Excess Nickel-Arsenides

Striking support for this model comes from the system Fe Ge, where 1.5 x 2.0 (703,704). In the compositional range 

---

The A2 metals and the elements of the earlier B subgroups (Bj metals) form the electron compounds already discussed. With the metals of the later B subgroups the A2 metals, like the Aj, tend to form intermetallic phases more akin to simple homopolar compounds, with structures quite different from those of the pure metals. The nickel arsenide structure has, like typical alloys, the property of taking up in solid solution a considerable excess of the transition metal. From Table 29.12... 

The widespread occurrence of the nickel arsenide and related structures in T2-B2 systems is illustrated by the data in table 13.05. Where the composition is given in the form MN it is to be understood that the simple nickel arsenide structure normally obtains, but that in general a range of solid solution on each side of the equiatomic composition is found. In a few cases, however, the range of homogeneity does not include the simple composition, and phases occur only with an excess of one component. The copper-tin system is an example of this, for the phase shown as CuSn does not exist at this composition but only with an excess of interstitial copper. A similar state of affairs obtains in the other systems marked with an asterisk in the table. 

A series of complex silico-arsenides has been obtained 6 by melting metals with silicon and an excess of arsenic under a layer of molten cryolite and sodium chloride. The following have thus been prepared copper silico-arsenide, a grey crystalline brittle mass zinc silico-arsenide, which behaved as above with hydrochloric acid iron, cobalt and nickel siMco-arsenides, of composition M2SisAs4, similar in appearance to the copper compound. When platinum was treated in the same way, a hard white product of indefinite composition was obtained, almost insoluble in nitric acid. 

---