1. Compare the diffraction patterns of SC, BCC & B2 structures
Compare the diffraction patterns of:  SC (e.g. Po)  BCC (e.g. Disordered Ni0.4Al0.6, W, cI2)  B2 (NiAl, ordered BCC  SC with two atom motif)
The disordered Ni0.4Al0.6, has a BCC lattice and hence by selection rules only peaks have (h + k + l) = even are observed and others are missing.
Both Po and NiAl (ordered) are simple cubic→ but there is a distinct difference in the diffraction patterns (next slide).
Po is “simple” simple-cubic, while NiAl is ordered simple cubic with a two atom motif!
Lattice
E.g.
(100)
(110)
(111)
(200)
(210)
(211)
(220)
(300) (221)
(310) SC Po 
BCC
Disordered Ni0.4Al0.6 
NiAl (ordered)
 (weak)
(weak)
Superlattice reflections

2. Note the correspondences
Po (SC)
All peaks present
Note the correspondences
Probabilistic occupation of each BCC lattice site: 40% by Ni, 60% by Al
Ni0.4Al0.6 (disordered, B2)
Superlattice reflections (weak)
NiAl (ordered, SC)