R. Comes

Zirconium deficiency in nasicon-type compounds: Crystal structure of Na5Zr(PO4)3

Abstract Single crystals of Na 5 Zr(PO 4 ) 3 have been prepared using a flux method. At high temperature (200|MoC), the crystal structure can be described as a 3D network of PO 4 tetrahedra corner shared with ZrO 6 and NaO 6 octahedra. In fact, this compound belongs to the Nasicon-type class which exhibits a large nonstoichiometry range by zirconium deficiency.

Structural investigation of K+ and Tl+ β-aluminas

Abstract Structures of K+ and Tl+ aluminas were investigated by single-crystal X-ray diffraction. In the final refinements, corresponding to R values of 0.034 and 0.058, respectively, the distribution of conducting ions was found to be similar to that of Na β-alumina. In K+ β-alumina clear evidence of additional electronic density in the spinel-like blocks is obtained and is interpreted as due to a Frenkel defect.

Crystal structure and ion-ion correlation of ion-rich β alumina type compounds I. Magnesium doped potassium rich β alumina

Abstract The 3-D crystal structure of magnesium doped potassium rich β alumina ( Al 11− y Mg Y K 1+ y O 17 ) is determined with an R factor of 3.36%. The compensation mechanism for non-stoichiometry is found to be exclusively provided by the Mg 2+ ion substitution of Al 3+ within the spinel blocks. The composition deduced from the refinement ( y = 0.62) corresponds to 1.62 K + ions per conducting plane and unit cell, which is in fair agreement with the model deduced independently from the analysis of the 2-D superstructure diffuse reflections of this material. In spite of a large in-plane coherence length of the K + ions (> 150 A), no interplanar correlation is observed.

Phase transition in Nasicon compounds Na3Sc2P3O12 and Na1+xZr2 (P3−xSix) O12

Abstract Nasicon type compounds are investigated with X ray diffraction and diffuse scattering. The single crystals of scandium compound are strictly rhombohedral in the whole temperature range studied and exhibit a continuous change of their lattice parameters and occupation factors. Single crystals of NASICON reveal 2 types of long-range order before the short range order is reached at about 130°C.

Crystal structure and ion-ion correlation in ion-rich β alumina type compounds. II. potassium β ferrite

Abstract The crystal structure analysis of βK ferrite (Fe10.92K1.55O17) refined to an R factor of 5.07% shows that the compensation mechanism for non-stoichiometry (y = 0.55) is achieved simultaneously by large deficiencies in the spinel blocks and by Fe2+ ions. The ion-ion correlation studied by X-ray diffuse scattering is analysed using a mixture of two models of short range order corresponding respectively to the type of order observed previously for stoichiometric β alumina and ion rich Mg2+ doped potassium β alumina.

Quasi-long range order in β rich, alumina with a composition close to that of β″: Consequences on ionic conductivity

Abstract β rich alumina compounds which correspond to the upper limit of existence of β alumina has a composition close to that of β″ alumina. But for Na and K compounds much higher correlation lengths and narrower conduction paths than in β″ lead to higher resistivities and activation energies.

The Fe1−xCoxS system (x < 0.25); transition and the high temperature phase

Abstract The α-transition in Fe 1− x Co x S materials is examined based on electrical, magnetic and structural properties. A decrease of the transition temperature is observed with increasing alloying rate up to x co = 0.24, where the transition is completely cancelled. Beyond this critical concentration the materials exhibit a high-temperature FeS type behavior. The existence of hk 0 reflections, with h and/or k = 2 n + 1 in the cobalt-stabilized superstructure, the high-temperature phase of FeS, establishes that the structure is hexagonal, space group P 6 3 mc , with two independent metal positions, in contrast to the previously proposed tri-twinned MnP type model. The physical and structural results are discussed within the framework of conductivity by polarons. The α-transition occurs when the polaron concentration, thermally created or introduced by impurities, reaches the critical value x p ≅ 0.08. The driving mechanism for the α-transition is a reduction of the polaron dissociation energy by a temperature increase or by cobalt alloying.

Structure and local order in monovalent and divalent β and β″'; alumina

From crystal strucuture determinations and X-ray diffuse scattering, local correlations in β (Mg) and β″ (Mg) alumina are examined for various ions - with Na+ the local correlations appear to be identical and are referred to a common 2D model with theoretical composition ϱ=53 Na+.β″ alumina is stabilized by a host lattice polarization. The calculated homogeneity ranges for β alumina ϱ<1.57 and for β″alumina 1.56 < ϱ < 1.66 are in good agreement with experimental results. - With divalent ions when the ions are only present in β″ alumina on m-O sites (9d), such as Sr2+, one observes a strictly 2D correlation. When ionic polarization terms are included a part of ions occupy the M(1) site which produces a thickness of the conduction slab and interplanar correlations. Finally with strong polarization effects for Pb2+ this results in a 3-D long range order.

Etude comparative des structures des alumines β stoechiometrique et non stoechiometrique: Transition de phase dans l'alumine β stoechiometrique (II)

The usual preparation methods of β alumina lead to a non stoichiometric compound (β“N.S.”) of formula 11Al2O3−(1 + x) B2O with x ≈-0.3; a metastable phase with a composition close to stoichiometry (β“S” and x ≅ 0) can however also be obtained. X-Ray diffuse scattering studies of this stoichiometric form of silver β alumina reveals a sharp order—disorder phase transition at about 307 K. The low temperature ordered state of the silver ions is found to correspond to a 3D hexagonal superstructure with the lattice constants a3, a3, c. Above the transition temperature 3D short range order is observed up to about 315 K, where a cross over occurs towards a higher temperature 2D short range state, similar to that previously observed at low temperature in βAg “N.S.”. Above 500 K the conducting silver ions are found to be in a 2D quasi liquid state. A similar type of order—disorder phase transition seems to occur in stoichiometric sodium β alumina at lower temperature. It is concluded that the very particular behaviour namely the absence of phase transition in the usual forms of β alumina is a direct consequence of non stoichiometry.

Diffuse X-ray scattering and order-disorder effects in the iodide chain compounds N,N′-diethyl-N,N′-dihydrophenazinium iodide, E2PI1.6, and N,N′-diebenzyl-N,N′-dihydrophenazinium iodide, B2PI1.6

Abstract Crystals of E2PI1.6(C16H18I1.6N2) and B2PI1.6(C26H22I1.6N2) contain stacks of the organic cations which form an array of parallel channels filled with iodide anions. These anions are mainly “ordered” in channel direction. The structure of the iodine chain was derived from a quantitative analysis of the X-ray diffuse scattering. It consists of linear symmetric triiodide anions with intramolecular distances of 2.99 A and average intermolecular distances of 3.72 A. There is no long range order along the channel direction and the triiodide chains behave like a one-dimensional liquid or an amorphous body. The fluctuation of distances between the center of gravity of 1st neighbour I−3 molecules is well reproduced by a gaussian distribution law of 0.30 A mean deviation. However, at room temperature, short range interchain transverse correlations exist between the average iodine positions in E2PI1.6, and between the triiodide sites in B2PI1.6. Longitudinal and transverse correlations increase at lower temperatures. The triiodide units of B2PI1.6 present also a displacive modulation of their positions with a long periodicity in channel direction.