Site Preference Of Cations Research Articles

Mixed cation effects and ionic equilibria in β-alumina solid electrolytes

Conductivity data have been determined for Na/K, Na/Li, Na/Ag and Na/Tl β-aluminas as part of an investigation into the mixed alkali effect. No simple pattern of behaviour is found, but the behaviour of individual systems can be understood by reference to the site preferences of particular cations. This approach is exemplified by analysis of new data for the Na/K system. The importance of collecting data in the dilute foreign cation region is stressed, especially for quantitative treatments based on the weak electrolyte theory.

The factors influencing cation site-preferences in spinels a new mendelyevian approach

Pseudopotential orbital radii rs, rp are used to construct an index, rσ=rs+rp, which characterizes the average potential experienced by atomic valence electrons. A plot of rAσverses rBσfor 172 chalogenide spinels (AB2X4, X=O, S, Se, Te) leads to two well defined areas, which separate normal and inverse spinels, with only four errors (a predictive success rate of 98%). The gross sorting is achieved without recourse either to the number of d-electrons or an orbital radius rd, from which it is inferred that it is the energies and extents of the cation s and p-orbitals which primarily determine coordination number in these systems. This approach to the problem of cation distribution in spinels is contrasted with the less generally applicable, traditional, crystal field ideas. The relevance of both rσ and crystal field stabilization energies to the thermodynamics of spinel reactions is also discussed.

A Mössbauer study of hyperfine interaction in the systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mossbauer study of systems CoxMn3−x−yFeyO4 and NixMn3−x−yFeyO4 for values ofx=0·1, 0·5, 1·0 andy ranging from 0·1 to 2·0 in steps of 0·2 have been made. At room temperature samples fory values ranging in between 0·1 to 0·5 exhibit paramagnetic behaviour while all spectra for values ofy between 0·6 to 0·8 show relaxation effects. Well-defined hyperfine Zeeman spectra are observed for all the samples withy>0·8 and resolved in two sextets corresponding to octahedral and tetrahedral site symmetries and a central doublet probably due to the presence of super-paramagnetic particles in the system. The hyperfine field at57Fe nucleus reduces with decreasing iron cobalt and nickel concentration. These observations have been explained in terms of site preference of cations and exchange interactions.

Relations between (130) spacing,chemical composition,and cation site preference of olivine.

The relation between the (130) lattice spacing (d130) and the chemical composition of olivine in the system Mg2SiO4(Fo)-Fe2Si04(Fa)-Mn2SiO4(Te) has been successfully established. The regression analyses based on the equation, d130(A)=A(1)×Te(mol %)+A(2)×Fa(mol %)+A(3)×Te2+A(4)×Fa2+A(5), have been carried out for 75 natural and 44 synthesized olivines, respectively. The resulting regression coefficients show that there are significant discrepancies between these relations for synthetic and natural olivines. The differences of the term Te2 is the most significant, suggesting that the cation site preference exerts influences on the relationships. Preliminary Mossbauer measurements were made on Mn-olivines. It is demonstrated for the first time that natural olivines exhibiting preferential ordering of larger Mn in the larger M2 site have larger d130 values than those of synthetic ones having relatively disordered cation arrangements. Thus d130 depends not only on the chemical composition, but also on the cation distribution in the M1 and M2 sites of olivine. On the other hand, the coefficient for the term of Fa2 corresponding to the site preference of iron is smaller than that of Te2, and contains relatively large errors. The difference of the coefficients between natural and synthetic olivines is meaningful, considering the facts that in the synthetic olivines, larger Fe ions are more enriched in the smaller MI site than in the natural olvines, as already reported by Shinno (1974). For synthetic olivines, the relation is nearly linear, whereas for natural olivines, it is not linear as well as in the case of Mn-olivines. For the olivine in the system Fo-Fa, the regression equation containing only 1-st order term, X-ray determinative curve, have been obtained for natural and synthetic olivines, respectively, and compared with the curves already reported by several researchers. The error attached to an individual measurement ranges from 1.6 to 2.4 mol %, depending on the composition.

