M.M. Reventós

X-ray powder diffraction phase analysis and thermomechanical properties of silica and alumina porcelains

Chemical and mineralogical characterization, using the Rietveld method, of some silica and alumina rich porcelains and its relationship with thermomechanical properties have been studied in this work. X-ray powder diffraction analysis allows to differ clearly between silica and alumina porcelains. X-ray study shows that both porcelains have a content of vitreous phase. This vitreous phase is higher in the silica than in the alumina porcelain. Dilatometric studies combined with powder diffraction methods shows a strong relationship between silica content and a lower expansion coefficients and between alumina content and a higher crash resistance. Lower contents in vitreous phase in porcelain yields to higher thermal expansion coefficients.

Quantitative Phase Analysis of Mixtures of Three Components using Rietveld and Rius Standardless Methods. Comparative Results

Eight samples, supplied by the Commission on Powder Diffraction of the International Union of Crystallography, through the Round Robin on quantitative phase analysis, were analized using standardless methods. Samples were mixtures of corundum, zincite and fluorite in different ratios. The Rietveld method, using the DBW 3.2 and FULLPROF software, and the Rius method, using MENGE-PC software, were used. Results obtained agree well with the real composition supplied (a posteriori) by the IUCr.

Crystal structure determination of 1-pentanol from low-temperature powder diffraction data by Patterson search methods

In the course of our research on normal alkanols, the crystal structure of 1-pentanol has been solved by applying Patterson-search methods to laboratory powder X-ray diffraction data recorded on a curved position-sensitive detector (CPS120) at 183 K. The crystal structure was refined with the rigid-body Rietveld least-squares method. The cell is monoclinic, space group P 2 1 ∕ c , Z =4, and the cell parameters are a =15.592(9) A, b =4.349(1) A, c =9.157(1) A, β =104.7(7)°, V =600.6(3) A 3 . There is one molecule in the asymmetric unit with the O–H bond in gauche conformation with respect to the alkyl skeleton. Packing is defined by the hydrogen bonds linking the 1-pentanol molecules along zigzag chains parallel to b .

A tangent formula derived from Patterson-function arguments. VII. Solution of inorganic structures from powder data with accidental overlap

Accidental overlap constitutes one of the principal limitations for the solution of crystal structures from powder diffraction data, since it reduces the number of available intensities for direct-methods application. In this work, the field of application of the direct-methods sum function is extended to cope with powder patterns with relatively large amounts of accidental overlap. This is achieved by refining not only the phases of the structure factors but also the estimated intensities of the severely overlapped peaks during the structure solution process. This procedure has been specifically devised for inorganic compounds with uncertain cell contents and with probable severe atomic disorder, a situation often found when studying complex minerals with limited crystallinity. It has been successfully applied to the solution of the previously unknown crystal structure of the mineral tinticite. Finally, an estimation of the smallest ratio (number of observations to number of variables) for the procedure to be successful is given.

A tangent formula derived from Patterson-function arguments. II. Practical tests with single-crystal intensity data

A series of test calculations of the tangent formula derived from Patterson-function arguments [Rius (1993). Acta Cryst. A49, 406–409] using single-crystal intensity data is presented. This new tangent formula has been compared with the results reported [Sheldrick (1990). Acta Cryst. A46, 467–473] for (a) the tangent formula incorporating the most reliable negative quartets and (b) its extension to the phase-annealing method. The success rate of the new tangent formula is an order of magnitude higher than that of (a), is better than that of (b) when the origin can float in at least one direction, and is similar to that of (b) for other space groups.

Microstructural evolution of mullites produced from single-phase gels

The crystalline microstructure of mullites obtained by heating monophasic gels has been investigated. Gels with alumina to silica molar ratio of 3:2 (as in secondary mullite) and 2:1 (as in primary mullite) were prepared by gelling mixtures of aluminium nitrate and tetraethylorthosilicate. Phase transformations were induced by heating the gel precursors, with different final treatment temperatures between 1173 and 1873 K. The mullites formed as a result of thermal treatment were studied by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The crystalline structure (unit-cell parameters) and microstructure were determined from X-ray diffraction patterns. The formation of mullites of homogeneous chemical composition and with unit-cell parameters depending almost linearly on the treatment temperature was found. Their compositions, expressed as alumina to silica molar ratio, were determined from the unit-cell parameters and were in the range of those characterizing primary and secondary mullites. Mullites processed at lower temperatures were accompanied by small amounts of vitreous phase. The crystalline microstructure of the obtained mullites was interpreted by means of a mathematical model of polycrystalline material, involving prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice strain distribution as model parameters. The model parameters were determined for each sample by modelling its X-ray diffraction pattern and fitting it to a measured pattern. Bimodality of the size distribution was observed and explained as a consequence of two crystallite nucleation and growth processes, which started from small alumina-rich and alumina-poor domains, spontaneously formed in a precursor gel at early stages of heating. Images produced by scanning and transmission electron microscopy agreed well with the characteristics obtained from the analysis of the X-ray diffraction patterns.

