Sr(4)N(3): a hitherto missing member in the nitrogen pressure reaction series Sr2N-->Sr4N3-->SrN-->SrN2

Pressure-stabilized polymerization of nitrogen in alkaline-earth-metal strontium nitrides

High-pressure technology can help us to obtain excellent materials. We have explored alkaline-earth-metal strontium nitrides under different pressures, theoretically. A variety of stable Sr-N structures were predicted by the structure searching method using CALYPSO code. Six new stoichiometries, SrN, Sr₂N₃, SrN₂, SrN₃, SrN₄, and SrN₅, were predicted. And our calculation proved that all these compounds were stable existing under ambient pressure up to 100 GPa. A rich variety of poly-nitrogen forms appeared in the newly predicted SrN_x compounds, including four nitrogen polymerization forms: ranging from N₂, N₃, N₄, and N₅ molecules, to zig-zag nitrogen chains and extended chains connected by puckered "N₆" rings. Significantly, the 1D extended polymeric chain of puckered "N₆" rings was firstly identified in the P1[combining macron]-SrN₃ structure at 60 GPa. Another N-rich C2/c-SrN₄ was stable only under the relatively high-pressure of 20 GPa, but this phase can be quenched under atmospheric pressure. The N-rich phase SrN₅ maintained structural stability when the pressure reached 50-70 GPa. The delocalization of π electrons from N atoms was the principal cause for its metallicity in SrN₅. In this paper, our calculated results indicated that the energetic poly-nitrides in alkaline-earth-metal nitrides can be obtained by the high-pressure method.

Host-guest interactions, uniform vs fragmented linear atom chains and likeliness of Peierls distortions in the (Ca7N4)[M(x)] (M = Ag, Ga, In) phases

The electronic structure of the recently reported (Ca(7)N(4))[M((x))] (M = Ag, Ga, and In) phases has been studied by means of first principles density functional theory (DFT) calculations. It is shown that under the assumption of very weak host-guest interactions: (a) four calcium atoms per formula unit may be considered as Ca(1.5+), whereas the remaining three may be considered as Ca(2+) so that the guest atoms would be neutral, and (b) the Peierls distortions which could set in the guest linear chains are unlikely. These results are compatible with the experimental information. However, the first principles DFT calculations clearly show that very sizable host-guest interactions occur and drastically modify this situation. As a result, there is a substantial electron transfer from the framework to the guest atoms, and all calcium atoms of the framework are better described as Ca(2+). The stoichiometry and structure of these systems result from a competition between the natural tendency of the bare guest atoms to form uniform linear chains within the reduced space of the channels and the attempt to optimize their positions within the channels through interactions with the calcium atoms. Model calculations suggest that indium has a weaker tendency to form uniform linear chains and interacts in a stronger way with the host. It is shown that, for the (Ca(7)N(4))[M(1.33)] (M = Ag and Ga) phases, a structure built from three repeat units of the Ca(7)N(4) host framework containing uniform linear chains with a repeat unit of four guest metal atoms is compatible with the strong interaction scenario and the lack of correlation between the different linear guest chains. These phases should be metallic conductors, and the carriers have both host and guest character. In contrast, the guest atoms in (Ca(7)N(4))[In(1.0)] prefer to occur as a series of trimeric units. Although this phase is found to have a metallic band structure, the conductivity should be smaller than those of the (Ca(7)N(4))[M(1.33)] (M = Ag and Ga) phases.

High-pressure chemistry of nitride-based materials

Besides temperature at one atmosphere, the applied pressure is another important parameter for influencing and controlling reaction pathways and final reaction products. This is relevant not only for the genesis of natural minerals, but also for synthetic chemical products and technological materials. The present critical review (316 references) highlights recent developments that utilise high pressures and high-temperatures for the synthesis of new materials with unique properties, such as high hardness, or interesting magnetic or optoelectronic features. Novel metal nitrides, oxonitrides as well as the new class of nitride-diazenide compounds, all formed under high-pressure conditions, are highlighted. Pure oxides and carbides are not considered here. Moreover, syntheses under high-pressure conditions require special equipment and preparation techniques, completely different from those used for conventional synthetic approaches at ambient pressure. Therefore, we also summarize the high-pressure techniques used for the synthesis of new materials on a laboratory scale. In particular, our attention is focused on reactive gas pressure devices with pressures between 1.2 and 600 MPa, multi-anvil apparatus at P < 25 GPa and the diamond anvil cell, which allows work at pressures of 100 GPa and higher. For example, some of these techniques have been successfully upgraded to an industrial scale for the synthesis of diamond and cubic boron nitride.

Crystalline Inclusion Compounds Constructed through Self-Assembly of Isonicotinic Acid and Thiocyanato Coordination Bridges

The synthesis, crystal structures, inclusion ability, and structural robustness of novel crystalline inclusion compounds of [Ni(SCN)2(isoH)2].xG (isoH = isonicotinic acid; G = aromatic guest) are described. The inclusion compounds are constructed by stacking identical 2D host layers that consist of SCN-, isoH, and Ni2+ with van der Waals contact separation. In the layer, two types of rectangular cavities (A-type and B-type) are formed, and the guests are included in the former cavity. The inclusion compounds were categorized into four stacking modes according to the difference in the stacking mode of the layers. A systematic investigation of the crystal structures of the 21 inclusion compounds clarified the close relationship between the molecular structure of the guest and the resultant stacking mode of the layers.