Our Solution - Fluxes

The vast majority of new nitride compounds have been prepared in molten fluxes (solvents). We pioneered the use of liquid Na/Alkaline earth (AE) alloys to prepare ternary compounds that typically contain the AE and a second metal: M = Co, Ni, Li, Zn, B, Al, Ga, Si, Ge. In most of these ternary phase diagrams at least several ternary compounds are found. With the exception of Co and Ni, these metals (AE and and M) are soluble in liquid Na at temperatures near 800^oC. The AE is known to greatly increase the solubility of N in the melt and in the vast majority of cases is essential to producing a ternary nitride. The reaction with N₂ is usually carried out at constant temperature. Small X-ray sized crystals grow slowly at the interface of the flux and the N₂ gas. Thus the flux acts more like a reaction medium than a solvent for the product, as is usually found and exploited in classical flux growth. The Na flux method rarely produces ternary compounds of metals that are not soluble in Na. While Co and Ni have very low solubility in Na, we hypothesize that the reaction proceeds by the formation of soluble intermediate nitride complexes of those metals.

More recently we have begun to explore the use of other metallic fluxes, such as elemental Zn, Ga, In and Sn and alloys of these main group metals with other species, such as Ge, Bi, Fe, Co and Ni. Each flux appears to enable the synthesis of ternary or quaternaries from different regions of the periodic table. Interestingly, we have also shown that some binary nitride pairs do not react, forming neither binary alloys nor ternary compounds. For example, binary Mg and La nitrides form even when Mg-La intermetallics or melts are reacted with either N₂ or NH₃. However, our understanding of the factors that determine multinary nitride compound formation is still rather poor

Some interesting compounds that we have made are shown below, 

Ag8Ca19N7 was made from a sodium flux and is an interesting example of a Zintl-like sub-nitride. Even if we count Ag as an electron acceptor (as Ag-), there are 2x19 - 3x7 - 8x1 = 9 excess electrons in this metallic compound, or equivalently not enough nitrogen (hence sub-nitride). The structure presents an interesting arrangement of cluster like units: Ag4 tetrahedra (in black) and Ca6N octahedra (tan).		Ln3TM2N6 (Ln = La, Ce or Pr; TM=V, Nb or Ta) compounds were first prepared as crystals up to several mm in dimensions in a Ga melt at 1600 oC, a temperature at which both Ln and TM have considerable solubility in Ga. The layered structure of this metallic compound has been reported previously for the superconducting oxide, La2-xSrxCaCu2O6 and for the insulating oxide Sr2Ln0.8Ca0.2Co2O6. In our nitride compounds the TM are all crystallographically equivalent with an oxidation state of 4.5+, so the materials must be metals. They are superconductors, but only below 4.2 K. The black circles are Ln atoms and the square planar units, TMN5		Micron sized crystals of Ca6[Cr2N6]H were made from a Li3N flux at 1000 oC. This compound confused us for a long time until we realised that instead of being a pure nitride it was a nitride-hydride i.e. both the hydrogen and nitrogen are anionic and are not bonded to each other. The hydrogen is in the centre of the green Ca6 octahedra (each vertex is a calcium atom), and the nitrogen and chromium are the lilac and purple spheres that make up the Cr2N6 unit

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Last updated July 2007.