The Conditions of Formation of Nitrides

The affinity of nitrogen for other elements is not manifested at ordinary temperatures, but on heating combination often occurs, especially in the case of the metals (and hydrogen), which combine in an exothermic manner. The energy of combination with the more electro-positive metals (especially those of the rare earths) is considerable; thus cerium heated initially to a dull red heat in nitrogen soon becomes incandescent.

With non-metals the combination is usually endothermic, and occurs at very high temperatures; and this is the case also with those elements which are electro-negative to nitrogen, and which sometimes can only be caused to combine with it by indirect means. The formulae of the nitrides, in the cases where they have been definitely established, are usually those which are to be expected from the ordinary valency of the second element and the trivalency of nitrogen.

Nitrides can therefore be regarded as salts derived from the anhydro-acid, ammonia. In their chemical behaviour they either resemble salts of a very weak acid, being completely hydrolysed with the production of ammonia, or they are stable substances which may owe their chemical inertness to a high degree of polymerisation.

Typical formulae are M₃N, M₃N₂, MN, M₃N₄, M₃N₅, in which the valency of the nitride-forming element M runs from 1 to 5; for example,

Li₃N, Cu₃N, Ag₃N, Au₃N;
Ca₃N₂, Sr₃N₂, Ba₃N₂;
BN, AlN, AsN, SbN, BiN;
Si₃N₄, Ti₃N₄, Th₃N₄, U₃N₄;
P₃N₅, Nb₃N₅, Ta₃N₅.

1. Direct combination at moderate or high temperature with the element (in the case of metals the amalgams may be used). When heated in a current of nitrogen, or when they are made the poles of an electric arc in an atmosphere of nitrogen, the following will combine:
   
   Li, Mg, Ca, Sr, Ba, B, Al, some metals of the rare earths, including Ce and Th, also Si, Ti, Zr, V, Nb, Ta, Cr, U, and Mn.
   
   Lithium is one of the most reactive metals towards nitrogen; it gives the nitride at a red heat, or even in the cold. The nitrides of the alkaline earths are formed by heating the amalgams of the metals to a dull red heat in nitrogen.
   
   Manganese nitride is formed in the same way from manganese amalgam. Magnesium absorbs nitrogen at a still lower temperature, as may easily be shown by a well-known experiment. Metallic lanthanum absorbs nitrogen at 850° to 900° C. up to 9 per cent, of the weight of the metal.
   
   The nitrides of these reactive metals are usually dark powders which are easily hydrolysed by cold water with evolution of ammonia, e.g.
   
   Mg₃N₂+3H₂O = 3Mg(OH)₂+2NH₃.
   
   The reaction is in some cases so energetic that the addition of a little water raises the nitride, e.g. cerous nitride, to a red heat:
   
   2CeN+4H₂O = 2CeO₂+2NH₃+H₂.
   
   The formation of nitrides by this method has been much used to separate nitrogen, as well as oxygen, from mixtures of air with the inert gases. They also supply theoretical methods of fixing nitrogen, although up to the present these methods are not commercially feasible, except perhaps in the case of AlN.
   
   Chromium and manganese nitrides are not easily hydrolysed. Boron and titanium nitrides are inert powders, and are unaffected by water or aqueous acids. Silicon nitride, which is only formed at a high temperature, is also a very stable compound.
   
   
2. The Action of Carbon and Nitrogen on Oxides. - Aluminium nitride is formed when the oxide, mixed with carbon, is heated to a high temperature in a current of nitrogen or producer gas (nitrogen with carbon monoxide). In most cases, as in those of the oxides of the alkaline earth metals, this procedure gives, in addition, cyanide and cyanamide. Nitrides of zirconium, scandium, and niobium have also been prepared by this method.
3. The Action of Gaseous Ammonia on Metals or their Oxides. - When ammonia is passed over cupric or cuprous oxide at about 300° C., a nitride, having a composition approximating to Cu₃N, can be separated from the reduced copper by dissolving the latter in a mixture of ammonia and ammonium carbonate. Ammonia reacts with zinc dust at 600° C.; the product contains rather less nitrogen than is required by the formula Zn₃N₂. Nickel and cobalt at, or slightly below, 500° C. are converted by ammonia into nitrides of variable composition.
   
   A nitride of iron, of inconstant composition, but corresponding approximately to Fe₄N₂, was made by passing ammonia over the finely divided metal at 460° C. The reaction was further studied and the composition established by many researches. It is possible that the formation of nitrides by this method always takes place through the intermediate formation of amides and imides.
4. The Decomposition of Amides and Imides by Heat - The amide of zinc yields nitride when heated to 200° C.:
   
   3Zn(NH₂)₂ = Zn₃N₂+4NH₃.
   
   Borimide decomposes below 130° C.:
   
   B₂(NH)₃= 2BN+NH₃.
   
   Each of the intermediate compounds has been isolated, and the conditions of the successive changes determined in the case of arsenic:
   
   2As₂(NH₂)₃ = As₂(NH)₃+3NH₃ (at 60° C.);
   
   As₂(NH)₃ = 2AsN+NH₃ (at 250° C.).
5. The Action of Aqueous Ammonia. - Aqueous ammonia at ordinary temperatures converts the oxides of silver and gold into the explosive nitrides, Ag₃N, Au₃N, and Au₃N₂ respectively.
6. Reactions in Liquid Nitrogen. - The nitrides of tin, lead, and cadmium have been prepared by passing an electric arc between electrodes of the metals immersed in liquid nitrogen.
7. Reactions in Liquid Ammonia. - Double decompositions which occur between halides and amides in liquid ammonia generally yield complex double amides, often with ammonia of crystallisation. But in some cases a nitride is produced, e.g. the nitride of bismuth, by the reaction between the bromide and potassium amide:
   
   BiBr₃+3KNH₂ = BiN+3KBr+2NH₃.
   
   
8. The Action of Dry Ammonia on Anhydrous Chlorides (Chlor-anhydrides). - This reaction is applicable more especially to the chlorides of the non-metals, and particularly those of the fifth and sixth groups.