Rutile structures

This result is significant because it implies that equation 174 is only valid for the oxides, and the Rc( d) must be increased in the presence of anion sublattices of greater polarizability. It also implies that any extrapolation of equation 174 to transition metals must allow for some increase in Rc(3d) from that given in equation 174. 

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Chromium dioxide. Cr02 (HjO plus O2 on Cr03 at high temperature). Black solid with the rutile structure forming chromates(IV) in solid stale reactions. Used in magnetic lap>es. 

Lead II) fluoride, PbFa, rutile structure, precipitated from aqueous solution. 

In203 has the C-type M2O3 structure (p. 1238) and InO(OH) (prepared hydrothermal ly from In(OH)3 at 250-400°C and 100-1500 atm) has a deformed rutile structure (p. 961) rather than the layer lattice structure of AIO(OH) and GaO(OH). Crystalline In(OH)3 is best prepared by addition of NH3 to aqueous InCl3 at 100° and ageing the precipitate for a few hours at this temperature it has the simple Re03-type structure distorted somewhat by multiple H bonds. 

Finally, many disulfides have a quite different structure motif, being composed of infinite three-dimensional networks of M and discrete Sj units. The predominate structural types are pyrites, FeSa (also for M = Mn, Co, Ni, Ru, Os), and marcasite (known only for FeS2 among the disulfides). Pyrites can be described as a distorted NaCl-type structure in which the rodshaped S2 units (S-S 217 pm) are centred on the Cl positions but are oriented so that they are inclined away from the cubic axes. The marcasite structure is a variant of the rutile structure (Ti02,... 

The dioxides of molybdenum (violet) and tungsten (brown) are the final oxide phases produced by reduction of the trioxides with hydrogen they have rutile structures sufficiently distorted to allow the formation of M-M bonds and concomitant metallic conductivity and diamagnetism. Strong heating causes disproportionation ... 

Ruthenium and osmium have no oxides comparable to those of iron and, indeed, the lowest oxidation state in which they form oxides is -t-4. RUO2 is a blue to black solid, obtained by direct action of the elements at 1000°C, and has the rutile (p. 961) structure. The intense colour has been suggested as arising from the presence of small amounts of Ru in another oxidation state, possibly - -3. 0s02 is a yellowish-brown solid, usually prepared by heating the metal at 650°C in NO. It, too, has the rutile structure. 

Of the anhydrous dihalides of iron the iodide is easily prepared from the elements but the others are best obtained by passing HX over heated iron. The white (or pale-green) difluoride has the rutile structure the pale-yellow dichloride the CdCl2 structure (based on cep anions, p. 1212) and the yellow-green dibromide and grey diiodide the Cdl2 structure (based on hep anions, p. 1212), in all of which the metal occupies octahedral sites. All these iron dihalides dissolve in water and form crystalline hydrates which may alternatively be obtained by dissolving metallic iron in the aqueous acid. 

For iridium the position is reversed. This time it is the black dioxide, Ir02, with the rutile structure (p. 961), which is the only definitely established oxide. It is obtained by heating the metal in oxygen or by dehydrating the precipitate produced when alkali is added to an aqueous solution of [IrCl6] . Contamination either by unreacted metal or by alkali is, however, difficult to avoid. The other oxide, Ir203, is said to be... 

Ru02 can be made by high-temperature oxidation of ruthenium. It has the rutile structure (Ru—O 1.942 A and 1.984 A) and forms blue-black crystals [49b]. 

Copper-coloured 0s02 also has the rutile structure it can be made from the metal and NO at 650°C. 

It exists in two stable forms, of which the a-form has the corundum (a-A12Oj) structure with octahedrally coordinated rhodium (Rh-0 2.03-2.07 A) the /3-form and a high-temperature form also have octahedral coordination. Black Rh02 has the rutile structure (Rh-0 1.95-1.97 A) and is best made by heating rhodium or Rh203 at 400-900°C under oxygen pressures up to 3500 atm. 

PdF2 is that rare substance, a paramagnetic palladium compound, explicable in terms of (distorted) octahedral coordination of palladium with octahedra sharing corners [15], It exists in two forms, both having /zeff 2.0 /xB, rather below the spin only value for two unpaired electrons. Bond lengths are Pd-F 2.172 A (two) and 2.143 A (four) in the tetragonal form (rutile structure). 

We shall illustrate this technique by application to two magnetic structures that exist in nature. Consider the rutile structure associated with the antiferromagnetic crystal MnF2. Figure 12-4 shows the non-... 

The Rutile Structure.—A large number of compounds MX crystallize with the tetragonal structure of rutile, TiCfe. In this structure the position of the ion X is fixed only by the determination of a variable parameter by means of the intensity of reflection of x-rays from various crystal planes. In accordance with the discussion in a following section, we shall assume the parameter to have the value which causes the distances between X and the three ions M surrounding it to be constant. With this requirement the inter-atomic distance R and the edges a and c of the unit of structure are related by the equation R = (a/4 /2) [2 + (c/o)2]. In this way the inter-atomic distances in Table XII are obtained. In the case of magnesium fluoride the agreement is satisfactory. 

The Fluorite and Rutile Structures.—Compounds of the type MXS also have their choice of two ionic structures, but the factors influencing them... 

It was shown by Hund (Ref. 23) that for small values of n (less than 6 or 9, depending upon the assumptions made) the rutile structure can bgpome stable. However, our discussion makes it probable that the transition is actually due to the radius ratio. 

We have accordingly shown that for values of the ratio of the crystal radius of the cation to that of the anion greater than 0.65 the fluorite structure is stable for values less than 0.65 the rutile structure is stable. 

Efforts to provide such a treatment for simple alternative structures, such as the sodium chloride and cesium chloride structures and the fluoride and rutile structures, have been made with the aid of the Bom potential expression and modifications of it. Assuming that all ions repel each... 

It may be pointed out that in some structures easily derivable with the coordination theory, such as the rutile structure, the anion arrangement approximates no type of dose-packing whatever. 

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