Compounds elpasolites

Fig. 2. Characteristic structural features in AI2BIREIIIX6-type compounds ( elpasolites ). Sections in [00.7] direction show the principal connection of [BXfi and REX octahedra (see text).

Table 12. Elpasolite type. Lattice constants of isostructwal compounds...

For A3MF6 compounds with larger alkali cations, only a few single-crystal investigations are known. It seems to be clear that K3M0F6 and KsWFe crystallize in the cubic elpasolite-type structure, as well as most of the (NH4)3M F6 compounds at room temperature. For (NH4)3ScF6, a monoclinic structure variant has been reported recently. Very complicated superstructures with five-fold axes have been fonnd in different laboratories for some K and Rb componnds (see citation in Ref 33). However, no stracture determinations are known to date. 

While all these stmctnres derive from the elpasolite aristotype, the Li3M F6 componnds (M = Al, Ga, Ti, V, Cr, Fe) exist at room temperatnre in two modifications. The a-form (e.g. a-LisAlFe ) is still a strongly distorted variant of the elpasolite/cryolite family in which the coordination of all lithium ions is reduced to six. The /3-form, determined at /S-LisVFe and confirmed by recent single-crystal work on the Al, Ga, Ti, and Cr compounds (Ref 82 and literatnre cited... 

Perovskite Relatives with Vacancies on Octahedral Sites. A special derivative of the elpasolite-type structure A A M Fe with half-occupation of the A site has only been found in compounds of Jahn-Teller ions Mn + and Cu +. K2(no,5MnJJ5)Mn F6 = K4Mn3Fi2, CsBa(Do.5CuJ) 5)Cu F6 = Cs2Ba2Cu3Fi2, and recently Cs2(Do.5P( 5)Mn F6 showthe vacancies on the octahedral sites ordered in layers like in the chiolite structure (see Section 3.2.5) and stacked and connected according to a 4i screw axis (s.g. I4i/amd, Figure 18(a)) to form a 3D network. 

Every second octahedral site of the elpasolite-type structure is unoccupied in compounds A2M F6 of the K2SiF6 type, known also as the K2PtCl6-type (s.g. Fm3m, Figure 18(b)). This structure can also be seen as an antifluorite structure if the whole complex [MFg] anion is compared with Ca. It can be observed for nearly all alkali hexafluorometallates(IV) of the first transition metal series as well as some noble metals like /3-Cs2PtF6. However, for many such compounds other polymorphs are known, too, and the cubic form is the HT-phase only. These alternative structures are mentioned in the next section. A survey on all of the main structme types mentioned up to now derived from the ReOs/VFs type structnre is given in Table 3. 

Table 4 Numbers and stacking schemes of close-packed [AF3] layers in trigonal and hexagonal fluoride perovskites , elpasolites , and derivatives and typical compounds (cubic perovskite included, structure types in italics, space groups in parentheses)...

Fig. 9 The d-d spectra of the rhombohedral elpasolite Cs2NaCo "F5 and of compounds ACo "F4 (adopted from [2,35]). The assignment and fitting of the quintet-quintet and of the spin-forbidden quintet-triplet (top-listing) transitions -better resolved in the case of the ACoF4-solids - is according to tetragonaUy compressed Co F -octahedra the ligand field parameters A, B, C and Si (82 was taken from DFT), derived for the elpasolite-type solids, are listed in Table 5...

We therefore developed a much simplified, but mathematically exact, version of a shell model which is particularly suitable for the analysis of cross relaxation in high symmetry systems such as the lanthanide elpasolites. Since the vibronic structure in the emission and absorption spectra of these compounds is both intense and broad (relative to the electronic origins), there are many cases where the vibronic structure of one electronic transition in emission overlaps with the vibronic structure of another electronic transition of a chemically identical ion in absorption. The emission of the excited ion may then be partially quenched by energy transfer. Implicit in this formulation is the assumption that the interionic coupling is weak compared with the vibronic coupling this is certainly true for the lanthanide elpasolites where the lanthanide ions are separated by distances of more than 0.7 nm. We refer to this process as cross relaxation by the electric dipole vibronic-electric dipole vibronic (EDVEDV) mechanism. 

This review covers the series La3+ (4f°) to Lu3+ (4f14), including also Y3+, which are collectively denoted by Ln3+. It focuses upon neat and doped hex-ahalide compounds with Ln3+ situated at equilibrium sites of octahedral symmetry with the elpasolite structure, M2ALnX6 (M+, A+ are univalent cat-... 

Perovskite Relatives with Vacancies on Octahedral Sites. A special derivative of the elpasolite-type structure with half-occupation of the A site only been found in compounds of Jahn-Teller ions Mn + and Cu +. K2(Do.5MnQ5)Mrf F6 = K4Mn3Fi2, CsBa(no,5CuQ 5)Cu F6 = Cs2Ba2Cu3Fi2, and recently... 

The local Jahn-Teller effect in isolated octahedra is the subject of the next chapter. Experimental examples of Cu doped into compounds of perovskite and elpasolite structure wiU be given. In Chap. Ill the cooperative Jahn-Teller effect as the consequence of high concentrations of Jahn-TeUer centers in a solid matrix will be considered. In Chap. IV we discuss as illustrative model examples the highly interest-... 

Fig. 8. Ferrodistortive order of elongated octahedra in elpasolite-type compounds A2BB X [B (open circles) non-Jahn-Teller cations]...

Experimental data with respect to cooperative Jahn-Teller ordering in perovskite and elpasolite compounds as well as in lattices with isolated octahedra are collected in Table 2 for transition metal ions with electronic Eg ground states. The most inter-Chaps. II and III can be summarized as follows ... 

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