Abstract
The transformation of nickel lattices under the influence of nitrogen as an insertion impurity has been followed by electron diffraction after nitriding thin monocrystalline evaporated films of nickel in an atmosphere of ammoniac at different temperatures.
As established previously, the nitriding process occurs as follows: Ni(f.c.c., a=3.52 Å)→Ni4N(f.c.c., a=3.72 Å)→Ni3N(hexagonal).
In this work, a detailed study of some anomalous diffraction spots reported, but not interpreted in a previous work, has been undertaken. It has been found that these anomalous diffraction spots appear rather clearly on the specimens nitrided at 230–240°C and they belong to a new form of Ni4N which is no longer cubic but tetragonal. Its lattice parameters are:
a=b=3.72 Å;c=2c*=7.28 Å
These interpretations provide evidence of existence of two forms of Ni4N: one cubic: Ni N(I) and the other, tetragonal: Ni3N(II). The tetragonal form Ni4N(II) is derived from the cubic form Ni4N(I). This occurs as follows: Two elementary lattices of cubic Ni4N(I) join together after contraction along [c] axis (from c=3.72 Å to c*=3.64 Å) and form a unit cell of the tetragonal form. The two other axes [a] and [b] conserve their initial value of 3.72 Å(c*⁄a=0.98).
