Chemical substitutions crystal chemistry

Available data from natural occurrences and synthetic materials have shown that apatites are capable of accommodating a large number of elements and molecules because of the remarkable tolerance of these phases to structural distortion and chemical substitution. The chemistry of apatites is further complicated by nonstoichiometry, order-disorder in all of the c-axis anion channel, tetrahedral and Ca sites, and the presence of elements with multiple valences (e.g., Cr, Eu, Mn, and S). The example on the uptake of REEs in FAp, OHAp, and ClAp showed that the complex compositional variation in apatites is controlled by both crystal-chemical and external factors. 

The purpose of this chapter is not to cover the entire chemistry of the high Tc oxides but instead is to focus on one specific point the study of these materials through cationic chemical substitutions in order to illustrate how this type of chemistry can aid in the understanding of some of the crystal-chemical aspects of these new high Tc... 

Stucki JW, Roth CB, Baitinger WE (1976) Analysis of iron-bearing clay minerals by electron spectroscopy from chemical analysis (ESCA). Clays Clay Minerals 24 289-292 Swope J (1997) Single crystal X-ray and neutron diffraction studies of the crystal chemical effects of OH=0 substitution in mantle ratile and of Cl-OH substitution in biotite, and the crystal chemistry of IM ferromagnesian trioctahedral micas. PhD dissertation, Dept Geological Sciences, University of Colorado, Botrlder, 85 p... 

The hardness concepts have recently been used as indices of aromaticity [97] and of the orientation of electrophilic aromatic substitution [98]. The principle of maximum hardness [20] requires higher-order derivatives of the electronic energy with respect to the electron population variables, and especially the hardness derivative Sq/0N [99]. Applications of the EE procedure and the CS concepts to the structural and reactivity problems of solids and clusters are becoming routine [40, 41, 47, 100, 101] and new sensitivity indicators of reactivity, addity/basisity in crystal chemistry are being developed [102]. A novel CS-type approach to the chemical reactivity has recently been proposed by Tachibana and Parr [103]. 

Two aspects of the crystal chemistry of natural and synthetic apatites need to be recognized. First, the HA in bone is nonstoichiometric, has a Ca/P ratio of less than 1.67, and contains carbonate ions, sodium, magnesium, fluorine, and chlorine (Posner, 1985a). Second, most synthetic hydroxyapatites actually contain substitutions for the phosphate and/or hydroxyl groups and vary tom die ideal stoichiometry and Ca/P ratios. Oxyhydroxyapatite [Ca,o(P04) 0], a-tricalcium phosphate (a-TCP), )3-tricalcium phosphate O-TCP) or )8-Whitlockite [Caj(P04)J, tetracalcium phosphate (Ca4P209), and octocalcium phosphate [Cag(HP04)2(P04)4 5H20] have all been detected via x-ray diffraction (XRD), Fourier transform into spectroscopy (FITR), and chemical analyses (Kohn and Ducheyne, 1992 Ducheyne et al., 1986, 1990 Koch et al., 1990). These compounds are not apatites per se since the crystal structure differs from that of actual apatite. 

The use of the azo-linking group (-N = N -) in liquid crystal chemistry is now well documented, and hundreds of compounds of this class have been cited in the chemical literature [47] the linkage is formed by an azo coupling reaction between a substituted aryl diazonium salt and a suitably activated substituted benzenoid compound [50]. 

The same requirement extends to the minerals considered in the calculation. Minerals in nature occur as solid solutions in which elements substitute for one another in the mineral s crystal structure, but thermodynamic datasets generally contain data for pure minerals of fixed composition. A special danger arises in considering the chemistry of trace metals. In nature, these would be likely to occur as ions substituted into the framework of common minerals or sorbed onto mineral or organic surfaces, but the chemical model would consider only the possibility that the species occur as dissolved species or as the minerals of these elements that are seldom observed in nature. 

In addition, many mineral species, such as the chain silicates, contain identical basic chemical units, and hence exhibit very similar crystal structures but have different chemical compositions. These minerals may be members of a mineral series if the chemistry varies predictably. The variations, known as atomic substitution, are based on Pauling s rules. 

Lindsey andco-workers [27,69,70], Weglarz and Atkin [32], and Metivier and co-workers [31,81] have all developed and applied Zymark robotic workstations to optimize chemistry. Lindsey and co-workers [69] completed a factorial design study (16 experiments) to examine the role of catalyst and reactant concentrations on porphyrin yield in less than 1 day of workstation time. Weglarz and Atkin at Dow Chemical Company [32] studied the effect of reaction parameters on (i) the alkoxy substitution of cellulosic ethers (ii) the base-catalyzed conversion ofphenethyl bromide to styrene and (iii) the onset of crystallization employing a fiber optic probe. Metivier and co-workers at Rhone-Poulenc [31,81] focused on the evaluation of catalysts, reagents, and solvents for process optimization work of numerous proprietary reactions. 

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