The nickel-rich portion of the nickel–cadmium system

The constitution of the Ni–Cd phase-diagram has been explored by X-ray diffraction and microscopy, principally at its high-nickel end, but also with reference to the overall system. No primary solid solubility of Cd in Ni exists. The NiCd (β) compound equilibrates directly with nickel, with no further compound intervening. The unknown crystal structure of NiCd has been determined as being of the η-carbide (E9₃) type, with a₀ (typically) = 11.387 Å and X-ray density 9.18 g.cm⁻³. NiCd has a small homogeneity range and decomposes peritectically at about 680°C. The only other compound occurring in the system is Ni₅Cd₂₁ (γ). An earlier reported `Ni₂Cd₅' (γ) could not be confirmed, though γ′ could compositionally be a dimorph to γ. Three-phase coexistence of Ni, NiCd and Ni₅Cd₂₁ can occur unstably or in segregation, the constitution being sensitive to heat-treatment and to the large vapour-pressure differences. NiCd seems to represent a new class of η-carbide type intermetallic phases, in containing a non-transition metal and excluding any early-Group transition metal (e.g. IVA) so far thought to be essential. The wider implications of NiCd having this type structure are discussed briefly, and the (high Fe, Co, Ni, Cu)–(Cd, Zn) phase relations compared in series.