Substitutions—Crystal Chemistry

In a previous section, it was shown how the substitution of Y3+ for Las+ was an important step towards the formation of new compounds for superconductivity research. This substitution attempt led to the discovery of the 1-2-3 (or 90 K) superconducting compound. Substitutions similar to that proposed by Muller and Bednorz are now outlined. The type of chemical substitutions, based on atomic radii, which were proposed for La2Cu04, are presented in the following Table 11. 

Many of these substitutions were carried out in Japan and in the U.S. immediately after the disclosure of high Tc superconductivity in the barium-doped La2Cu04 samples. 

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The lowest coordination number of tantalum or niobium permitted by crystal chemistry formalism is 6, which corresponds to an octahedral configuration. X Me ratios that equal 3, 2 or 1 can, therefore, be obtained by corresponding substitutions in the cationic sub-lattice. A condition for such substitution is no doubt steric similarity between the second cation and the tantalum or niobium ion so as to enable its replacement in the octahedral polyhedron. In such cases, the structure of the compound consists of oxyfluoride octahedrons that are linked by their vertexes, sides or faces, according to the compound type, MeX3, MeX2 or MeX respectively. Table 37 lists compounds that have a coordination-type structure [259-261]. 

Chemistry departments should start offering a concentration area in materials chemistry, and the requirements for this degree include the substitution of 15 credits of phase equilibria, crystallography, crystal chemistry, and materials characterization to be taken in the materials or geoscience departments. Where local chemistry faculty can teach... 

It is well known that, as previously mentioned, close structural relationships exist between phosphates and silicates, as the crystal chemistry of both families is based upon similar X04 tetrahedral networks. The introduction of nitrogen within the P04 tetrahedra further increases the similarities when the cross-substitution is considered ... 

The crystal chemistry of BajRC C has been systematically studied by single-crystal and powder diffraction methods with R = La, Pr,... Yb, in addition to the conventional yttrium compound [(52)(53) (54) and references therein]. With the exception of La, Pr, and Tb, the substitution of Y with rare-earth metals has little or no effect on the superconductivity, with the values of Tc ranging from 87 to 95K. Also, a relatively small change is observed in the cell constants of these compounds. The La, Pr, and Tb-substituted materials are not superconductors. A detailed structural analysis of the Pr case (52) did not show any evidence of a superstructure or the presence of other differences with the atomic configuration of the yttrium prototype. 

Many of the tertiary bonds reported by Preiser et al. (1999) are likely artefacts of their calculations since these were based on the use of formal ionic charges. Substituting a more physically reasonable value for the formal ionic charge will reduce the total flux starting at the cation and eliminate many of the tertiary bonds around the highly charged cations where most tertiary bonds were found. However, there are some cases where tertiary bonds undoubtedly do occur and these can provide important information about the crystal chemistry. 

The current hypothesis is that layer collapse depends uniquely on the crystal chemistry of Fe2 in the octahedral sheet of the clay or on changes in crystal chemistry that accompany the process of Fe reduction. This hypothesis excludes the possibility that layer collapse occurs in response to an increased electrostatic charge on the clay layer due to the substitution of a divalent for a trivalent ion. Such exclusion is justified on the basis of observations by Mulla (36) that the surface area of smectites depends only on octahedral Fe2, tetrahedral Al, and interlayer K contents. Noticeably absent from this dependence is Mg2, which has the same valence and about the same size as Fe2, but has no apparent influence on surface area. 

Therefore, four types of atomic substitution may contribute to the crystal chemistry of todorokite- and vemadite-bearing manganese nodules. 

Most authors who have studied small particle hematite have, as a first approximation, considered their samples to be small pieces of the ideal bulk material a-Fe20s, where the changes in physical properties are predominantly controlled by particle size (e.g., Vandenberghe et al. 2000). In contrast, Dang et al. (1998) have stressed that even bulk hematite has a complex crystal chemistry involving the known OH/vacancy substitutional mechanism. 

Palmer DC, Dove MT, Ibberson RM, Powell BM (1997) Stractural behavior, crystal chemistry, and phase transitions in substituted leucite High-resolution neutron powder diffraction studies. Am Mineral 82 16-29... 

A second feature of silicate crystal chemistry is peculiar to silicates alone, and arises accidentally from the particular value of the radius of the aluminium ion. The A1 0 radius ratio of 0 36 is so close to the critical value of 0 3 for transition from 6- to 4-co-ordination that this ion can occur in both conditions, sometimes in the same structure. When 4-co-ordinated the aluminium ion replaces silicon, and such replacement is purely random and may be of indefinite extent. For every aluminium ion so introduced a corresponding substitution of Ca2+ for Na+, Al3+ for Mg2+ or Fe3+ for Fe2+ must simultaneously occur else-... 

