Deane K. Smith

Quantitative X-Ray Powder Diffraction Method Using the Full Diffraction Pattern

A new quantitative X-ray powder diffraction (QXRPD) method has been developed to analyze polyphase crystalline mixtures. The unique approach employed in this method is the utilization of the full diffraction pattern of a mixture and its reconstruction as a weighted sum of diffraction patterns of the component phases. To facilitate the use of the new method, menu-driven interactive computer programs with graphics have been developed for the VAX series of computers. The analyst builds a reference database of component diffraction patterns, corrects the patterns for background effects, and determines the appropriate reference intensity ratios. This database is used to calculate the weight fraction of each phase in a mixture by fitting its diffraction pattern with a least-squares best-fit weighted sum of selected database reference patterns. The new QXRPD method was evaluated using oxides found in ceramics, corrosion products, and other materials encountered in the laboratory. Experimental procedures have been developed for sample preparation and data collection for reference samples and unknowns. Prepared mixtures have been used to demonstrate the very good results that can be obtained with this method.

Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography

Cristobalite and tridymite are distinct forms of crystalline silica which, along with quartz, are encountered in industrial operations and industrial products. Because the International Agency for Research on Cancer has designated “crystalline silica” as an IARC Group 2A (probable carcinogen) and quartz and cristobalite as a Group 1 (carcinogen) , it is important to properly identify and quantify the silica phase in all materials used in production and encountered in products. Opal is a form of hydrated silica which is also encountered in industry. Although some forms of opal mimic cristobalite and tridymite, they are not truly crystalline. The term “silica” in the industrial sense is used to mean any material whose composition is SiO 2 whether it is crystalline or noncrystalline. Some people also consider silica to include hydrated SiO 2 . There are many forms of SiO 2 which have both long-range and short-range order and are recognized as crystalline phases among which are quartz, cristobalite, and tridymite. The hydrated silicas, on the other hand, pose an enigma. Only a few forms show sufficient long-range and short-range order to be considered crystalline. The mineral silhydrite is an example. Opal in all its forms lacks sufficient order to be considered crystalline. Even opal-C, which produces a X-ray pattern similar to the diffraction pattern of cristobalite, lacks not only sufficient order to be considered crystalline but also contains water in the structural make-up. This paper discusses a classification and nomenclature for these forms which is critical to proper regulation. It also reviews the recent literature on tridymite, cristobalite, and opal, and provides an extensive bibliography. Modern studies have shown that opal-A is disordered, but opal-CT and opal-C contain ordered domains that mimic stacked sequences of cristobalite and tridymite sheets such that X-ray patterns show features similar to the crystalline cristobalite and tridymite. There is debate on whether the ordered regions have lost the water that characterizes the opals. In fact, heating studies have shown that all opals show changes on heating characteristic of materials that lose water in the process. The TEM evidence showing domains in the range 10–30 nm in a matrix of disordered opal suggest that the proper term for this system is paracrystalline analogous to inorganic and organic polymers.

The crystal and molecular structure of beryllium hydride

Abstract The crystal and molecular structure of BeH 2 has been determined from high-resolution powder diffraction data obtained at a synchrotron radiation source. Computer indexing methods gave the unit cell as body-centered orthorhombic with a = 9.082(4), b = 4.160(2), c = 7.707(3) A V = 291.2(2) A 3 , with systematic absences corresponding to space groups, Ibam or Iba2. Essentially, single crystal methods were used for the structure determination: Patterson synthesis to locate the Be atoms and a “heavy-atom” electron-density synthesis to confirm the location of the H atoms. The crystal structure is based on a network of corner-sharing BeH 4 tetrahedra rather than flat infinite chains containing hydrogen bridges previously assumed. The Be-H bond distances are 1.38(2) A around Be(1) and 1.41(2) A around Be(2). The H-Be-H tetrahedral bond angles range from 107° to 113° and the Be-H-Be bond angle is approximately 128°. The space group in Ibam, and there are 12 BeH 2 molecules in the unit cell. The theoretical density is 0.755 g/cm 3 .

