Isovalent Cations

The only data for MgO appear to be for isovalent Ni additions [204]. The hardening rate at ambient temperature is somewhat higher than that for Ti + in sapphire, and the data best fit a c law. The plateau regime begins at about 400 °C for MgO (at least for Fe additions [205]), and so the yield stress behavior is in the low-temperature region where the solution-hardening rate is temperature-dependent. 

Pletka et al. [203] analyzed the solution-hardening rate in sapphire using the Labusch model [202], where the increment in yield stress due to solution hardening 

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A modification of the chemical composition by introducing adequate dopants provided some improvements. The most used dopants are alkaline earth cations (Mg2+, Sr2+) and isovalent cations as Al3+ and Mn3+. 

Let us consider first isovalent cations. MgO can dilute ions of similar size, as for instance Ni or Co forming NiO-MgO and CoO-MgO solid solutions with an infinite range of composition. The effect of progressively replacing Mg by Ni or similar cations (Co ", Cu ) on the surface properites has been investigated both experimentally [83,177,178] and theoretically [179,180]. The presence of Ni cations diluted in the MgO matrix results in an efficient catalyst for nitrous oxide, N2O, decomposition this has been attributed to the different bond strength of the Ni-0 and Mg-0 bonds at the surface [177]. Plane wave calculations on Ni-doped MgO have shown that the presence of Ni atoms on the... 

An appreciation of the mechanisms of trace element-bone interaction can be gained by comparing uptake of REE in bone with experimental and analytical studies of high temperature apatite-melt REE partitioning behavior. Apatite-melt partition coefficients derived from experimental studies, when plotted against ion radius, show parabolic traces with maxima around Nd-Sm (Fig. 3). These REE ions have radii closest to Ca, and indicate that REE are held in Ca sites in the lattice. Blundy and Wood (1994) developed a predictive model for calculation of equilibrium partition coefficients of trace elements between crystals and melt in igneous systems. In this model, the partitioning behavior of isovalent cations between a mineral and liquid at a particular pressure, temperature, and composition of interest can be explained by a version of the Brice (1975) equation, in which the size and elasticity of the crystal lattice sites play a critical role ... 

The addition of yttrium and other oversized isovalent cations to alumina has also been shown to enhance creep strength (Figure 5-3) (French et al., 1994). The oversized ions segregate to the alumina grain boundaries (Figure 5-4). One... 

Teppen, B. J., and D. M. MiUer. 2006. Hydration energy determines isovalent cation exchange selectivity by clay minerals. Soil Science Society of America Journal 70, no. 1 31-40. 

The substitution of Ce by isovalent cations, such as Zr, does not necessitate the formation of additional (charge-compensating)... 

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

Experimental clinopyroxene-melt (Brooker et al. 2003) and wollastonite-melt (Law et al. 2000) partitioning data can be fitted to the electrostatic model (see Fig. 9). Only species entering the large cation site (e.g., M2 in clinopyroxene) are considered. For each isovalent group (actinides, lanthanides etc.) derived as shown in Figure 4, is... 

What shape would an early transition-metal hydride adopt if the ionic component were reduced Structural analysis of the cation HfH3+ (which is isovalent with LaH3) provides insight. The molecular cation exhibits bond angles (98.1°) that are nearly 10° less than those of LaH3, even though the Hf—H bond ionicity (lOOcHf2 = 36.35%) still deviates appreciably from the covalent limit. 

Although beyond the main focus of this chapter, it is interesting to compare some of the properties of the cationic complexes to isovalent or isoelectronic neutral analogues, such as boron or aluminum pentahydride. Both BHj and AIH3 are electron deficient, and their stable forms are and AUH,. But how much do... 

Carbon disulfide is isovalent to carbon dioxide and it also has a bent monomer anion. While gas-phase CO2 has negative EAg of —0.6 eV [24], for CS2, EAg is +0.8 eV [34]. Despite this very different electron affinity, Gee and Freeman [34] observed long-lived electrons in CS2 (with lifetime > 500 psec) with mobility ca. 8 times greater than that of solvent cations. Over time, these electrons converted to secondary anions whose mobility was within 30% of the cation mobility. Between 163 and 500 K, the two ion mobilities scaled linearly with the solvent viscosity, as would be expected for regular ions. Of course, Gee and Freeman s identification of the long-lived high-mobility solvent anions as electron is just a manner of speech Obviously, quasifree or solvated electrons cannot survive for over a millisecond in a positive-EAg liquid. 

Mixed group 15 group 16 polycations are rare and tend to form clusters. For example, the arsenic-chalcogen cations [As3E4]" (E = S, Se) have polycyclic structures similar to those of the well-known isovalent species P4S3 and the bismuth-tellurium cation [Bi4Te4]" " adopts a slightly distorted cubic... 

The structures of the cations [SyX] (X = I, Br) show bond length alternations similar to those found in the isovalent neutral molecule S7O (Figure 12.5). The bond elongations are most pronounced near the source of the perturbation, i.e. the three-coordinate sulfur centre. The S-S bond length of ca 2.39 A in [87 ] is especially long, indicative of a bond order of ca 0.4. ... 

NiO is a cation deficient semiconductor. The fraction of its cation vacancies and compensating electron holes depends on the oxygen potential as discussed in Section 2.3. The isovalent Ca2+ ions can replace Ni2+ ions in the cationic sublattice of the fee matrix by chemical interdiffusion. TiOz and NiO form NiTi03 which dissolves to some extent in the fee matrix of NiO as Ti and Vmc. The counterdiffusion of Ti02 and CaO in the NiO solvent leads to the encounter of the different solute cations (Fig. 9-12a). With increasing overlap of their concentration profiles, the concentration of the product will eventually surpass the solubility limit (and the nucleation barrier). Precipitation of the rather stable CaTi03 compound as an internal reaction product in the NiO matrix is the result. 

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