The concentration dependence of the room-temperature lattice constant and the Néel temperature of [formula omitted] [formula omitted] disordered and ordered alloys was investigated in order to clarify the difference in the antiferromagnetic properties. The room-temperature lattice constant increases with increasing x and the difference between the disordered and ordered alloys was hardly observed. The Néel temperature increases with increasing x in both the disordered and ordered states. This behavior is not explained by the variation of the strength of the exchange interaction as a function of the Mn–Mn distance but by the number of [formula omitted] electrons in the Mn site. The Néel temperature in the ordered state is 130–160 K higher than that in the disordered state, indicating that the exchange interaction is enhanced by ordering.