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 Ni2Mo

Pt2Mo and Ni3Mo DO22. It also simplifies the
visualization of the diffraction patterns for these

phases, c.f., section 2.3.4.2.

2.2.1.2 Ni3Mo


 The Ni3Mo 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 Cu3Ti. 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 D022

tetragonal cell. The prominent 420 planes of Mo are now

separated by three 420 planes of Ni instead of four Ni

planes as in Ni4Mo. Figure 2.8b describes this packing.

2.2.1.3 Ni2Mo


 The Ni2Mo phase was first discovered by Saburi et al.

(1969). It is a Pt2Mo type superlattice, as shown in