John G. Thompson

The crystal chemistry underlying ferroelectricity in Bi4Ti3O12 Bi3TiNbO9, and Bi2WO6

Abstract The recent rerefinements of the crystal structures of Bi4Ti3O12, Bi3TiNbO9, and Bi2WO6 differ substantially from earlier reported structure refinements. A comparison of the major structural distortions present in each of these materials is presented and the main structural cause of ferroelectricity (in Bi4Ti3O12 and Bi3TiNbO9) is shown to be the a-axis displacement of Bi atoms in the perovskite A sites with respect to the chains of TiO6 octahedra and not the perovskite B atoms moving toward an octahedral edge. By using a modulated structure approach, the driving force for this displacement and other structural features can be simply understood in terms of the need to satisfy bond valence requirements.

The synthesis and structure of Ba2Cu3O5+δ

Ba2Cu3O5+δ has been prepared with ∼99% purity and possesses two related structural forms depending on δ. Both low-(δ = 0.6 to 1.0) and high- (δ ⋍ 0.5) Ba2Cu3O5+δ possess C-centered orthorhombic ‘average’ unit cells with ab ⋍ √3 and 1.91, respectively. Low-Ba2Cu3O5+δ also has a doubled c axis. Incommensurate satellite reflections are observed for both the low- and high-Ba2Cu3O5+δ structures. The modulation wave-vector varies with δ both in magnitude and direction. A structural model for low-Ba2Cu3O5+δ is proposed and the reason for the modulation discussed.

Atmospheric degradation of the high-temperature superconductor, YBa2Cu3O7−x

The superconducting oxides YBa2Cu3O7−x and HoBa2Cu3O7−x degrade in the presence of atmospheric water. Planar defects appear in the crystal lattice normal to c, followed by gross distortion of the lattice, then complete breakdown to give a mixture of insulating products. Corrosion is enhanced by increasing the relative humidity and temperature.

The structure and microstructure of α-cristobalite and its relationship to β-cristobalite

Recent transmission electron microscopy (TEM) electron diffraction investigation of β-cristobalite has revealed a strong characteristic diffuse intensity distribution. In this work a similar, but less intense, diffuse scattering is reported for synthetic α-cristobalite. This material also displays both tetragonal twinning and characteristic striations correspond to planar boundaries which may exist on any {1, 0, l}tetragonal plane.

Selective gas adsorption by amorphous clay mineral derivatives

Amorphous derivates prepared by aqueous reaction of various aluminosilicate clay minerals with concentrated KF solution at 80–110°C were studied for their gas adsorption properties. The four clay minerals studied are halloysite, a well-crystallized kaolinite, a poorly crystallized kaolinite, and a montmorillonite. The gases tested are N2, O2, CH4, CO, CO2, and C2H2. The kaolin-group mineral derivatives are characterized by substantial reduction in particle size, high specific surface, and significant selectivity towards CO2 and C2H2 relative to the other gases. The montmorillonite derivative shows no increase in adsorption over the starting material, however, for all the materials high adsorption of CO2 and C2H2 was observed. Levels of gas adsorption and gas adsorption ratios are comparable to pillared clays.

Interpretation of Solid State 13C and 29Si Nuclear Magnetic Resonance Spectra of Kaolinite Intercalates

