20
every fifth 420 plane of the FCC parent lattice (Okamoto
and Thomas, 1971), as described in Figure 2.7. This
stacking sequence is pertinent. With slight variation it
can also describe the stacking sequences of both N2M0
Pt2Mo and N3M0 DO22 It also simplifies the
visualization of the diffraction patterns for these
phases, c.f., section 2.3.4.2.
2.2.1.2 N3M0
The N3M0 phase exists stoichiometrically as both an
orthorhombic equilibrium phase, where a = 5.064 Angstroms
b = 4.448 Angstroms, and c = 4.224 Angstroms, and as a
metastable DO22 superlattice phase where a* = b' = 3.560
Angstroms, and c' = 7.12 Angstroms. Note that
c(FCC)=c'/2=3.560 Angstroms. The orthorhombic structure
was determined by Saito and Beck (1959) and was shown to
be isostructural with CU3T. The DO22 phase is an
equilibrium phase in the Ni-V system (Tanner, 1968). In
the Ni-Mo system it is not. Figure 2.8a shows the DO22
tetragonal cell. The prominent 420 planes of Mo are now
separated by three 420 planes of Ni instead of four Ni
planes as in N4M0. Figure 2.8b describes this packing.
2.2.1.3 N ip Mo
The N2M0 phase was first discovered by Saburi et al
(1969). It is a Pt2Mo type superlattice, as shown in