Al coordination numbers

The EXAFS results reported for the untreated samples (see Section 8.3.4) led to the conclusion that Zn may form highly ordered inner-sphere sorption complexes with gibbsite surfaces or substitute into an octahedral Al-hydroxide layer of some sort. The use of sequential extraction enabled more concrete conclusions to be made. For the nonextracted soil samples (bulk and coarse), second-shell Al coordination numbers did not exceed four, in fine with the dioctahedral structure of gibbsite sheets (only two out of three metal positions are occupied). Elsewhere, a gradual increase was observed in Al coordination up to six with each extraction step, indicating that Zn is part of a fully occupied, trioctahedral Al-Zn2+ layer and not part of gibbsite or another dioctahedral Al compound.67 While dioctahedral Al-hydroxide layers are... 

A Al MAS NMR study of a series of lanthanum aluminium phosphate glasses has shown the presence of Al in 4-fold, 5-foId and 6-fold coordination. The average Al coordination number increases with increasing Al/La ratio and decreases with increasing 0/P ratio, as can be understood in terms of the tendency of the structure to avoid forming Al-O-Al bonds (Karabulut et al. 2001). 

The coordination number for the Al ion is typically between four and six. Use the anion coordination number to determine the Al coordination number in the following compounds ... 

Aluminum hydroxide and aluminum chloride do not ionize appreciably in solution but behave in some respects as covalent compounds. The aluminum ion has a coordination number of six and in solution binds six molecules of water existing as [Al(H20)g]. On addition of a base, substitution of the hydroxyl ion for the water molecule proceeds until the normal hydroxide results and precipitation is observed. Dehydration is essentially complete at pH 7. 

Betyllium, because of its small size, almost invariably has a coordination number of 4. This is important in analytical chemistry since it ensures that edta, which coordinates strongly to Mg, Ca (and Al), does not chelate Be appreciably. BeO has the wurtzite (ZnS, p. 1209) structure whilst the other Be chalcogenides adopt the zinc blende modification. BeF2 has the cristobalite (SiOi, p. 342) structure and has only a vety low electrical conductivity when fused. Be2C and Be2B have extended lattices of the antifluorite type with 4-coordinate Be and 8-coordinate C or B. Be2Si04 has the phenacite structure (p. 347) in which both Be and Si... 

In these reactions the coordination number of Al has been assumed to be 6 throughout though direct evidence on this point is rarely available. Amphoterisni is also exhibited In anhydrous reactions, e.g. ... 

The structure of LiTa02F2, as reported by Vlasse et al. [218], is similar to a ReC>3 type structure and consists of triple layers of octahedrons linked together through their vertexes. The layers are perpendicular to the c axis, and each layer is shifted, relative to the layer below, by half a cell in the direction (110). Lithium atoms are situated in the centers of the tetragonal pyramids (coordination number = 5). The other lithium atoms are statistically distributed along with tantalum atoms (coordination number = 6) at a ratio of 1 3. The sequence of the metal atoms in alternating layers is (Ta-Li) - Ta - (Ta-Li). Positions of oxygen and fluorine atoms were not determined. The main interatomic distances are (in A) Ta-(0, F) - 1.845-2.114 Li-(0, F) - 2.087-2.048 (O, F)-(0,F) - 2.717-2.844. 

The structure leads to a general formula for the micas namely, KXMY.1Oio(OH,r )2, with 2 < < 3, in which X represents cations of coordination number 6 (Al+3, Mg+, Fe++, Fe+3, Mn++, Mn+3, Ti+ Li+, etc.) and Y cations of coordination number 4 (Si+4, A1+3, etc.). The subscript n can have any value between 2 (hydrargillite layer) and 3 (complete octahedral layer). K+ can be partially replaced by Na+ and possibly to some extent by Ca++. This formula represents satisfactorily the.numerous recently published mica analyses almost without exception.6 The distribution of the various ions X and Y must be such as to give general agreement with the electrostatic valence rule. 

The electrostatic valence rule is satisfied. The bond strength from S7+4, A +3, and Na+ are 1, -, and respectively, since the cations all have the coordination number 4. Each oxygen ion is in contact with 1 Si + i, 1 Al+i, and 1 Na+, giving JbV = 2, and each chlorine ion in contact with 4 Na+, giving Xs = 1, in agreement with their valences. 

Bearing in mind that the relative sizes of the ions permit isomorphous replacement of OH by F but not by Cl, we write with considerable confidence the formula (Si, Al, Fe, P)18O20(OH, F)laCl, which agrees well with analyses 1, 2, and 3. Inasmuch as aluminium (as well as phosphorus) may replace silicon with coordination number 4, it is evident that there are at least five silicon atoms in the unit, corresponding to the chemical formula... 

The ions that tend to be involved in AB cements include such species as Al , Mg, Ca and Zn. These are all capable of developing a coordination number of six, and hexaquo cations are known to be formed by each of these metal ions (Huckel, 1950). The typical requirements for an ion to develop such coordination characteristics are that the ion should exist in the -I- 2 or -1-3 oxidation state, and in this state should be of small ionic radius (Greenwood Earnshaw, 1984). 

Figure 1.4 Proposed steps in the chemisorption of OH on/in Pt, starting with arrays of OH groups over the uppermost metal atom layer, increasing the coordination number of the adsorbed OH by place exchange, and next generating a mixed, metal/oxygen overlayer while further oxidizing to form O atoms. From Conway et al. [1990].

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