Elpasolite-type structure

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. 

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. 

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... 

Another relationship to the elpasolite-t5T)e was mentioned earlier in the text the NaSbFe-structure, which has the same NaMeFe basic sceleton (page 8). The fluoride ions in both types occupy three quarters of the sites of cubic close-packing. In the elpasolite type the additional potassium ions fill up the remaining quarter. 

It is obvious that the elpasolite type of structure is preferred with A" " and B /R cations of (very) different sizes because is part of the closest packing and B and r3+ occupy the octahedral sites within the packing. Therefore, structural difficulties must arise when B" " becomes chemically equal with A+ as is the case in A3RX6 (= A2ARX6)-type halides. These difficulties are dealt with roughly in two ways ... 

Seifert and Yuan (1991) characterized the ABr-LaBr3 (A = Na, K, Rb, Cs) system and provided thermochemical data. The A = Na system is eutectic. The AjLaBrj (A = K, Rb, and Cs) compounds are stable at ambient temperature, but decomposed peritectically for A = Cs. At temperatures above 430°C A3LaBrg (A = Rb, Cs) crystallize in the cubic elpasolite structure common to the heavier lanthanides at ambient temperature the monoclinic K3MoClg-type structure prevailed, with A = Rb metastable. Cs2LaBr7 was stable over the limited 459 551 °C range. 

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. 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).

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