Abstract13C and 29Si nuclear magnetic resonance spectroscopy with magic-angle spinning bas been used to study the short-range ordering and bonding in the structures of intercalates of kaolinite with formamide, hydrazine, dimethyl sulfoxide (DMSO), and pyridine-N-oxide (PNO). The 29Si chemical shift indicated decreasing levels of bonding interaction between the silicate layer and the intercalate in the order: kaolinite: formamide (δ) = -91.9, ppm relative to tetramethylsilane), kaolinite: hydrazine (-92.0), kaolinite: DMSO (-93.1). The 29Si signal of the kaolinite:PNO intercalate (-92.1) was unexpectedly deshielded, possibly due to the aromatic nature of PNO. The degree of three-dimensional ordering of the structures was inferred from the 29Si signal width, with the kaolinite: DMSO intercalate displaying the greatest ordering and kaolinite: hydrazine the least. 13C resonances of intercalating organic molecules were shifted downfield by as much as 3 ppm in response to increased hydrogen bonding after intercalation, and in the kaolinite: DMSO intercalate the two methyl-carbon chemical environments were non-equivalent (δ = 43.7 and 42.5).РезюмеСпектроскопия ядерного магнетического резонанса 13C и 29Si использовалась для исследования короткодействующего упорядочения и связи в структуре прослоек каолинита с формамидом, гидразином, диметиловой сероокисью (ДМСО) и пиридино-N-окисью (ПNO). Химический сдвиг 29Si указывал на уменьшающиеся уровни взаимодействия связи между силикатными слоями и включаемым веществом в порядке: каолинит: формамид (δ = -91,9, частей на миллион по отношению к тетраметилсилану), каолинит: гидразин (-92,0), каолинит: ДМСО (-93,1). Сигнал 29Si прослойки каолинит: ПNO (-92,1) оказался неожиданно не защищенным, вероятно, в результате ароматической природы ПNO. Степень пространственного упорядочения структур была обнаружена при помощи ширины сигнала 29Si. Прослойка каолинит: ДМСО имела наибольшее упорядочение, а каолинит: гидразин—наименьшее. Резонансы 13C включаемых органических молекул перемещались вниз на величину порядка 3 частей на миллион в результате увеличивающейся водородной связи после прослаивания. В случае прослойки каолинит: ДМСО, две химические группы метил-углерод были неравновесны (δ = 43,7 и 42,5). [E.G.]Resümee13C. und 29Si nuklearmagnetische Resonanzspektroskopie mit “Magic-angle Spinning” wurde verwendet, um die Nahordnung und die Bindung in den Strukturen von Wechsellagerungen von Kaolinit mit Formamid, Hydrazin, Dimethylsulfoxid (DMSO), und Pyridin-N-Oxid (PNO) zu undersuchen. Die chemische Verschiebung von 29 Si deutete auf abnehmende Niveaus der Bindungswechselwirkung zwischen der Silikatschicht und der Einlagerung hin, in der Reihenfolge: Kaolinit: Formamid (δ = -91,9, ppm in Vergleich zu Tetramethylsilan), Kaolinit: Hydrazin (-92,0), Kaolinit:DMSO (-93,1). Das 29Si-Signal der Kaolinit: PNO-Wechsellagerung (-92,1) war unerwartet wenig abgeschirmt, wahrscheinlich aufgrund der aromatischen Natur von PNO. Der Grad der dreidimensionalen Ordnung der Strukturen wurde aus der Breite des 29Si-Signals abgeleitet, wobei die Kaolinit: DMSO-Wechsellagerung den höchsten Ordnungsgrad und die Kaolinit: Hydrazin-Wechsellagerung den niedrigsten zeigte. Die 13C-Resonanzen der eingelagerten organischen Moleküle wurden bis zu 3 ppm nach geringerer magnetischer Feldstärke verschoben als Auswirkung einer zunehmenden Wasserstoffbindung nach der Einlagerung, und in der Kaolinit: DMSO-Wechsellagerung waren die zwei chemischen Methyl-Kohlenstoff-Milieus nicht gleich (S) = 43,7 und 42,5). [U.W.]RésuméLa spectroscopic de résonance magnétique nucléaire de 13C et de 29Si avec spin d’angle magique a été utilisée pour étudier l’ordre à court terme et les liaisons dans les structures d’intercalates de kaolinite avec la formamide, l’hydrazine, la sulphoxide diméthyle (DMSO), et l’oxide-N-pyridine (PNO). Le déplacement chimique de 29Si a indiqué des niveaux décroissants d’interaction de liaisons entre la couche silicate et l’intercalate dans l’ordre: kaolinite: formamide (δ) = -91,9, ppm relatives à la tétraméthylsilane), kaolinite: hydrazine (-92,0), kaolinite: DMSO (-93,1). Le signal 29Si de kaolinite: hydrazine (-92,1) était découvert de manière inattendue, possiblement à cause de la nature aromatique de PNO. Le degré d’ordre à trois dimensions des structures a été inféré à partir de la largeur du signal 29Si, avec l’intercalate kaolinite: DMSO montrant le plus grand ordre et la kaolinite: hydrazine, le plus petit. Les résonances 13C de molécules organiques intercalantes étaient deplacées vers le bas du champ par autant que 3 ppm en reponse à une liaison d’hydrogene augmentée après l’intercalation, et dans l’intercalate kaolinite:DMSO, les deux environements chimiques méthyl-carbone étaient non-équivalents (δ = 43,7 et 42,5). [D.J.]

