Atomic-Scale Structure and Stability of the Low-Index Surfaces of Pyrochlore Oxides

The multifunctional properties of complex ternary oxides such as pyrochlores are often influenced by surface structure. Optimizing the surface-driven attributes of these materials necessitates a detailed understanding of the structure and chemical composition of those surfaces. Here we report atomistic simulations elucidating the diverse atomic-scale structures of a set of low-index surfaces [(100), (110), (111), and (112)] in pyrochlore compounds as a function of both A and B cation chemistry. In pyrochlores, the low-index facets are all dipolar, requiring the introduction of surface defects to eliminate the surface dipole. We find that, due to the corresponding higher coordination of the surface cations, the (110) facet is the most energetically stable in all of the compounds considered, an interesting contrast to fluorite, in which the (111) surface is most stable. We also reveal a correlation between the surface energy and the energy to disorder the pyrochlore as a function of B cation chemistry, implying a similar physical origin for the two phenomena. Further, we find that surface rumpling is common across all pyrochlore compounds. An even more interesting feature emerging at these surfaces is the formation of extended structural defects such as steps and trenches, which are found to be stable after high-temperature annealing. As the formation of these features is a consequence of surface defects introduced to eliminate the surface dipole, we propose that the superior surface properties of materials of pyrochlores are due to these extended structural features, which are a direct consequence of the inherent dipole at the surfaces

Identifying the Molecular Properties that Drive Explosive Sensitivity in a Series of Nitrate Esters

Energetic materials undergo hundreds of chemical reactions during exothermic runaway, generally beginning with the breaking of the weakest chemical bond, the “trigger linkage.” Herein we report the syntheses of a series of pentaerythritol tetranitrate (PETN) derivatives in which the energetic nitrate ester groups are systematically substituted by hydroxyl groups. Because all the PETN derivatives have the same nitrate ester-based trigger linkages, quantum molecular dynamics (QMD) simulations show very similar Arrhenius kinetics for the first reactions. However, handling sensitivity testing conducted using drop weight impact indicates that sensitivity decreases precipitously as nitrate esters are replaced by hydroxyl groups. These experimental results are supported by QMD simulations that show systematic decreases in the final temperatures of the products and the energy release as the nitrate ester functional groups are removed. To better interpret these results, we derive a simple model based only on the specific enthalpy of explosion and the kinetics of trigger linkage rupture that accounts qualitatively for the decrease in sensitivity as nitrate ester groups are removed