Long range order in Ag3Mg

The long range order in a powder sample of Ag3Mg has been studied by X-ray diffraction as a function of quenching temperature below the critical ordering temperature of 665°K. By measuring the positions and integrated intensities of fundamental, superlattice and satellite reflections, the latter arising from the periodic anti-phase boundary structure, the variation with temperature of the overall long range order, antiphase boundary order, and anti-phase boundary periodicity have been determined. It appears that the anti-phase boundaries disorder slightly more with increasing temperature than the average superlattice, and that the boundary spacing, nominally equal to two (M = 2), increases towards that ideal value with increasing disorder below the critical point. These resalts are qualitatively interpreted by extending the reasoning of Sato & Toth on equilibrium long period superlattices to include the case where the Fermi surface becomes progressively more diffuse with increasing disorder.