Site preference

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

All pipework must be installed to the appropriate standard and codes of practice. The degree of insulation will be suitable for the temperature of the pipework, although the finish may vary, depending upon site preferences. Valve boxes are recommended, although on some low-pressure installations these are not always included. 

The affinity of different cations to the octahedral coordination can be estimated based on the Octahedral Site Preference Energy [279-281]. The above-defined ratio characterizes the way in which the octahedrons are linked [282]. Table 41 illustrates the correlation between the value of the X Me ratio and the way in which the octahedrons are linked. 

Some cations with an octahedral-site preference (such as Ni2+, Co3+, and Cr3+) are expected to occupy the 16d sites of the spinel with Mn, whereas cations with a strong tetrahedral-site preference (such as Zn2+) are expected to occupy the 8a sites and to dislodge corresponding lithium ions into the 16d sites. In cases where Mn is substituted by transition metal ions (such as Co, Ni, and Cr) that can partake in the electrochemical reaction, voltage plateaus between 4 and 5V have been observed [135, 136],... 

DNA polymerase, or reverse transcriptase—can be integrated into chromosomes of the mammalian cell. The integration of the animal vims DNA into the animal genome generally is not site-specific but does display site preferences. 

MitcheU RS, Beitzel BF, Schroder AR, Shinn P, Chen H, Berry CC, Ecker JR, Bushman FD (2004) Retroviral DNA integration ASLV, HIV, and MLV show distinct target site preferences. PLoS Biol 2(8) E234... 

Bottazzi P, Tiepolo M, Vaimucci R, Zanetti A, Brumm R, Foley SF, Oberti R (1999) Distinct site preferences for heavy and light REE in amphibole and the prediction of " p Dree. Contrib Mineral Petrol 137 36-45... 

Tiepolo M, Vaimucci R, Zanetti A, Bramm R, Foley SF, Bottazzi P, Oberti R (1998) Fine-scale stractural control of REE site-preference the case of amphibole. Mineral Mag 62A 1517-1518 Tiepolo M, Varmucci R, Oberti R, Foley SF, Botazzi P, Zanetti A (2000a) Nb and Ta incorporation and fractionation in titanian pargasite and kaersutite crystal-chemical constraints and implications for natural systems. Earth Planet Sci Lett 176 185-201... 

We have already used the HSAB principle as it applies to linkage isomers in metal complexes. This application to bonding site preference can also be used to show the behavior of other systems. For example, the reactions of organic compounds also obey the principles when reacting with nucleophiles such as SCN- or N02 ... 

Similar methods have been used in other cases as well, and a recent example is an analysis of the cation distribution in the complex oxide, LaSr2 xCaxCu2Ga07. Here site preference enthalpies for La, Sr and Ca have been derived [21]. 

For a review on site preference of substitutional additions to CsCl-type inter-metallic compounds see Kao et al. (1994). In this work dilute additions to NiAl, FeAl and CoAl are especially discussed. The addition of a third element to ordered Ni3Al (cP4-AuCu3 type) occurring in different ways (Ochiai etal. 1984) is another... 

NixAlj x (homogeneous between 42 and 69 at.% Ni) with good mechanical and oxidation resistance properties. By quenching from high temperatures the formation of an ordered martensite is obtained which can be considered for its shape memory behaviour. For a discussion on substitutional additions to CsCl-type alloys (site preference for dilute additions to NiAl, FeAl, CoAl, etc.) see Kao et al. (1994). 

Figure 8.1. Lock and key model (a) geometrical fit, (b) complementary pattern of functional groups, (c) site preference due to the solvent effect. The ligand L may better fit site A, but it binds preferentially to site B due to the solvent effect...

Effect of Valence on Site Preference of Other 3d 4527 Transition Metals... 

The focus of this paper is on the role electronic structure plays in determining the site preference and mobility of 3d transition-metal ions in an oxide and how these factors in turn affect the resistance of metastable 3d transition-metal oxides against transformation. This is a relevant topic to the Li rechargeable battery field because 3d transition-metal oxides are often used as positive electrode materials. 

It appears that for the layered Mn and Co oxides considered, ionic size effects do not play a significant role in the preference for octahedral or tetrahedral sites nor in the activation barrier to hops between the two. Consequently, size effects probably do not play a significant role in determining the mobility of Mn or Co through a ccp oxide framework. In contrast, the results indicate that valence and electronic structure are more decisive factors in the site preference of Mn or Co and hence in their propensity to migrate through a ccp oxide framework. This is consistent with the work of Goodenough that found valence to be an important determinant of the site preference of 3d TM ions in oxides. 

The role of electronic structure in Mn and Co site preference and mobility can to some extent be understood through ligand-field theory (LFT). LET qualitatively explains how the degeneracy of the 3d orbitals is broken when a free TM ion is surrounded by coordinating anions. The ligand-field splitting of d orbitals in octahedral and tetrahedral coordination is pictured in Figure 6. ... 

Summaiy of Important Factors Influencing Co and Mn Site Preference in ccp Oxides... 

A series of first-principles results will now be shown that demonstrate the effect of a variety of chemical substitutions on the valence of Mn and how this in turn affects the site preference of Mn. 

It is assumed that as with pure Mn oxides, the site preference for Mn at a given valence will be reflected by the energy difference between the chemically substituted compounds with and without tetrahedral Mn. The use of simple structures such as ps-(LiMn)tet-(Li51/63)oct08 with tetrahedral Mn instead of large supercells with tetrahedral Mn defects, like those used in section 4, greatly reduces the calculation time. 

Since the results of the previous sections indicate that the site preference and tendency toward migration of Mn or Co is strongly affected by the electron occupancy of the d levels split by ligand-field effects, it is possible that this may be the case for all of the 3d TM ions. 

The results given in Tables 2 and 3 support the decisive role of LFSE in the site preference and most likely the mobility of 3d metal ions in a ccp lattice. For Mn. Ni. and Co the results in Table 2 are consistent with the experimental work of Choi. Man-thiram et al. who found increasing resistance of /-LL-MO2 against transformation into spinel as M is... 

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