Fig. 65: FeCo alloy in the ordered CsCl structure (left) and in the disordered bcc phase (right). The ordered phase is composed of alternating layers of Fe and Co atoms stacked along the [001] direction; in the disordered phase each lattice point is occupied by Fe or Co atoms with equal probability. Fe and Co are neighbours in the periodic table resulting in a very weak one-electron X-ray scattering contrast.