0 K ground-state structures through ϳ10 5 cation/anion configurations. Our approach correctly identifies the experimentally observed compound Zr 3 Y 4 O 12 and also predicts an asyet-unobserved ordered structure with Zr 4 Y 2 O 11 stoichiometry. In the latter structure, vacancies are at second nearest neighbors from yttrium and every Zr has seven oxygen first nearest neighbors. The Zr-O bond lengths and oxygen coordination around Zr in the Zr 4 Y 2 O 11 ground state are nearly the same as those in monoclinic ZrO 2 . We also predict structures at Zr 5 Y 2 O 13 and Zr 6 Y 2 O 15 that are stable with respect to cubic ZrO 2 but metastable with respect to monoclinic ZrO 2 .
NEWS & ANALYSIS RESEARCH/RESEARCHERSof a bottom mirror to refl ect downward emitted photons back to the nanowire tip. The researchers integrated a gold mirror at the nanowire base by transferring the nanowires into a fl exible and fully transparent polymer fi lm and coating it with the metal by evaporation. Through this novel approach the researchers were able to achieve a 20-fold enhanced singlephoton emission fl ux.Joan J. Carvajal
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.