Perovskite fluoride

Cobaii(II) fluoride, C0F2. Forms 4,3,2,0 hydrates and gives perovskites, e.g. KC0F3. 

Manganese(If) fluoride, Mnp2. Forms hydrates (Mnp2 obtained by heating NH MnFs telrahydrate from aqueous solution). Solutions are hydrolysed. Insoluble perovskites, e.g. KMnFs, formed from solution. 

F can he suppressed hy the high site symmetry of the central atom In many perovskite-like structures of the ABO3 type the lone pair of the B-cat-ion leads not to a structural distortion. In CsPbF3 under ambient conditions no lone-pair activity observed [27], but upon cooling a phase transition is observed that leads to less symmetrical surrounding of Pb by fluoride [28]. 

RbCaF3 has the perovskite structure with the Ca in the center of the unit cell. What is the electrostatic bond character of each of the Ca-F bonds How many fluoride ions must surround each Ca2+ ion What is the electrostatic bond character of each Rb-F bond How many F ions surround each Rb+ ... 

The structures of the compounds AMeFs are closely related to each other and can be derived from the well known perovskite structure. Therefore they may be generalizing referred to as fluoroperovskites, although some deformations of the cubic perovskite t e may occxir orthorhombic, tetragonal and hexagonal structures have been observed in ternary fluorides, in addition to the basic cubic type. 

The two mentioned ternary fluorides of cadmium with their tolerance factors of 1.00 and 0.88 resp. mark quite accurately the field of existence of cubic perovskites. As may be seen from the following Table 25 the tolerance factors of all cubic fluoroperovskites of the transition metals hitherto known lie within the range of these limits. 

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 structure of these compounds was elucidated by the work of Okazaki and Suemune (236) on the fluoride KCuFj. The unimolecular tetragonally compressed perovskite cells of previous reports (90) do not account for additional reflections observed in single crystal work. Instead one has to conceive a unit cell containing z = A formula units... 

The same orthorhombic deformation of the perovskite structure, that Geller 111) reported of the ternary oxide GdFeOs, is according to Rudorff et al. 268, 270, 271), also present in the sodium compounds NaMeFs. The unequal sizes of the Na+- and fluoride-ions bring about a considerable distortion of their common close-packing. To describe the structure a... 

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. 

There exists quite a number of hexagonal oxidic perovskites 183, 332), but there seem to be only three types in the case of ternary fluorides. Their occurrence again clearly depends on the tolerance factor wich thus proves to be useful in classifying the hexagonal perovskites also. After having described their structures in detail they will be further discussed under a common point of view. 

The structure of the hexagonal oxide perovskite BaRuOa, recently described by Donohue et al. 84), is also adapted by the ternary fluoride CsCoFs 11). The positional parameters (not listed above) are almost the same in both compounds. 

On the whole ternary fluorides of the bivalent transition metals are least affected by polarisation. This is partly the reason why in the fluoro-perovskites Goldschmidt s tolerance relation is much more definite, than... 

Machin, D. R. L. Martin, and R. S. Nyholm Preparation and magnetic properties of some complex fluorides having the perovskite structure. J. Chem. Soc., 1490 (1963). 

Permanent electrical multipole term Uq, lattice energies and, 1 176-177 Pemitrous acid, 22 147 Perovskites, ordered, 35 354, 370-371 Perovskite type fluorides, 20 152-166 Peroxides, see also specific compounds fluorinated, 16 109-168 fluoroaUcyl, spectral properties of, 16 154, 155... 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

The salts KCrF3 and RbCrF3 have distorted perovskite structures, and from analysis of powder data the former contains tetragonally compressed CrFe octahedra in which two Cr—F bond distances are 2.00 A and four 2.14 A.244 The ionic radius of Cr23- in fluoride perovskites has been estimated at 0.73 A as a weighted average from lattice constants.245... 

Fig. 13-11.—The structure of the cubic crystal KMgF3. Potassium ions are represented by large shaded circles. They are at the corners of the unit cube. The fluoride ions, represented by large open circles, are at the face-centered positions, and the magnesium ions, represented by small circles, are at the center of the cubes. This structure is often called the perovskite structure perovskite is the mineral CaTiOj.

Fig. 16.2 Idealized structures of some perovskite-related layered oxides and their fluoride relatives.2)...

In all the perovskite compounds, the A ions are large and comparable in size to the oxygen or fluoride ion. The B ions must have a radius for six-coordination by oxygen (or fluorine). Thus, the radii of A and B ions must lie within the ranges 100 to 140 pm and 45 to 75 pm, respectively. 

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