A new interpretation and practical aspects of the direct-methods modulus sum function. VIII

Since the first publication of the direct-methods modulus sum function [Rius (1993). Acta Cryst. A49, 406-409], the application of this function to a variety of situations has been shown in a series of seven subsequent papers. In this way, much experience about this function and its practical use has been gained. It is thought by the authors that it is now the right moment to publish a more complete study of this function which also considers most of this practical knowledge. The first part of the study relates, thanks to a new interpretation, this function to other existing phase-refinement functions, while the second shows, with the help of test calculations on a selection of crystal structures, the behaviour of the function for two different control parameters. In this study, the principal interest is focused on the function itself and not on the optimization procedure which is based on a conventional sequential tangent formula refinement. The results obtained are quite satisfactory and seem to indicate that, when combined with more sophisticated optimization algorithms, the application field of this function could be extended to larger structures than those used for the test calculations.

Quantitative X-ray diffraction phase analysis of airborne particulate collected by a cascade impactor sampler using the Rietveld full-pattern fitting method

Samples of airborne particulate were collected at the “El Ingenio” site in Castellon (Spain) using a cascade impactor sampler. Quantitative analysis of present phases in the aerosol was performed using the full-pattern fitting Rietveld method. Quantitative information was obtained from refined individual scale factors and unit-cell volumes, obtained with a Rietveld refinement program. Quartz, calcite, and gypsum were encountered as major phases, and their size distribution and concentration in the atmosphere were calculated.

Molecular structure of haemanthamine, an alkaloid from Narcissus confusus

CITHIgNO4, M r = 3 0 1 . 3 4 , orthorhombic, P212~2 l, a = 14.055 (4), b = 11.056 (5), c = 9.608 (3) A, V = 1493 (2) A 3, Z = 4, D x = 1.341 Mg m -3, F ( 0 0 0 ) = 640, M o K a radiation, 2 = 0.71069 A, p(Mo Ka) = 0.057 mm -~. The structure was solved by direct methods and refined to a final R value of 0.041 for 1369 observed reflections, confirming the anti position of C ( 1 1 ) O H with respect to the aromatic ring, and the half-chair conformation of ring C. Experimental. Platy white crystals, 0.07 x 0.09 x 0.10 mm. Philips PW 1100 computer-controlled singlecrystal diffractometer, graphite-monochromated Mo K~t radiation, w--20 scan. Cell parameters from setting angles of 25 reflections having 10 < 0 < 15 °. Data collection at 293 K: index range 2 < h _< 16, 0 _< k _< 13, 0 <_ 1 _< 11 with 20 < 50 °, three standard reflections (800, 040, 004) measured every 60 min showed only random deviations from mean intensity, 0108-2701/88/081497-02503.0

Crystal structure and microstructure of synthetic hexagonal magnesium–cobalt cordierite solid solutions (Mg2−2xCo2xAl4Si5O18)

Co(2+)-containing cordierite glasses, of nominal compositions (Mg(1-x)Co(x))2Al4Si5O18 (with x = 0, 0.2, 0.4, 0.6, 0.8 and 1), were prepared by melting colloidal gel precursors. After isothermal heating at 1273 K for around 28 h, a single-phase α-cordierite (high-temperature hexagonal polymorph) was synthesized. All materials were investigated using X-ray powder diffraction and field-emission scanning electron microscopy. The crystal structure and microstructure were determined from X-ray diffraction patterns. Rietveld refinement confirmed the formation of magnesium-cobalt cordierite solid solutions. The unit-cell volume increased with the increase of cobalt content in the starting glass. The crystalline microstructure of the cordierites was interpreted using a mathematical model of a polycrystalline material and characterized by prevalent crystallite shape, volume-weighted crystallite size distribution and second-order crystalline lattice-strain distribution. Hexagonal prismatic was the prevalent shape of α-cordierite crystallites. Bimodality in the size distribution was observed and interpreted as a consequence of two paths of the crystallization: the nucleation from glass of μ-cordierite, which transformed into α-cordierite with annealing, or the nucleation of α-cordierite directly from glass at high temperatures. Scanning electron microscopy images agreed well with crystalline microstructure characteristics determined from the X-ray diffraction line-profile analysis.