The rutile structure

More information on this structure, and the relationship between the A1 and A3 structures, is given in Chapter 7. 

The general formula of crystals with the halite structure is MX. The mineral halite, which names the group, is sodium chloride, NaCl, also called rock salt. 

This structure is adopted by many oxides, sulphides, halides and nitrides with a formula MX. 

This structure is relatively common and adopted by a number of oxides and fluorides with a formula MAS. 

This tetragonal structure (Fig. 6.5(a)) is named after one of the polymorphs of Ti02 it is also referred to as the cassiterite (Sn02) structure. The coordinates of the atoms are  

The structure consists of chains of TiOg octahedra, in which each octahedron shares a pair of opposite edges (Fig. 6.5(b)), which are further linked by sharing vertices to form a 3D structure of 6 3 coordination as shown in the projection of Fig. 6.5(c). With the above coordinates each 0 has three coplanar neighbours (2 at the distance d and 1 at e), Ti has six octahedral neighbours (4 at the distance d and 2 at e), and all Ti-Ti distances (between centres of octahedra along a chain) are 

The rutile structure (a) unit cell, (b) parts of two columns of octahedral TiO coordination groups, (c) projection of structure on base of unit cell, (d) corresponding projection 

The normal rutile structure is restricted to the difluorides of Mg and certain 3d metals, a number of dioxides, some oxyfluorides (FeOF, TiOF, VOF), and MgH2 it is not adopted by disulphides, by other dihalides, or by intermetallic phases. In 

Dioxides and difluorides with the rutile or fluorite structures 

---

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. 

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,... 

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. 

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. 

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. 

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. 

Stishovite is a high-pressure modification of Si02 having the rutile structure. Should it have longer or shorter Si-O bond lengths than quartz ... 

Although CaF2 has the fluorite structure, MgF2 has the rutile structure. Explain this difference. 

The naturally occurring form of Sn02 (cassiterite) has the rutile structure (see Chapter 7), and it is an amphoteric oxide as illustrated by the following equations ... 

Like Sn02 Pb02 has the rutile structure, but unlike Sn02 it is a strong oxidizing agent that can be produced by the reaction... 

Byway of example, Figure 11.8 shows a comparison [3] of calculated and experimental expansion coefficients for MgF2, which has the rutile structure. 

As a specific example, Li can be intercalated into several oxides with the rutile structure (Murphy, DiSalvo, Caricles and Waszczak, 1978). In Li,jMo02, the structure changes from monoclinic to octahedral and back to monoclinic as x goes from 0 to 1 (Dahn and McKinnon, 1985). In both monoclinic structures. Mo atoms are shifted to form Mo-Mo pairs along the chains (Cox, Cava, McWhan and Murphy, 1982), and these pairs disappear in the octahedral structure at intermediate x (Dahn and McKinnon, 1985). The rutile LijtW02 has been considered as a replacement for metallic Li in electrochemical cells (Murphy et al, 1978). 

Stishovite is very interesting because it has the rutile structure with octahedral coordination of silicon. In all other forms of silica, each silicon atom is surrounded tetrahedrally by four oxygen atoms. 

---