Brigatti MF, Galli E, Poppi L (1991) Effect of Ti substitution in biotite-IM crystal chemistry. Am Mineral 76 1174-1183... 

Kato T, Mima Y, Yoshii M, Maeda K (1979) The crystal stracture of IM-kinoshitalite, a new barium brittle mica and IM-manganese trioctahedral micas. Mineral J 9 392-408 Keppler H (1990) Ion exchange reactions between dehydroxylated micas and salt melts and the crystal chemistry of the interlayer cations in micas. Am Mineral 75 529-538 Keusen HR, Peters T (1980) Preiswerkite, an Al-rich trioctahedral sodium mica from the Geisspfad ultramafic complex (Penninic Alps). Am Mineral 65 1134-1137 Knurr RA, Bailey SW (1986) Refinement of Mn-substituted muscovite and phlogopite. Clays Clay Minerals 34 7-16... 

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... 

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. 

Rey C, Collins B, Goehl T, Dickson IR, Glimcher MJ (1989) The carbonate enviromnent in bone mineral A resolution-enhanced Fourier transform infrared spectroscopy study. Calcif Tissue Inti 45 157-164 Roeder PL, MacArthur D, Ma XP, Palmer GR (1987) Cathodoluminescence and microprobe study of rare-earth elements in apatites. Am Mineral 72 801-811 Ronsbo JG (1989) Coupled substitution involving REEs and Na and Si in apatites in alkaline rocks from the Illimaussaq intmsions. South Greenland, and the petrological implications. Am Mineral 74 896-901 Rouse RC, Dunn PJ (1982) A contribution to the crystal chemistry of ellestadite and the sihcate srrlfate apatites. Am Mineral 67 90-96... 

This argument was explored by Reynard et al. (1999), using values of E and Vg obtained from the experimental partitioning data of Fujimaki (1986). Reynard et al. (1999) used Equation (1) to predict equilibrium REE-apatite partition coefficients at surface temperature and pressure, assuming that the crystal chemistry of bone apatite is broadly similar to that of HAP, and that crystal-melt partition coefficients can be used to estimate crystal-water partitioning. Reynard et al. (1999) then compared the predicted partition coefficients with measured adsorption coefficients for the REE between seawater and HAP derived by Koeppenkastrop and DeCarlo (1992), and concluded that incorporation of REE into bone via a substitution mechanism produces bell shaped REE patterns with relatively little fractionation between La and Lu. Incorporation of REE into bone via an adsorption mechanism, on the other hand, produces significant fractionation between La and Lu (La/Lu = 5). Based on REE patterns found in fossil fish teeth, they concluded that REE uptake in fossil bone was dominated by adsorption mechanisms, but that subsequent recrystallization may superimpose a degree of substitution-related fractionation over the initial, adsorption related REE pattern. It is important to note, however, that these predictions are based on crystal chemistry of hydroxyapatite and fluorapatite, and not dahllite and francolite. Variations in E and Vo will affect relative adsorption and/or partition coefficients, and may alter the predicted partition coefficient ratios (e.g., La/Lu and La/Sm). 

Ternary Compounds REiMi(Si0i) 02. In order to follow the correlation between cation substitution and variation of cell dimensions in 2 8 mixed-cation apatites, we bring forward crystal data for 4 8 hydroxy apatites from 44). It is especially atractive to introduce the 4 6 apatite compounds into the discussion of crystal chemistry because their mixed-cation ratio exactly corresponds to the ratio of the symmetry-... 

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. 

Ya. Mudryk, P. Rogl, C. Paul, S. Berger, E. Bauer, G. Hilscher, C. Godart, H. Noel, A. Saccone, R. Ferro, Crystal chemistry and thermoelectric properties of clathrates with rare-earth substitution. Phys. B 328, 44-48 (2003)... 

Rational synthesis of materials requires knowledge of crystal chemistry besides thermodynamics, phase equilibria and reaction kinetics. There are several examples of rational synthesis. A good example is SIALON [11], where A1 and oxygen were partly substituted for Si and nitrogen in SijN. The fast Na ion conductor NASICON, NajZr PSi Ojj (Fig. 1.3), was synthesized with a clear understanding of the... 

Sassi, R., Cruciani, G., Mazzoh, C, Nodali, L., Craven, J. (2008) Multiple titanium substitutions in biotites from high-grade metapelitic xenohths (Euganean Hills, Italy) complete crystal chemistry and appraisal of petrologic control. Am. Mineral, 93,339-350. 

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