An electron diffraction study of the polymorphs of siO2-tridymite

The results of a detailed TEM study of the low frequency modes of distortion of high temperature SiO2-tridymite and their relationship to the extensive structural polymorphism of tridymite are presented. It is found that low energy modes of distortion of the ideal tridymite tetrahedral framework structure give rise to a strong and extremely characteristic diffuse intensity distribution (which can be broken into two component types) for the two highest temperature polymorphs of tridymite. Experimental results strongly suggest that this observed diffuse distribution is not a result of irreversible beam damage but is rather an intrinsic property of the ideal tridymite tetrahedral framework structure. The diffuse intensity distribution is closely related to the lower temperature polymorphs of tridymite — in particular, the primary modulation wave-vectors of these low temperature polymorphs always fall on the higher temperature diffuse distribution. The first type of diffuse distribution appears to result from coupled tetrahedral edge rotation of 〈110〉 columns of corner-connected SiO4 tetrahedra (uncorrelated from column to column as a result of the tetrahedral connectivity of the ideal tridymite framework structure). The real space structural origin of the second curved type of diffuse distribution, however, remains unclear.

Selective gas adsorption by metal exchanged amorphous kaolinite derivatives

Metal exchanged amorphous kaolinite derivates are studied for their gas adsorption properties. The kaolinite derivatives were prepared by aqueous reaction of the clay with concentrated KF solution at 80–110°C. The K+ ions were then exchanged by Cs+, Sr2+, La3+, Fe3+ or Cu2+. Fe(II)- and Cu(I)-containing derivatives were prepared by reduction under H2. The gases tested were N2, O2, CH4, CO, CO2, and C2H2. Correlations between gas retention and various physical and chemical variables are investigated. Gas retention levels normalised against BET surface show that level of adsorption is at least partly determined by surface chemical composition. The highest gas retention levels per unit area are achieved for alkali metal exchanged derivatives or alternately for exchange cations with the largest ionic radius.

Diffuse scattering in yttria-stabilized cubic zirconia

Abstract We have used a position-sensitive detector (PSD) system to make measurements of the diffuse X-ray scattering on a cubic Y 2 O 3 -stabilized zirconia, Zr 0.61 Y 0.39 O 1.805 , in far greater detail than has hitherto been reported. In addition to the fairly prominent diffuse peaks visible in 〈1 1 0〉 sections that have been the center of discussion in many previous studies we see a number of other characteristic diffraction features. We report results of some Monte Carlo simulations in which we attempt to find the origins of these various features. We believe one feature, in the form of sets of dark planes normal to 〈1 1 0〉, occurs because of size-effect -like strains induced along 〈1 1 0〉 intermetal vectors, and is only visible because of the similarity of the scattering factors for Zr and Y. A second feature, in the form of bow-tie -shaped regions of scattering, originates from the same basic strains but the symmetry of these requires that displacements in the [1 1 0] direction are out of phase with those in the [1 – 1 0], implying a distortion of the basic metal coordination such that an expansion along [1 1 0] is accompanied by a contraction along [1 – 1 0].

A transmission electron microscope and group theoretical study of the new Bi-based high-Tc superconductors and some closely related Aurivillius phases

The Bi-based superconductors of approximate compositions Bi2Sr2CuO6 and Bi2Sr2CaCu2O8, and the closely related Aurivillius phases, Bi2BaNb2O9 and Bi4Ti3O12, are discussed from a group theoretical point of view. The structural complexities associated with these phases are treated in terms of successive modulation of an appropriately chosen parent structure. The space-group symmetries of the two Aurivillius phases and the average-structure space-group symmetries of the two superconductors are interpreted by such an approach. It is proposed that compositional ordering of the oxygens midway between adjacent Bi layers is responsible for the enlarged square root 2ap* square root 2ap average-structure basal-plane cell of the superconductors. In the case of the incommensurate q= gamma a*+ delta c* modulation, the dimensional inflexibility of the CuO4 square-planar sheets is seen as the underlying structural case.