Apical vacancy

If one removes from an octahedron the center metal M6 and an apical oxygen, four planar oxygens remain available for coordination. If this is done from 2 W6 units, there are 8 oxygens and a cation vacancy that can coordinate a large cation. The first example of such a structure contained Ce(IV), and later U(IV) (6-7). In these structures the 8-0 s approximate an antiprism for Ce(IV) which is distorted in the U(IV) case. 

Hole-based superconductors have an average copper oxidation state of approximately 2.2. The holes are normally introduced by oxygen vacancies in the apical oxygens of the copper-oxygen layers. 

Excess OH in partially dioctahedral micas of the K20-Mg0-Be0-Si02-H20 system, described as a TMM-KMg3(Si3.5M o.5)Oio(OH)2 solid solution series with tetrahedrally coordinated cations, were reported by Robert et al. (1995). This is demonstrated by the increase of a broad band at 3700 cm The most likely hydrogen acceptors are the strongly underbonded apical oxygens of (Be or Mg) tetrahedra adjacent to two octahedrally coordinated Mg and an octahedral vacancy. 

The three transition-metal molybdate layers defining the unit cell are stacked at a separation of c/3, analogous to the rhombohedral LDH precursor. In these layers, the position of the ordered vacancy capped by the molybdate groups follows the sequence A-B-C-A (where A, B, and C are the three threefold axes at x,y = 0,0 2/3,l/3 and l/3,2/3). This three layer arrangement is shown in Figure 5. Between each layer, in the space defined by the apical oxygens of six tetrahedral molybdates, lie ammonium ions. The positions of the nitrogen atoms follow the sequence C-A-B-C. The ammonium ions do not serve to connect the array of layers in the [00/] direction, the distance between the... 

Observations by electron microscopy have shown that, when polymerization is carried out on well-formed crystals of a-TiQj, polymer formation occurs only on the lateral faces and the edges of crystals, but not on the basal faces (83). The layer structure of a-TiCIa appears to be such that on the lateral faces and the edges there are exposed titanium atoms, i.e. atoms bearing coordination vacancies, while the basal faces consist of planes of chlorine atoms overlying lattice planes occupied by nonexposed titanium atoms, i.e. atoms located at the center of a coordination polyhedron whose apices are occupied by six Q s, each one shared by two titanium atoms (Fig. 6). The titanium... 

The cubic unit cell of the Th3P4 type structure with space group I43d-Tg (No. 220), see Fig. 2, p. 36, contains four M3 xDxSe4 ( = cation vacancy). The ideal composition M3Se4 (x = 0) does not contain vacancies. Each M is surrounded by 8 Se atoms at the apices of a triangular dodecahedron. This can be resolved Into two distorted interlocked tetrahedra, one being... 

Filling anion vacancies with inserted fluorine anions completes the coordination polyhedron of the transition metal cation to an octahedron, which in the case of Jahn-Teller active cations leads to anisotropic coordination with four short equatorial and two longer apical metal-oxygen bonds. 

The active center is thought to comprise a titanium atom coordinated with four chlorine atoms and an alkyl group in an octahedral configuration with an empty site. An incoming ethylene molecule coordinates with the titanium at the vacant site, thereafter inserting between the metal and the alkyl. A new vacancy is thus generated at the apical position. Repetitive addition of ethylene molecules generates a polyethylene chain. 

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