Modification of the anion sublattice in metal nitrides

The main goal of solid-state chemistry is the synthesis and characterisation of new compounds with technologically exploitable properties. To this aim, one established chemical route is the modification of known inorganic materials, in most cases oxides, via substitution or insertion of cations different from the original makeup. An alternative, and less frequently adopted, approach is the manipulation of the anion sublattice to yield mixed-anion materials. Recent examples of materials with structural and physical properties tunable via both anionic and cationic substitutions are bringing more attention to the potential of this approach as an alternative and/or complementary chemical approach to cationic modifications. Within this review, structural relationships and differences between nitrides and mixed-anion nitrides, such as nitride-halides, nitride-sulfides, nitride-carbides and nitride-borides will be highlighted to set the scenery and the future challenges to a fuller exploitation of the ‘anionic route’ as a strategy towards the design of new materials

Long-range magnetic ordering in Ba2CoS3: A neutron diffraction study

Neutron powder diffraction has been used to determine the magnetic structure of the quasi-one-dimensional compound Ba2CoS3, which contains linear [001] chains of vertex-sharing CoS4 tetrahedra, spaced apart by Ba2+ cations. At 1.5 K the Co2+ cations in the chains are antiferromagnetically ordered with an ordered magnetic moment of 1.97(4) μB per cation aligned along [100]. Each Co2+ cation is ferromagnetically aligned with four cation in neighbouring chains and antiferromagnetically aligned with two others. © 2007 Elsevier Inc. All rights reserved

Investigation of the stability of Co-doped apatite ionic conductors in NH3

Hydrogen powered solid oxide fuel cells (SOFCs) are of enormous interest as devices for the efficient and clean production of electrical energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the larger scale use of SOFCs. Identifying alternative fuel sources to act as intermediate during the transition to the full use of hydrogen is, therefore, of importance. One excellent alternative is ammonia, which is produced on a large scale, is relatively cheap and has the infrastructure for storage and transportation already in place. However, considering that SOFCs operate at temperatures higher than 500°C, a potential problem is the interaction of gaseous ammonia with the materials in the cathode, anode and solid electrolyte. In this paper, we extend earlier work on high temperature reactions of apatite electrolytes with NH3 to the transition metal (Co) doped systems, La9.67Si5CoO26 and La10(Si/Ge)5CoO26.5. A combination of PXRD, TGA and XAFS spectroscopy data showed a better structural stability for the silicate systems. Apatite silicates and germanates not containing transition metals tend to substitute nitride anions for their interstitial oxide anions, when reacted with NH3 at high temperature and, consequentially, lower the interstitial oxide content. In La9.67Si5CoO26 and La10(Si/Ge)5CoO26.5 reduction of Co occurs as a competing process, favouring lower levels of nitride-oxide substitution

Formation of apatite oxynitrides by the reaction between apatite-type oxide ion conductors, La8+xSr2-x(Si/Ge)6O26+x/2, and ammonia

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, La8+xSr2−x(Si/Ge)6O26+x/2, and NH3 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/2) 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 NH3 at elevated temperatures

Stereostructural behaviour of N–N atropisomers:two conglomerate crystallisations and a crystallisation-induced deracemisation

The solid state behaviour of a number of compounds which show hindered rotation around an N–N bond, in some cases leading to axial chirality is described. A diacyl hydrazine, bisanthranoly hydrazine, 1 crystallises in the chiral space group P212121, presenting an example of conglomerate crystallisation. A tetra-acyl hydrazine derived from lactic acid, 2, shows kinetic resolution by crystallisation, as of the two isomers observed in the solution NMR, only one crystallises, again in the space group P212121. Two cyclic acyl hydrazines in the form of biquinazolinones are studied: 2,2′diphenyl-3,3-biquinazolinone, 3 crystallises in the achiral space group Pbca, while 3,3′-dimethyl-2,2′-biquinazoline-4′-thio-4-one, 4 crystallises in the chiral space group P21 giving another example of a conglomerate crystallisation. The single crystal structures of each of the species have been compared to powder XRD data to confirm that the single crystal structures are representative of the bulk material

Probing local and electronic structure in Warm Dense Matter: single pulse synchrotron x-ray absorption spectroscopy on shocked Fe

Understanding Warm Dense Matter (WDM), the state of planetary interiors, is a new frontier in scientific research.There exists very little experimental data probing WDM states atthe atomic levelto test current models and those performed up to now are limited in quality. Here, we report a proof-ofprinciple experiment that makes microscopic investigations of materials under dynamic compression easily accessible to users and with data quality close to that achievable at ambient. Using a single 100 ps synchrotron x-ray pulse, we have measured, by K-edge absorption spectroscopy, ns-lived equilibrium states ofWDM Fe. Structural and electronic changes in Fe are clearly observed forthe first time at such extreme conditions.The amplitude ofthe EXAFS oscillations persists up to 500GPa and 17000K, suggesting an enduring local order. Moreover, a discrepancy exists with respectto theoretical calculations in the value of the energy shift of the absorption onset and so this comparison should help to refine the approximations used in models