We study the electronic band structure, dielectric, and lattice dynamical properties of FeSi, RuSi and OsSi from first principles using density functional theory. We performed the band structure calculations for these compounds to confirm they are indirect band-gap semiconductors. Density functional perturbation theory is employed to evaluate the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the Γ-point, and the phonon dispersion curves as well as the corresponding density of states. These calculated results are in reasonable agreement with the experimental data and theoretical values available. From the present calculation we also predict that there exist longitudinal-transverse optical (LO-TO) splittings by 2-32 cm −1 for these three compounds, which were not observed in previous experiments.

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Following growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs), we have investigated the possible reaction between the apatite silicate/germanate electrolytes, La 8+x Sr 2-x (Si/Ge) 6 O 26+x/2 , and NH 3 gas. We examine how the composition of the apatite phase affects the reaction with ammonia. For the silicate series, the results showed a small degree of N incorporation at 600 ○ C, while at higher temperatures (800 ○ C), substantial N incorporation was observed. For the germanate series, partial decomposition was observed after heating in ammonia at 800 ○ C, while at the lower temperature (600 ○ C), significant N incorporation was observed. For both series, the N content in the resulting apatite oxynitride was shown to increase with increasing interstitial oxide ion content (x) in the starting oxide. The results suggest that the driving force for the nitridation process is to remove the interstitial anion content, such that for the silicates the total anion (O+N) content in the oxynitrides approximates to 26.0, the value for an anion stoichiometric apatite. For the germanates, lower total anion contents are observed in some cases, consistent with the ability of the germanates to accommodate anion vacancies. The removal of the mobile interstitial oxide ions on nitridation suggests problems with the use of apatite-type electrolytes in SOFCs utilising NH 3 at elevated temperatures.