For pt.I see ibid., vol.1, p.3565 (1989). The frequency dependence of the critical fluctuations at the antiferrodistortive phase transition exhibited by the disordered crystals KMn_0.99Mg_0.01F₃ and KMn_0.9Mg_0.1F₃ has been studied using neutron scattering techniques. In pure KMnF₃ the soft R₂₅ mode is overdamped for temperatures below approximately (T_c+40) K. A similar behaviour is observed in the disordered crystals, but the damping constant is increased by the presence of the impurities. The amplitude of the central peak observed in the spectral response of pure KMnF₃ is not significantly increased by the addition of the impurities. This is consistent with measurements of the anomalous component observed close to T_c in complimentary X-ray scattering studies, which showed that this component was reduced by the addition of impurities. The results further indicate that although the temperature dependence of the anomalous scattering is similar in both the X-ray and neutron scattering experiments, the neutron central peak persists over the temperature range T_c to (T_c+20) K while the X-ray anomaly was only observed for a few degrees above T_c. Thus, while the anomalous scattering measured using the two techniques is related to the impurity concentration in the disordered crystals, the two features must be to some extent distinct.