Some new sulfo-spinels containing iron-group transition metals

The study of the cation site preferences related to Rh 3+ or Sn 4+ ions in the spinel lattice leads to a series of new spinels. Various sulfo-spinels containing some of the transition elements such as Mn, Fe, Co and Ni, have been synthesized and the cation distributions and u-parameters were determined. Some magnetic and electrical properties were measured for Fe 2RhS 4 and Cd 1−xFe 1+xSnS 4 (0 ≤ x ≤ 0.9).

Mössbauer Spectra of Mixed Crystals; Diamagnetically Substituted Yttrium Iron Garnet in the Critical Region

There is a range of temperature where the M\ossbauer spectra of $^{57}\mathrm{Fe}^{3+}$ in diamagnetically substituted magnetically ordered iron oxides show broad lines and may be reproduced by local-molecular-field theory. The lines can exhibit a well-defined structure if the number of neighboring cation sites is less than six, and the spectra can then be used to determine whether the cation distribution is random. Such is the case for ${\mathrm{Y}}_{3}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x})}_{2}{[\mathrm{Fe}]}_{3}{\mathrm{O}}_{12}$ with $x=0.10 \mathrm{and} 0.25$. The site preferences of other trivalent cations in yttrium iron garnet (YIG) are measured, including ${\mathrm{Rh}}^{3+}$ which goes onto $a$ sites. At higher temperatures, central peaks appear in the spectra of mixed oxides, and they grow at the expense of the magnetically split spectrum until it finally disappears at ${T}_{c}$. The spectra of ${\mathrm{Y}}_{3}{({\mathrm{Fe}}_{0.75}{\mathrm{Sc}}_{0.25})}_{2}{[\mathrm{Fe}]}_{3}{\mathrm{O}}_{12}$ were studied in and around this broad critical region. An applied field produces increased hyperfine splitting, which is correlated with the static magnetic susceptibility. The mixed spectra cannot be explained by a range of ordering temperatures in different parts of the crystal, or by independent relaxation of the ionic spins. Short-range collective behavior is indicated, with coupled moments of $({10}^{2}\ensuremath{-}{10}^{4}){\ensuremath{\mu}}_{B}$ around ${T}_{c}$. A computer study of the geometry of the random lattice shows that there is no tendency to form localized clusters of the right size. The short-range order is probably evanescent.

Studies on reaction kinetics in magnetic oxide minerals

Reactions within spinel-type oxides are examined by consideration of (i) the law of mass-action and its limits, (ii) the structural field situation (Mossbaufr) and the (high- or low-) spin state, (iii) the cation site preferences. The conditions for electron transport, cation and vacancy diffusion during oxidation are determined. Effects of defect diffusion on domain-wall mobility and ordering processes during thermal influences are discussed.

Cation distributions in multisublattice ionic crystals and applications to solid solutions of ferrimagnetic garnets and spinels

An equilibrium expression of cation transfer among oxygen-coordination lattice sites is given which takes into account the solid solution concentration parameters and the stoichiometric restrictions in multisublattice crystals. Some energy considerations are made about cation site preferences in ionic crystals. Ionic distributions in sublattices are inferred. Comparison of derived distribution expressions for some magnetic garnet and spinel solid solutions is made with experimental magnetic moments from literature.

Some effects of charge-compensating ions on cation site preferences in complex oxides∗

The influence of anion charge compensation on the site preferences of Ni 2+ and Co 2+ in oxides such as spinels and magneto-plumbites is compared with that of cation charge compensation. By means of diffuse reflectance measurements on Co 2+ and Ni 2+-substituted BaAl 12O 19 and BaGa 12O 19, it is concluded that the use of fluoride ion as a charge compensator confers additional stability to bivalent ions in tetrahedral sites. Magnetization data on BaCo x Fe 12− x O 19− x F x and Cu 1+ x Fe 2− x O 2− x F x are in accord with this hypothesis. Spectroscopic investigation of Ni 2+ and Co 2+ substitution in the garnets Y 3Al 5O 12 and Y 3Ga 5O 12 do not show stabilization of the tetrahedral sites, but indicate that some of the bivalent ions appear to have entered yttrium (dodecahedral) sites.