Trirutile

Unit cells of the rutile and the trirutile structures. The positions of the twofold rotation axes have been included... 

The rutile and trirutile compounds of table 8 are characterised by a missing cation order and a cation order which is not vertical to oxygen layers, respectively. 77-bonding may be effective in these lattice types, however (3). As was stated at the end of section F, zJ-effects opposite to those observed for a cation order vertical to oxygen layers are expected in these cases. Indeed the substitution of a d - by a d -cation of high oxidation state is accompanied by a distinct decrease in J, which is larger for the ordered than for the disordered phases (Table 8). 

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... 

Dichromium tungsten(VI) oxide is a deep-purple powder possessing the trirutile type of structure. The compound crystallizes in the space group/>42/mnm. with Z = 2,a0 =4.582 A., c0 = 8.870 A. 

Figure 5.35. The structure of ZnSb2C>6, shown as trirutile based on the rutile (Ti02) structure.

Cr2NbOsF M1CrM204 Nb02F + Cr203 at 900°C X. trirutile structure i... 

Figure 15 (a) LiMnF4 structure (large grey spheres Li) (b) trirutile... 

The structure and distorted variants of it are also found for a number of mixed transition metal oxides. In the trirutile structure, the tetragonal c axis of the simple unit cell is tripled and the titanium sites are no longer equivalent. The mineral tapiolite, FeTa206, has this structure in which the Fe and Ta occupy separate crystallographic positions. The ferrous ion can be replaced by Mg+ and other divalent transition metal cations such as Co+ and Ni+. Antimonates of the form MSb206 (M+ = Mg, Fe, Co, Ni, Zn) also form with the trirutile structure. Examples are known in which the transition metals are mixed on the two sites. This occurs for WCr206, which might more correctly be formulated Cr(Cro.5 Wo.5)206. 

The cryochemical method is also rather useful for the preparation of various metastable compounds. Thus thermal decomposition of freeze-dried oxalate complexes resulted in the formation of metastable monoclinic CuNb20g and CuTa20g with trirutile structure. An unusual polymorph of FeOOH—akageneite—was found during analysis of freeze-dried hydrated ferric oxides obtained from different precursors under various precipitation conditions. Soft oxidation of... 

FIG. 4.28. The structures of compounds M M X (a) trirutile, (b) LijZrF, (c) Na2SiI 4. The open and filled circles represent M and M atoms in octahedral holes in a h.c.p. assembly of X atoms. Clear and shaded octahedra contain cations at heights 0 and c/2. The sketches at the right show the types of atoms between successive pairs of c.p. layers. 

Several of these compounds are polymorphic for example, the low-high transitions of Li2SnF6 (li2ZrF6 to trirutile) and Ii2GeF5 (trirutile to Na2Sip5). 

Since both A and B ions occupy octahedral interstices in all of these structures they are suitable only for the smaller metal ions, as may be seen from the following examples ZnSb20g (trirutile), (Fe, Mn)Nb20g (niobite), and NiU206 (Na2SiFg structure). 

M Sb20fi. Three types of structure have been found for these compounds. The small ions of Mg and certain 3d metals form the trirutile structure (p. 203), sometimes described as the tapiolite structure, after the mineral of that name, FeTa20fi. Compounds with this structure include ... 

The Ni<6 OMv2[Pg.673]

Figure 5 Diffuse reflectance spectra of octahedral Ni2+ in various oxide ceramics Zn(4 Zn0 75Ni0 25Ge)(6)04 (I A = 9500cm- (3 0.83 spinel), (NiTa2)(6)06 (II A = 7800cm / = 0.82 trirutile), (Bao5Sr15)12 (NiTe)(6)06 (III A-6500cm, — 0.83 elpasolite) and Ba2(12)(Ni0.iCao.9Te)(6)06 (IV A-4900cm, ...

Al,Sb,V)204 likewise Sbo.9Vo.9O4 has a rutile-related structure and for both phases only the basic rutile-type reflections are observed with powder X-ray diffraction. Selected area electron diffraction, on the other hand, admits the easy distinction between (Al,Sb,V)204 and Sbo.9Vo.9O4 (see Fig. 1). The diffraction pattern in Fig. lb of (Al,Sb,V)204, or (Al,V)Sb04, shows a 3-fold supercell of trirutile-type [18]. 

The active structure in the Al-Sb-V-0 system is a bulk phase, which is directly formed in the catalyst synthesis. It is a trirutile-like phase Ali.xSbVx04 (0 < x < 0.5), and the presence in the synthesis of an excess of aluminium is critical for its formation. Thus, the aluminium is not only a catalyst support in the form of 8-AI2O3, but it is also an element in the active phase. The Sb V ratio in the trirutile usually falls in the range 2-5 (cf. Fig. 6). 

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