Rutile Type Structure

Transition metal difluorides are known mainly for first transition series elements, with palladium and silver difluorides from the second series, and no examples from the third. All these compounds have either the rutile structure, or, for chromium, copper, and silver, a distorted variant, which can be correlated with a Jahn-Teller distortion of the octahedral coordination of the ions. This rutile structure type is associated with smaller cations and, for comparison, although zinc difluoride has the same rutile structure, cadmium and mercury difluorides have the cubic fluorite structure with eight coordination of the cations (12). 

TcO., like RcO., adopts the molybdenum dioxide structure, a distorted rutile structure type [31] of the space group P2ilc. The approximate cell dimensions of the mono-... 

ZrOi crystallizes in the Rutile structure type, like TiOi... 

The rutile structure type is adopted by Sn02 (cassiterite, the most important tin-bearing mineral), P-Mn02 (pyrolu-site) and Pb02. 

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

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. 

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. 

M-M multiple bonding has long been known in metal oxide structures. The first Mo=Mo bond was seen in one crystalline form of Mo02 which has a distorted rutile structure wherein the Mo(4+) ions occupy adjacent octahedral holes throughout the lattice ( 4). The octahedra are distorted because of the short Mo-Mo distances 2.51 X. La. RejO has a fluorite type structure in which 02 is substituted for F and four of the five Ca2 sites are occupied by La3 ions. The remaining Ca2 site is occupied by an (Re=Re)8 unit with an Re-Re distance 2.259(1) A... 

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. 

In materials in which a metal-insulator transition takes place the antiferromagnetic insulating state is not the only non-metallic one possible. Thus in V02 and its alloys, which in the metallic state have the rutile structure, at low temperatures the vanadium atoms form pairs along the c-axis and the moments disappear. This gives the possibility of describing the low-temperature phase by normal band theory, but this would certainly be a bad approximation the Hubbard U is still the major term in determining the band gap. One ought to describe each pair by a London-Heitler type of wave function... 

In some circumstances the magnitudes of the translation vectors must be taken into account. Let us demonstrate this with the example of the trirutile structure. If we triplicate the unit cell of rutile in the c direction, we can occupy the metal atom positions with two kinds of metals in a ratio of 1 2, such as is shown in Fig. 3.10. This structure type is known for several oxides and fluorides, e.g. ZnSb20g. Both the rutile and tlie trirutile structure belong to the same space-group type PAjmnm. Due to the triplicated translation vector in the c direction, the density of the symmetry elements in trirutile is less than in rutile. The total number of symmetry operations (including the translations) is reduced to... 

When the ratio of the radii increases, then a new type of crystal structure occurs, in which each divalent ion is surrounded by six univalent ions, and each univalent by three divalent ones. This arrangement, shown in Figure 14, is called the rutile type, named after the mineral Ti02. While CaF2 still has the structure with 8 4 coordination, the rutile structure is observed in MgF2 because of the smaller radius of the Mg2+ ion. After the rutile type there is a further reduction of the coordination to 4 2 this type of structure occurs in BeF2 and Si02 in the different modifications of silica, each silicon ion is surrounded by 4 O2" ions, and each O2 ion is between two Si4+ ions. The ionic ratio r+/r can also be decreased if,... 

Fig. 2.9 Rutile-type structure (II) (a) The top and bottom are the projection on (100) and (001) of a real rutile structure, respectively, (b) The top and bottom are the projection on (100) and (001) of an idealized rutile structure, i.e. the oxygen packing of rutile is assumed to be HCP.

In the halides bridging anions lead to distorted CrXe octahedra sharing corners in which there are four short approximately equal Cr—X bonds and two long trans Cr—X bonds. There is also angular distortion of the octahedra. The fluoride has the tetragonal (distorted) rutile structure and the other halides chain-type structures. The mixed crystal (Mn, Cr)I2 is isotypic with monoclinic Crl2 (Table 34). 

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