Rutile Type

Binary fluorides MeF2 shall not be discussed here in detail. With the exception of the fluorite structures of CdFa and HgFa they crystallize in the tetragonal rutile type. In this structure the MeFg-octahedra share opposite edges and form chains that are at the same time 3-dimensionally linked in corners. The Me—F—Me angles in the chains are 90° and between them about 130°. 

MonocUnic distorted variants of the rutile tj e are found in the difluorides of the ions Cr + and Cu2+, subjected to the Jahn-Teller effect 33, 172). In both cases the MeFg-octahedra appear elongated. The same structure is assumed of AgFg, unfortunately obtained in an amorphous state usually 223). 

The lattice constants of the fluorides of the rutile type, the x-ray work on which is owing to Stout and Reed 306) and Baur 23, 24, 25) are listed in the following Table 29. Finally Bartlett and Maitland 19) reported the rutile structure for PdFg also. 

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Sn02, cassiterite, is the main ore of tin and it crystallizes with a rutile-type structure (p. 961). It is insoluble in water and dilute acids or alkalis but dissolves readily in fused alkali hydroxides to form stannates M Sn(OH)6. Conversely, aqueous solutions of tin(IV) salts hydrolyse to give a white precipitate of hydrous tin(IV) oxide which is readily soluble in both acids and alkalis thereby demonstrating the amphoteric nature of tin(IV). Sn(OH)4 itself is not known, but a reproducible product of empirical formula Sn02.H20 can be obtained by drying the hydrous gel at 110°, and further dehydration... 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

Co2Nb03F3 was obtained as a result of the thermal treatment of CoNbOF5, predominantly prepared by the hydrofluoride method [129]. This compound crystallizes in a rutile-type structure that can be achieved due to the statistical distribution of cations within the oxyfluoride octahedrons. 

Oxyfluoroniobates, M2Nb05F, containing trivalent metals (where M = Ti, V, Cr) have the same type of structure [264], except for Cr2Nb05F, which has a tri-rutile type structure. This exception is related to the ordered, rather than statistical, distribution of chromium and niobium cations in the oxyfluoride octahedrons, which leads to a corresponding increase in cell parameter c. 

Inter-Atomic Distances in Rutile Type Crystals and in Anatase... 

Probably the high calculated inter-atomic distances in the oxides are due to our method of using the crystal radii. The substitution in Equation 13 of z = 4 for the cation and z = 2 for the anion, instead of z = V8 for each, would lead to high calculated values in case the anion is much smaller than the cation, as in the rutile type crystals and anatase. 

The data for rutile type crystals are given in Table XVI. 

This theoretical result is completely substantiated by experiment. Goldschmidt,31 from a study of crystal structure data, observed that the radius ratio is large for fluorite type crystals, and small for those of the rutile type, and concluded as an empirical rule that this ratio is the determining factor in the choice between these structures. Using Wasastjerna s radii he decided on 0.67 as the transition ratio. He also stated that this can be explained as due to anion contact for a radius ratio smaller than about 0.74. With our radii we are able to show an even more satisfactory verification of the theoretical limit. In Table XVII are given values of the radius ratio for a large number of compounds. It is seen that the max-... 

The Radius Ratio for Fluorite Type and Rutile Type Crystals... 

In this discussion, two mutually canceling simplifications have been made. For the transition value of the radius ratio the phenomenon of double repulsion causes the inter-atomic distances in fluorite type crystals to be increased somewhat, so that R is equal to /3Rx-5, where i has a value of about 1.05 (found experimentally in strontium chloride). Double repulsion is not operative in rutile type crystals, for which R = i M + Rx- From these equations the transition ratio is found to be (4.80/5.04)- /3i — 1 = 0.73, for t = 1.05 that is, it is increased 12%. But Ru and Rx in these equations are not the crystal radii, which we have used above, but are the univalent crystal radii multiplied by the constant of Equation 13 with z placed equal to /2, for M++X2. Hence the univalent crystal radius ratio should be used instead of the crystal radius ratio, which is about 17% smaller (for strontium chloride). Because of its simpler nature the treatment in the text has been presented it is to be emphasized that the complete agreement with the theoretical transition ratio found in Table XVII is possibly to some extent accidental, for perturbing influences might cause the transition to occur for values a few per cent, higher or lower. 

