Compounds of the formMn2−xCrxSbshow a first-order ferrimagnetic to antiferromagnetic phase transitionwith decreasing temperature. Sublattices composed of the three adjacent ferrimagnetically aligned (001)planes Mn(II)-Mn(I)-Mn(II) are ferromagnetically ordered above the transition temperatureTsandantiferromagnetically ordered belowTs. The exchange interaction between these sublattices changes signat a critical interlayer distance. The discontinuous change in interlayer distance atTsis proportional to thesquare of the magnetization of the sublattice in agreement with Kittel's conclusions.