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

Abstract

The frequency dependence of the critical fluctuations at the antiferrodistortive phase transition exhibited by the disordered crystals KMnn 99Mgn and KMno ,Mg" IF3 has been studied using neutron scattering techniques. In pure KMnF, the soft RaS mode is overdamped for temperatures below -(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 for pure KMnF, is not significantly increased by the addition of the impurities. This is consistent with measurements of the anomalous component observed close to T, 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, to (Tc + 20) K while the x-ray anomaly was only observed for a few degrees above T,. 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.

A High-Energy Monochromatic Laue (MonoLaue) X-ray Diffuse Scattering Study of KMnF3 using an Image Plate

“…4. The presence of this diffuse scattering is related to the existence in the perovskite compound of the flat RM branch of soft phonons, which has been extensively investigated in the past (Com6s, D6noyer, Deschamps & Lambert, 1971;Shapiro, Axe, Shirane & Riste, 1972;Almairac, Rousseau, Gesland, Nouet & Hennion, 1977;Cox & Cussen, 1989). A similar effect is observed in Fig.…”

On the question of dynamic domains and critical scattering in cubic methylammonium lead triiodide

“…[3] We note that the CP area and ξ do not diverge as T → T C as is observed in SrTiO 3 , KMnF 3 , RbCaF 3 , and CsPbBr 3 . [12,16,22,25,40,41] Critical exponents for these systems range from 1 to 2 for the CP intensity and 0.5 0.84 for the correlation lengths, both much larger than what we observe. In fact, the temperature dependence of the CP in d 6 -MAPI above T C is better described with a linear model.…”

The effect of impurities on the R-point instability in KMnF3. I. An X-ray scattering study

“…The relationship between the central peak observed in neutron scattering experiments and the anomalous component observed in x-ray scattering experiments is of interest since although the two features are not immediately comparable, they may be manifestations of a common physical mechanism. The work presented here, and in the companion paper (Cox and Cussen 1989), attempts to correlate the two phenomena and also to further clarify the effect of impurities on a displacive structural phase transition.…”

“…With pyrolitic graphite monochromator and analyser crystals in position, the The samples for x-ray work were cut perpendicular to a crystallographic (001) direction from the grown crystals and consisted in each case of a slice -2 mm thick. (The remainder of the grown crystals provided samples for the neutron scattering experiments described in Cox and Cussen (1989).) The active surface of the slices was polished and then etched for ten minutes in ortho-phosphoric acid to remove any surface damage.…”

“…The present work is an experimental investigation of the transition in two crystals of KMnF3 containing known proportions of the compound KMgF,. This paper reports the results of x-ray scattering experiments, and a companion paper describes neutron scattering measurements (Cox and Cussen 1989).…”