Jahn-Teller site deformations

One of plausible candidates for the entropy source is a dynamic structural disorder in the HS phase, which should be settled down in the LS phase. The crystallographic data for [Mn(taa)] [11] provide a clue, i.e., the presence of C3 axis in the HS molecule. An Mn(III) ion in the 5E state is a well-known Jahn-Teller ion [19]. Since the C3 site-symmetry cannot lift the orbital degeneracy of the 5E term (Fig. 1(b)), it is likely that the Mn ion is subjected to the E e Jahn-Teller effect, which gives rise to three energetically equivalent deformation structures. The apparent C3 symmetry should be observed in a time-averaged structure over three deformed structures. 

The site preferences shown by cations in the spinel structure demonstrate that transition metal ions prefer coordination sites that bestow on them greatest electronic stability. In addition, certain cations deform their surrounding in order to attain enhanced stability by the Jahn-Teller effect. These two features suggest that similar factors may operate and cause enrichments of cations in specific sites in silicate structures, leading to cation ordering or intersite (intracrystalline ) partitioning within individual minerals which, in turn, may influence distribution coefficients of cations between coexisting phases. 

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° ... 

---