Nitrides and Pnictides

Synopsis:

Nitrides are synthetically-challenging, often requiring specialised preparative techniques. This has meant that, until recently, known nitrides were relatively rare. We have focused on developing new synthetic routes to these compounds and on unraveling the frequently curious and unique relationships between structure, bonding and physical properties in these materials.

We have succeeded in synthesising a number of new nitrides composed of alkali or alkaline earth metals and transition metals. The structures of these ternary nitrides can be predominantly oxide-like, carbide-like or unique to nitrides themselves, emphasising the intermediate nature of nitride bonding and inviting us to question our concepts of valence and coordination in inorganic solids. More details of some of these systems are given below.

Some current areas of interest:

Lithium nitridometallates Lithium nitride, Li3N, is the only stable binary alkali metal nitride and has a unique layered structure. Lithium-nitrogen [Li2N] layers are separated by layers of lithium atoms. Low concentrations of lithium vacancies in the [Li2N] layers give rise to high Li+ ion conductivity. The suitability of the material as a component in batteries is chiefly limited by its low decomposition voltage. Our present studies are focused on ways of improving the ionic conductivity and stability of these materials. By substituting appropriate transition metals for Li between [Li2N] layers, we can create new ternary compounds with increased lithium vacancy concentrations (e.g. fig. 1, 2). The vacancy concentration is related to the preparative conditions in each system but can be tuned via the redox chemistry of the transition metal substituent.	Figure 1. Structure of the ternary lithium nitridonickelate, LiNiN Figure 2. SEM micrograph of Li2-x-yCuxN crystallites.
Layered ternary nitrides Two dimensional layered structures are prevalent in some of the most interesting and useful inorganic oxides and chalcogenides. (e.g. superconductors, fast ion conductors etc.). Layered nitrides are at present far less numerous, yet the structural principles can be very similar to those seen in these materials. Our interests here are in exploring the range of ternary and higher nitrides that yield 2D structures and in determining how these different structure types relate to often quite unexpected physical properties. For example, our recent work has shown SrZr(Hf)N2 to form the ubiquitous hexagonal NaFeO2 structure. DFT calculations (in collaboration with Dr Régis Gautier at Rennes) and experimental data show these materials are intrinsically semiconducting (fig. 3). SrTiN2, however, which forms the much rarer KCoO2 structure, is an unexpected paramagnetic metal. DFT calculations rationalise the observed properties.(fig. 4). .	Figure 3. Structure of SrZrN2. Figure 4. Structure of SrTiN2.
Two dimensional subnitrides and new intercalation materials The group 2 metals from calcium downwards unexpectedly form subnitrides, A2N and in the case of Sr and Ba, no equivalent "ionic" A3N2 compounds have yet been proven to exist. The subnitrides are fascinating - they maintain a charge balance via the existence of free electrons within the wide van der Waals gaps in their 2D structures (fig. 5). This intrinsic feature leads to anisotropic metallic properties but also creates highly reactive compounds as a basis for extensive reaction chemistry. Ongoing projects in this area focus on the challenging synthesis of these subnitrides and of other low dimensional variants. Our aims are twofold: (1) to characterise these compounds and elucidate links between structure and anisotropic properties and (2) to utilise these low dimensional hosts for unusual anion intercalation reactions, observing how incorporation of different and quite complex guest species can dramatically change the chemical and physical properties of the host (fig. 6).	Figure 5. Structure of Ca2N. Figure 6. Structure of intercalated Ca2N(Cl,Br).

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