Ordering patterns antiferrodistortive

Fig. 8 Orbital ordering in pseudo-cubic perovskite KCUF3 with vertex-sharing octahedrons [CuFe]. Type a is layer-antiferrodistortive and Type d is layer-ferrodistortive pattern. (After Kugel and Khomskii [19], Reinen and Friebel [16], and Binggeli and Altarelli [20])...

In general the antiferrodistortive pattern is more stable than the ferrodistortive order in a 3-dimensional frame-work of comer-connected octahedra (jperov tes, lead nitrocomplexes). As discussed just before the antiferrodistortive order leads to appreciable deformations of those octahedral sites which are occupied by non-Jahn-Teller ions in elpasolite-type stmctuies. Thus the ferrodistortive order may be stabilized also (oxidic elpasolites, alkaline earth nitrocomplexes). The lower transition temperatures of the lead compared to the alkaline earth compounds indicate weaker cooperative-elastic interactions and are possibly caused by the less distinct distortion of the CuNg polyhedra (Table 3). Finally it should be mentioned that the g tensor in the (001) planes of the 7-modifications has an orientation [K 15° ... 

We interpret this behavior as caused by excited vibronic levels vdiidi are partly occupied at higher temperatures. After all the angular dependence of the g tensor in the /3-phases [Eq. (30)] still indicates the presence of a static antiferrodistortive pattern as in the T modifications udiidi is, however, superimposed by a planar dynamic Jahn-Teller effect The g(ic) parameters, characterizing the planar delocalization in the excited state ( planar dynamics ) [Eq. (31)] and the static antifenodistortive order [Eq. (25)], respectively, are identical. Only the small lower munetry component of the unit cells vthich induces Ihe cos 2 9 term in Eq. (30) allows to suggest the superimposed planar dynamic behavior as discussed. Further evidence for a delocalization of this type is obtained from the EPR line width vdiidi exhibits a pure cos ... 

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