Fluorides structure

Crystals of the intermetallic compound magnesium stannide, MgjSn, have been prepared and investigated by means of Laue and spectral photographs with the aid of the theory of space-groups. The intermetallic compound has been found to have the calcium fluoride structure, with dwo = 6.78 0.02 A. U. The closest approach of tin and magnesium atoms is 2.94 0.01 A. U. 

FIGURE 7.5 The calcium fluoride structure (also known as the fluorite structure). 

This type of orthorhombic perovskite structure appears, if the tolerance factor of Goldschmidt is smaller than t — 0.88. The example of the compound NaMnFs [t = 0.78), showing doubled lattice constants a and h (287), is likely to mark the lower limit of the field in which orthorhombic fluoro-perovskits of the GdFe03-t3q>e may occur. Fluoroperovskites which have a smaller tolerance factor than t = 0.78 never have been observed so far, nor do fluoride structures of the ilmenite type seem to exist, which might be expected for ya = Me, corresponding to 1=1/1/2=0.71. 

The anions in ternary fluorides always exceed the other ions present in number and often in size, too. In most fluoride structures close-packing (including very often the A-ions), is determined by the fluoride ions. The smaller Me-ions are embedded in the matrix thus given. 

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). 

Ludekens, W. L. W., and A. J. E. Welch Reactions between metal oxides and fluorides some new double fluoride structures of type ABF,. Acta Cryst. 5, 841 (1952). 

Table 3. Examples of oxide, nitride and fluoride structures in which the cation arrays are alloy structures...

Three oxide halides T10X (X = F, Cl, Br) are known.1 Preparations must be carried out at low temperatures1 387 since the heavier oxyhalides in particular readily decompose. There is no structural information on either TlOCl br TlOBr, but X-ray crystallography shows that TlOF has a calcium fluoride structure, with Tl3+ ions in a cube of F" or 02 ions.388... 

The elements form oxides M20, which have the anti-fluoride structure for li-Rb. Cs20 has the very... 

Sulfonyl Fluorides. Sulfonyl fluorides inhibit serine proteases by reacting with the active-site serine residue. Previously we investigated the rates of inhibition of human leukocyte elastase and cathepsin G by a variety of sulfonyl fluorides and found relatively little selectivity or reactivity (38). However, we have discovered recently that the introduction of fluoroacyl groups into the sulfonyl fluoride structure gives considerable reactivity and selectivity for elastase (39). 

In the mixed chloride/fluoride structure [NH4]2[SbCl3F2] (39) (53), a polymeric structure related to L is present, as shown in M, the main difference being that the bridging chlorides (all fluorides are terminal) are bent rather than linear, as in L. 

Gabuda, S. P., Gagarinskii, Yu. V., Polischuk, S. A. NMR in Inorganic Fluorides Structure and Chemical Bonding. Moscow Atomizdat 1978... 

Table 3 Main fluoride structure types [italics) derived from ReOs-type (left) and its distortion variant VFs-type (right) with site occupation schemes, space groups in parentheses...

In the tellurium(IV) fluoride structure with chains of corner-sharing TeFs tetragonal pyramids (100), the lone pair (E) is stereochemically much more active and occupies one position in the (/(-octahedral TeFsE polyhedra, in accordance with simple VSEPR considerations (Fig. 2). 

The unit cell volume is in all cases close to 500 A which on the basis of Zachariasen s criterion[30] that the cell volume of a fluoride structure is primarily determined by the number of fluorine atoms (of effective volume 16 A in Irp3[12] and Rhp3[19]) indicates 8 formula units per unit cell. Certainly the observed line intensities are consistent with eight metal atoms in the special positions 0,0,0 7, 7, 7 (2,1.O y,0, i and 0, 7, t). Thus each metal atom (as in the UCl structure) is surrounded by a flattened tetrahedral set of metal atoms at a distance of. y/ao + bo+Co /4A. The ideal values of important distances and angles are given in Table 5. Calculations... 

Figure 2. Idealized structures of some perovskite-related layered oxides and their fluoride relatives. Fluoride structures adapted from Ref. [3].

Efremova and Matizen (11) reported that the phase transition for SrF occurs over a short temperature interval (1421-1484 K). There are no discontinuities in their enthalpy data in this temperature range. Other alkaline earth dihalides (1J ), which have the calcium fluoride structure, are known to exhibit similar behavior, and this fact would seem to rule out the possibility that the observed transition resulted wholly from impurities. We speculate that the two crystalline forms, a and 0, of SrPg are practically... 

Haloanions of bismuth(V) are limited to the fluorides. Halobismuthates(III) and organohalobis-muthates(III) exhibit a range of structures and among the heavier halogens there is considerable similarity to their antimony (III) analogues. The halobismuthate(III) (not fluorides) structures have been reviewed by Fisher and Norman. ... 

The early characterization of a low-volatility radon-fluorine compound by combination of radon with fluorine at 400 °C 68 l01) was followed by work which showed that 222Rn is oxidized by chlorine and bromine fluorides, IF7102 105> or [NiF6]2- 103 -105> in HF to give stable solutions of radon fluoride. On the basis of electromigration studies it has been suggested that the radon in these solutions is present as [RnF]+ or [Rn]2+ 104-105), but [Rn2F3]+ also seems a likely possibility. The application of relativistic quantum mechanics to radon fluoride structures also shows that ionic formulations are favoured 106>. 

---