The effect of impurities on the R-point instability in KMnF3. II. A neutron scattering study

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Abstract
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 KMn0.99Mg0.01F3 and KMn0.9Mg0.1F3 has been studied using neutron scattering techniques. In pure KMnF3 the soft R25 mode is overdamped for temperatures below approximately (Tc+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 Tc 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 Tc to (Tc+20) K while the X-ray anomaly was only observed for a few degrees above Tc. 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.