Toyoda T, Tsuboya 1, Shen Q (2005) Effect of rutile-type content on nanostructured anatase-type Ti02 electrode sensitized with CdSe quantum dots characterized with photoacoustic and photoelectrochemical current spectroscopies. Mater Sci Eng C 25 853-857... 

The catalyst composition has a role in the control of selectivity. The rutile-type V/Sb/(Nb) mixed oxide activates the hydrocarbon and ammonia. However, most of the ammonia is burnt to N2, rather than being inserted on the hydrocarbons this likely occurs because the catalyst is not veiy efficient in the generation of the selective Me=NH species when reaction temperatures lower than 400°C are used (11). In fact, with all catalysts the selectivity to A -containing compounds increased when the reaction temperature was increased, and the selectivity to N2 correspondingly decreased (Figure 40.6). The dilution of the active phase with tin... 

In order to specify the structure of a chemical compound, we have to describe the spatial distribution of the atoms in an adequate manner. This can be done with the aid of chemical nomenclature, which is well developed, at least for small molecules. However, for solid-state structures, there exists no systematic nomenclature which allows us to specify structural facts. One manages with the specification of structure types in the following manner magnesium fluoride crystallizes in the rutile type , which expresses for MgF2 a distribution of Mg and F atoms corresponding to that of Ti and O atoms in rutile. Every structure type is designated by an arbitrarily chosen representative. How structural information can be expressed in formulas is treated in Section 2.1. 

Several additional, more complicated structure types are known for ionic compounds. For example, according to the radius ratio, one could expect the rutile type for strontium iodide (rSr2+ /i = 0.54). In fact, the structure consists of Sr2+ ions with a coordination number of 7 and anions having two different coordination numbers, 3 and 4. 

According to this rule, rutile and, at high pressures, the modification with the oc-Pb02 structure are the most stable forms of Ti02. Numerous compounds crystallize in the rutile type and some in the oc-Pb02 type, whereas scarcely any examples are known for the brookite and the anatase structures. 

Use ionic radius ratios (Tables 6.3 and 6.4) to decide whether the CaF2 or the rutile type is more likely to be adopted by NiF2, CdF2, Ge02, K2S. 

Ferroelasticity is the mechanical analogon to ferroelectricity. A crystal is ferroelastic if it exhibits two (or more) differently oriented states in the absence of mechanical strain, and if one of these states can be shifted to the other one by mechanical strain. CaCl2 offers an example (Fig. 4.1, p. 33). During the phase transition from the rutile type to the CaCl2 type, the octahedra can be rotated in one or the other direction. If either rotation takes place in different regions of the crystal, the crystal will consist of domains having the one or the other orientation. By exerting pressure all domains can be forced to adopt only one orientation. 

One aspect that remains to be explored is the extent to which the packing of X atoms alone governs the structural characteristics of the FeS2—m type. As variously pointed out in the literature e.g. (9, 10)], the location of the X atoms in the FeS2—m type structure, in common with the TiOa—r (r=rutile) type, bears some resemblance to hexagonal close-packing. 

The X-ray diffraction pattern of the solid phase obtained by complete neutralization of an acidic solution of SnCl4-5H20 is presented in Fig. 13.18. Cassiterite Sn02 (rutile-type structure) was identified and Laue-Scherrer s law gives an average particle size of 2 0.2 nm in good agreement with transmission electron microscopy observations. 

Table 2.1 shows the crystal structure data of the phases existing in the Mg-H system. Pnre Mg has a hexagonal crystal structure and its hydride has a tetragonal lattice nnit cell (rutile type). The low-pressure MgH is commonly designated as P-MgH in order to differentiate it from its high-pressure polymorph, which will be discussed later. Figure 2.2 shows the crystal structure of p-MgH where the positions of Mg and H atoms are clearly discerned. Precise measurements of the lattice parameters of p-MgH by synchrotron X-ray diffraction yielded a = 0.45180(6) mn and c = 0.30211(4) nm [2]. The powder diffraction file JCPDS 12-0697 lists a = 0.4517 nm and c = 0.30205 nm. The density of MgH is 1.45 g/cm [3]. 

BaurW. H. and Kahn A. A. (1971). Rntile-type componnds, IV Si02, Ge02, and a comparison with other rutile-type compounds. Acta Cryst, B27 2133-2139. 

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