Cation Radius Ratio Research Articles

Ionic conduction and disorder in the Gd 2Sn 2O 7 pyrochlore system

Electrical conductivity measurements were performed on Gd 2Sn 2O 7 as a function of temperature, Po 2 and Ca doping concentration. An effective frenkel constant and oxygen vacancy mobility were derived. The high level of intrinsic anion disorder is consistent with expectations based on the cation radius ratio ( r A r B ) in A 2B 2O 7 pyrochlore compounds. These measurements were extended to the solid solution Gd 2(Ti 1 − x Sn x ) 2O 7 and Gd 2(Zr 1 − x Sn x) 2O 7. Intrinsic anion disorder was correlated with x or r A r B . Systematic decreases in oxygen vacancy mobility with increasing Sn content were observed. An apparent second ionic plateau in a plot of σ vs. Po 2 was found at low Po 2in Gd 2(Ti 1 − xSn x) 2O 7 ( x ≤ 0.6) which appears to be connected with the reduction of Sn 4+ to Sn 2+. The variations in ionic carrier density and mobility with composition are discussed in light of appropriate defect models and structural evolution of the lattice.

Crystal structures of synthetic 7 angstrom and 10 angstrom manganates substituted by mono- and divalent cations.

The crystal structures of synthetic 7 angstrom and 10 angstrom manganates, synthetic birnessite and buserite, substituted by mono- and divalent cations were investigated by X-ray and electron diffractions. The monoclinic unit cell parameters of the subcell of lithium 7 angstrom manganate, which is one of the best ordered manganates, were obtained by computing the X-ray powder diffraction data: a = 5.152 angstroms, b = 2.845 angstroms, c = 7.196 angstroms, beta = 103.08 degrees. On the basis of the indices obtained by computing the X-ray diffraction data of Li 7 angstrom manganate, monovalent Na, K and Cs and divalent Be, Sr and Ba 7 angstrom manganates were interpreted as the same monoclinic structure with beta = 100-103 degrees as that of Li 7 angstrom manganate, from their X-ray diffraction data. In addition, divalent Mg, Ca and Ni 10 angstrom manganates were also interpreted as the same monoclinic crystal system with beta=90-94 degrees. The unit cell parameters, especially a, c and beta change possibly with the type of substituent cation probably because of the different ionic radius, hydration energy and molar ratio of substituent cation to manganese. However, these diffraction data, except for those of Sr and Ba 7 angstrom and Ca and Ni 10 angstrom manganates, reveal only some parts of the host manganese structure with the edge-shared [MnO6] octahedral layer. On the other hand, one of the superlattice reflections observed in the electron diffractions was found in the X-ray diffraction lines for heavier divalent cations Sr and Ba 7 angstrom and Ca and Ni 10 angtrom manganates. The reflection presumably results from the substituent cation position in the interlayer which is associated with the vacancies in the edge-shared [MnO6] layer and indicates that the essential vacancies are linearly arranged parallel to the b-axis. Furthermore, the characteristic superlattice reflection patterns for several cations, Li, Mg, Ca, Sr, Ba and Ni, manganates were interpreted that the substituent cations are regularly distributed in the interlayer according to the exchange percentage of substituent cation to Na+. In contrast, the streaking in the a-direction observed strongly in the electron diffractions for heavier monovalent cations, K and Cs, manganates probably results from the disordering of their cations in the a-direction in the interlayer.

Electrical Conductivity in (Gdl-xCax)2Sn2O7±δPyrochlore System

AbstractElectrical conductivity measurements were performed on Gd2Sn207as a function of temperature, P02and Ca doping concentration. An effective frenkel constant and oxygen vacancy mobility were derived. The high level of intrinsic anion disorder found in this study is consistent with expectations based on the cation radius ratio (rA/rB) which was earlier found to be important in determining anion disorder in A2B207pyrochlore compounds [1]. The magnitude of the ionic conductivity in Gd2Sn2O7was found to be depressed relative to Gd2(Til-xZrx)207based systems due to a high oxygen vacancy migration energy.

Solvatation en milieu organique des cations intercales dans les xerogels de V 2O 5

V 2O 5 xerogels (V 2O 5, 1.6H 2O) undergo topotactic reversible exchange reaction at room temperature in organic solvents containing monovalent alkali or divalent (Mn 2+) cations. Basal spacings are dependent on solvent type and charge to radius ratio of guest cations. From interlayer distances, two solvation stages have been inferred, depending on the nature of the solvent and of the cation, except with Cs + for which no intracrystalline swelling by organic solvents is observed.

Crystal structure of dehydrated LiNa-A type zeolites

The crystal structures of dehydrated Li-exchanged A-type zeolites, determined by neutron and X-ray diffraction powder methods, indicate that the majority of the Li-cations are located at the six-ring centres. The framework distortion is progressive from d-K-A to d-Na-A and d-Li-A and follows the charge to radius ratio of the exchangeable cations. The shape of the framework eight-rings changes accordingly.

Ionic dynamics in the LiCl–KCl system at liquid state

The mixing effects in the fused system LiCl–KCl are studied by molecular dynamics simulation. The predominant contribution of the Coulombic interaction is clearly detected, its nonlinear variation gives rise to the main part of the excess internal energy. This change reflects the structural ionic arrangement; at high potassium concentration a significant decrease of the Li+–Cl− distance is observed. The anion–cation radius ratio has a marked effect: the lithium ions remain fourfold coordinated by chloride ions throughout the concentration range while the potassium ions are about sixfold coordinated. It is shown that the ion motion proceeds from two main mechanisms: an oscillatory displacement in the force field of the surrounding particles which tends to restore the electroneutrality and a collective mode which concerns the ion and its ionic atmosphere. This approach is described by the generalized Langevin equation and may open a better understanding of the transport processes at liquid state. It provides a link between the structure and the dynamical properties, the strong Li+–CL− interactions leading to a significant decrease of the Li+ mobility when the potassium concentration increases.

Cationic radius ratio and formation of K 2NiF 4-type compounds

A survey of known compounds which can assume K 2NiF 4-type structure at least over a certain P, T range was made. The cationic radius ratios r A r B of this family of compounds were found to vary over a relatively narrow range of values around 2.00, in contrast to the much wider ranges known for other important A 2 BX 4 structure groups, e.g., β-K 2SO 4, thenardite, olivine, etc., providing thereby a constraint on the crystal chemical variabilities in this group of compounds. Usefulness of the simple size parameter r A( IX) r B( VI) in predicting formation of new K 2NiF 4-type compounds is discussed.

Crystal chemistry of AB 2X 4 (X = S, Se, Te) compounds

An attempt is made to correlate the crystal structures of ternary chalcogenides of composition AB 2X 4 with the cationic radius ratio and a pseudo force-constant involving their electronegativities. The resultant diagram adequately resolves structures based on the types K 2SO 4, monoclinic, olivine, MnY 2S 4, Th 3P 4, and CaFe 2O 4 but structure types based on spinel, Cr 3Se 4, and Ag 2HgI 4 are not resolved. Crystal chemical arguments are used to explain these observations and to advance reasons for the successes and failures of this method for predicting structure types.