Second‐Harmonic Generation and the Ordering Reaction in NH 4 Cl

The order‐disorder transformation in ammonium chloride is taken to be a first‐order phase change. The asymptotic behavior of the second‐harmonic intensity is determined on the assumption that the growth of the ordered domains is controlled by the stress field around crystal defects. By a suitable choice of the characteristic temperature T 0 the data can be reconciled with nucleation along edge dislocations. The location of T 0 relative to the hysteresis region suggests that the irreversibility of the transition is associated with the formation of vacancies.