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.

In this work, high pressure was used as a tool to induce structural transition and prepare metastable polymorphs of ternary sulfides. Structural transformations under high pressure of compounds belonging to the Ba(2)Co(1-x)Zn(x)S(3) (0 ≤ x ≤ 1.0) series were studied using X-ray diffraction and electron microscopy. All members of the Ba(2)Co(1-x)Zn(x)S(3) series show the Ba(2)CoS(3)-type one-dimensional structure, but, after heating under pressure, the Ba(2)CoS(3) compound (x = 0) separates into BaS and the two-dimensional BaCoS(2-δ) (δ ≈ 0), while Ba(2)Co(1-x)Zn(x)S(3) compounds with x ≥ 0.25 maintain their one-dimensional features but rearrange into polymorphs showing the Ba(2)MnS(3)-type structure. All structural transformations can be linked to shortening in interchain metal-metal distances caused by the high pressure, and the role of the zinc in preventing loss of one-dimensionality is discussed.

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.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

In terms of application, mixed anion compounds have attracted far less attention than mixed cation systems. Interest in the catalytic properties of nitrides has been driven, in part, by their accessibility through new, improved synthesis routes. Although attention in this area extends beyond simple binary systems, to our knowledge, studies have not yet probed the influence of mixed anion compounds. Accordingly, in this review we describe the structures and synthetic routes to transition metal pnictide-halides, which are predominantly nitride-based. It can be anticipated that some of these materials may have interesting catalytic properties, although they have yet to be explored.

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.

Ce(2)MnN(3)F(2-delta) has been synthesised via low-temperature fluorination of the ternary nitride Ce(2)MnN(3). To the best of our knowledge, Ce(2)MnN(3)F(2-delta) is the first quaternary nitride-fluoride to be reported. The reaction of Ce(2)MnN(3) with 10% F(2)/N(2) at 95-115 degrees C yields the fluorinated phase Ce(2)MnN(3)F(2-delta), with tetragonal symmetry (P4/nmm a = 3.8554(4) A and c = 13.088(4) A based on neutron powder diffraction), accompanied by starting material. Rietveld refinement supports a staged fluorine insertion reaction (into alternate rocksalt layers) to give a product with stoichiometry Ce(2)MnN(3)F(2-delta). A comparison with the formation of Sr(2)TiO(3)F(2) and the isostructural LaSrMnO(4)F indicates that two F(-) anions are inserted but no F(-)/N(3-) substitution takes place. Magnetic susceptibility measurements show a transition from Pauli paramagnetic behaviour, in Ce(2)MnN(3), to paramagnetic behaviour upon fluorination. The effective magnetic moment in Ce(2)MnN(3)F(2-delta), mu(eff) = 5.38 mu(B), is consistent with an intermediate value between that of Mn(3+) (4.9 mu(B)) and Mn(2+) (5.9 mu(B)) supporting the proposed stoichiometry, Ce(2)MnN(3)F(2-delta).

There is growing interest in the use of ammonia as a fuel in Solid Oxide Fuel Cells (SOFCs). However, the possible reaction between the electrolyte and ammonia, and its potential effect on performance, has received little attention. In this paper, we report an investigation of the high temperature (950 degrees C) reaction of the apatite-type oxide ion conductor, La9.33Si6O26, and ammonia. The results show that such treatment leads to nitridation of the sample, with evidence for Si loss leading to an increased La: Si ratio in the final product. From neutron diffraction studies, the composition of the final product was determined to be La9.7(1)Si6O22.6(2)N2.7(2), with structural and Si-29 NMR data suggesting the presence of N both within the apatite anion channels, and bonded to Si. An interesting feature of the structural studies are the relatively low atomic displacement parameters compared to the comparable apatite oxide systems, La-9.33 (+) xSi6O26 (+ 3x/2), which can be related to the lack of interstitial anions in the oxynitride. Further studies on samples heated in ammonia at lower temperatures (600, 800 degrees C) suggest lower N incorporation, particularly for the 600 degrees C treatment. Considering the correlation of ionic conductivity, and interstitial oxide ion content in apatite systems, the data suggests the potential use of apatite-type electrolytes in SOFCs utilising NH3 as the fuel should be limited to temperatures < 800 degrees C.

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 P2(1)2(1)2(1), 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 P2(1)2(1)2(1). 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 P2(1) 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.

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 1.5K the Co2+ cations in the chains are antiferromagnetically ordered with an ordered magnetic moment of 1.97(4)mu(B) per cation aligned along [100]. Each Co2+ cation is ferromagnetically aligned with four cation in neighbouring chains and antiferromagnetically aligned with two others. (C) 2007 Elsevier Inc. All rights reserved.