Fluorides Jahn-Teller distortion

The powder diagrams of the ternary mangan(III)-fluorides LiMnF4 [163) KMnF4 and RbMnF4 [166) could not be indexed yet. Possibly the Jahn-Teller distortion not only affects the MnFe --octahedra, but also the symmetry of the unit cells, which, at least in the case of the potassium-and rubidium-compounds, should be very similar to each other. 

The same structure with a Jahn-Teller distortion even more pronounced was observed in RbCuFs 8). It is therefore beyond doubt, that the remaining compounds of Cu + and Cr2+ crystallize in the KCuFg-structure type, even if x-ray powder diagrams do not prove this. The lattice constants of the fluorides belonging to this group — some of the chromium compounds have been prepared only recently by Vollmer 316) — are summarized in the following Table 26. 

Jahn-Teller distortions of octahedra are expected to be much smaller than in the cases mentioned above, if the orbital degeneracy occurs in the non-bonding d e-orbitals. Indeed no structures of fluorides are known, with the possible exception of FeF2 (24,130), in which distortions without doubt are effected by this degeneracy of rfs-orbitals. Weinstock and Claassen [322, 323) reported evidence of d5mamic Jahn-Teller effects on the vibrational properties of hexafluoride molecules, however. 

The complex fluoride CrZrFe (Table 31) is an example of a class of compound A M Fs which undergoes phase transitions as the temperature changes. In the chromium(II) and copper(II) compounds additional modifications occur because of local and cooperative Jahn-Teller distortions. From the three spin-allowed d-d transitions for high-spin Cr in strongly distorted CrFe octahedra (Table 31) an octahedral A value of 7600 cm is calculated. Transitions between the monoclinic IT, pseudotetragonal I and cubic I phases have been... 

Schwerdtfeger, P. Second-order Jahn—Teller distortions in group 17 fluorides EF3 (E = Cl, Br, I, and At). Large relativistic bond angle changes in AtFa. J. Phys. Chem. 100, 2968-2973 (1996)... 

Young has studied the Fe(II) spectrum in fluoride melts his results are shown in Table XXII and the spectrum in Li2BeF4 at 540°C in Fig. 18. The results in Table XXII show that the species is relatively insensitive to changes in both temperature and melt composition. The presence of two bands in the spectrum (rather than the single transition expected for an octahedral ion) was suggested to be due to a Jahn-Teller distortion of the octahedron. 

Silver(II) fluoride AgF2 is a genuine silver(II) compound exhibiting Jahn-Teller tetragonal distortion (4F at 2.07 A 2F at 2.59 A) it exhibits a low... 

A group of 8 ternary fluorides containing the transition metal ions Cr2+ and Cu + crystallizes in a tetragonedly distorted perovskite lattice. This distortion is caused by the Jahn-Teller effect displayed by the configurations d% d (Cr +) and d d (Cu2+) resp., rather than by geometrical reasons. As for their space requirements the ions Cr + and Cu + are very close in size to Mn2+ and Co + resp. and as a consequence the corresponding compounds do not differ in their tolerance factors. 

The lowering of symmetry in fluoride structures that contain Jahn-Teller ions instead of normal ones is explained this way. All cases of high-spin configurations d (Cr2+, Mn +) and (Cu2+) provide examples of distorted MeFa-octahedra. The overall distortion always leads to elongated octahedra, as accounted for theoretically by Opik and Pryce (234). 

Considering the influence of electronic configurations on crystal structures it may be asked, whether certain structure t5rpes are restricted to fluorine compounds of the transition elements. Apart from the structure types distorted by the Jahn-Teller effect such a limitation is not obvious at all. On the contrary quite a number of structure prototypes are represented by compounds of the main group elements. Bonding thus must be similar in both, main group and transition element fluorides, at least as for the factors that influence crystal structmes. 

The Jahn-Teller effect enhances the structural diversity of Cu(II) compounds [68], Most of the octahedral complexes of Cu2+, for example, show elongated tetragonally distorted geometry. Crystalline cupric fluoride and cupric chloride both have four shorter and two longer copper-halogen interatomic distances, 1.93 vs. 2.27 A and 2.30 vs. 2.95 A, respectively [69],... 

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