Rutile network

The relationship between interpenetration and selfpenetration is best illustrated by considering a series of closely related structures. In the compounds, o -M(dca)2, 3,6-connected rutile networks are formed (Figure 18a). " ... 

The compounds which crystallize as rutile networks are shown in Table 4.9. 

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,... 

In the structure of rutile, the Ti atoms constitute deformed TiOe octahedra, in which the Ti-O distance are 194.85 pm (four) and 198.00 pm (two). The TiC>6 octahedra share edges to form chains, which further share vertices to yield a three-dimensional network, as shown in Fig. 10.2.6. 

Another simplification consists in the fact that among the various structural types of fluorides, the involved d-transition elements generally possess the coordination number 6. The crystallographic features can be deduced from the arrangement of (MF6) octahedra5,6. Besides in three-dimensional (3-D) networks such as found in perovskite, rutile or pyrochlore types for instance, fluorides crystallyze in two-dimensional (2-D) layer structures, one-dimensional (1-D) chain structures and isolated unit arrangements. 

The existence of at least nine phases in the molybdenum-oxygen system is well established. Their crystal structures are briefly described and it is shown that they can be classified into four main families dependent on whether they possess a basic structure of rutile type, ReOs type, or MoOs type, or have complex structures where polygonal networks can be distinguished. The known tungsten and mixed molybdenum tungsten oxides fit into this scheme. Because of their complicated formulas many of these compounds may be termed "nonstoichiometric," but variance in composition has not been observed for any of them. 

Whereas carbon forms the molecular oxides CO and C02 with multiple bonding (see Topics Cl and C5), stable oxides of Si and Ge are polymeric. Silica Si02 has many structural forms based on networks of corner-sharing Si04 tetrahedra (see Topic D3). Ge02 can crystallize in silica-like structures as well as the rutile structure with six-coordinate Ge. This structure is stable for Si02 only... 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Monolithic zirconia networks can also be formed using a similar procedure giving porous 2xQ>2 structures [9]. As the titania and zirconia precursors are miscible, binary inorganic networks of various Ti Zr ratios could be produced [9]. The crystallinity and photocatalytic properties of the mixed material were studied X-ray amorphous materials were produced for Ti Zr ratios of 2 8 to 7 3, and the binary material containing 10% zirconia (the presence of which inhibited crystal transformation to the rutile phase) showed the highest photocatalytic activity for the photodecomposition of sahcylic acid and 2-chlorophenol [9]. 

Mikkelsen J (1984) Self-diffusivity of network oxygen in vitreous Si02. Appl Phys Lett 45 1187-1189 Millot F, Niu Y (1997) Discussion of O in Fc304 An experimental approach to study the behavior of minority defects in oxides. J Phys Chem Solids 58 63-72 Millot F, Picard C (1988) Oxygen self-diffusion in non-stoichiometric rutile Ti02-x at high temperature. Solid State Ionics 28-30 1344-1348... 

Magnesium oxide, y-aluminum oxide, synthetic rutile, thorium dioxide, silica gels, barium sulfate, activated carbons, metallic network supporting structures, various silicates (especially of Mg, Al), etc. 

Depending on the voliune filHng factor of the matrix material, substantial shrinkage of the porous network accompanied by crack formation may occur for low filHng fractions during template removal. Furthermore, annealing the repHca material at elevated temperatures may lead to a transformation from one modification or crystal phase to another that is thermodynamically more stable at these temperatures, as shown for the thermal conversion of amorphous titania or its anatase phase to the rutile structure [79]. 

Closely related to the salt infiltration method discussed above is the utilization of metal alkoxide precursors in a sol-gel process to fill the interstitial sites in an opal template. The template is first infiltrated with an alkoxide (neat or as solution), then the hydrolysis and condensation reaction following the infiltration transforms the alkoxides into an extended network of the corresponding metal oxide. By this method Ti02 networks besides other oxides were initially prepared with a strong focus on their optical properties as photonic crystals [30,50]. For this purpose a high refractive index material is desired, which would be the rutile modification of titania with the highest refractive index. Nevertheless, the sol-gel procedure provides an amorphous or anatase modification with lower refractive index. By calcination of the anatase... 

An interesting series of structures based on the rutile net illustrates the relationship between interpenetration and self-penetration. The compounds are constructed using octahedral metal ions and the trigonal ligands tcm [tricyanomethanide, C(CN) ] and dca [dicyanamide, N(CN)2 ]. It is important to note that dca is a smaller ligand than tcm. Thus, while the compounds M(tcm)2 have two interpenetrating rutile-related networks,the... 

Again, it is the six-membered rings that are penetrated, this time by rods from other parts of the same network. The structure of this network is closely related to the rutile... 

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