Preparation and X-ray characterization of CsAlSiO4

A systematic study of the preparation of CsAlSiO/sub 4/ using various cesium oxide, alumina, and silica sources and a typical set of firing conditions was performed. The object was to determine effective methods of preparing phase-pure CsAlSiO/sub 4/. The reaction of Cs/sub 2/CO/sub 3/ with metakaolin at 600/sup 0/C (decomposition and calcining), 850/sup 0/C (prefiring) and 1050/sup 0/C (crystallization) produced the only phase-pure CsAlSiO/sub 4/ obtained by these methods. None of the eighteen sets of starting materials yielded a phase-pure CsAlSiO/sub 4/ in the 1100/sup 0/C and 1200/sup 0/C firings. CsAlSiO/sub 4/ was determined to be isomorphous with RbAlSiO/sub 4/ as reported by Klaska and Jarchow. CsAlSiO/sub 4/ is orthorhombic with lattice parameters of a/sub 0/ = 8.907(2)A, b/sub 0/ = 9.435(1)A, and c/sub 0/ = 5.435(1)A. The space group in Pna2/sub 1/, with Z = 4. Single crystals of CsAlSiO/sub 4/ were grown hydrothermally from a 1Cs/sub 2/O:1A1/sub 2/O/sub 3/:2SiO/sub 2/ gel in a 3M CsOH solution. The reaction conditions were 770/sup 0/C and 11,700 psi. The crystal habit of CsAlSiO/sub 4/ is needle-like.

Evaluation of the detectability and quantification of respirable crystalline silica by X-ray powder diffraction methods

X-ray powder diffraction is one of the most sensitive methods for the analysis of crystalline forms of silica. In addition to detection and quantification, it can determine the specific crystalline species in the sample. The principal limitations of the method depend on the effective volume of the sample in the X-ray beam and the number of crystallites in the proper orientation to diffract. Detection limits are usually reported as 2 μg in thin-film filter mounts and 0.1% in bulk samples that are free of interference from associated minerals. Filter methods are most often used for air quality monitoring and several standardized procedures have been certified. Standard procedures for bulk samples are difficult to certify because of the variability of the matrices and their potential interferences. All of the methods of quantification require calibration with known samples of quartz or cristobalite. Certification of standard samples involves characterization of the particle and crystallite size and size distribution and amorphous content as well as determining the X-ray diffraction response. Although quartz is readily available and cristobalite is easy to synthesize, preparation of quantities of sufficient uniformity and stability is a limiting factor in certifying such samples for reasonable costs. Conventional diffraction equipment can be used for crystalline silica analysis at the present detection limits required by safety standards. Relatively simple modifications of the diffractometer will increase its sensitivity to small amounts of silica and improve the lower limits of quantification.

The Role of Boron in Monitoring the Leaching of Borosilicate Glass Waste Forms

In the absence of any identified solid phase host (other than the original glass), boron has been assumed to accumulate in the fluid during the reaction of borosilicate glass waste forms with aqueous fluids. Using this assumption, it is possible to define a boron index which can be used to monitor the amount of glass that has been dissolved and to provide a worst-case measure of the degradation of the primary glass waste form. Several boron-containing silicate phases have been identified thus invalidating the assumption that boron does not precipitate. The effect is apparently small and the assumption that boron release is a direct measure of degree of alteration of borosilicate glass is still probably a good one.

JCPDS — International Centre for Diffraction Data Sample Preparation Methods in X-Ray Powder Diffraction

The aim of any diffraction experiment is to obtain reproducible data of high accuracy and precision so that the data can be correctly interpreted and analyzed. Various methods of sample preparation have been devised so that reproducibility, precision and accuracy can be obtained. The success of a diffraction experiment will often depend on the correct choice of preparation method for the sample being analyzed and for the instrument being used in the analysis. A diffraction pattern contains three types of useful information: the positions of the diffraction maxima, the peak intensities, and the intensity distribution as a function of diffraction angle. This information can be used to identify and quantify the contents of the sample, as well as to calculate the materials crystallite size and distribution, crystallinity, and stress and strain. The ideal preparation for a given experiment depends largely on information desired.

Use of full diffraction spectra, both experimental and calculated, in quantitative powder diffraction analysis

The use of the full powder diffraction trace over a selected diffraction range for quantitative analysis has advantages over using single peaks in that it compensates for the effects of peak overlap and low levels of preferred orientation. Using a data base composed of experimental and calculated traces, the phase composition of an unknown may be determined by determining the least-squares best-fit sum of the appropriate data base patterns to the pattern of the unknown. Weight fractions are calculated from the pattern weighting factors using the reference-intensity-ratio method.

Particle statistics and whole-pattern methods in quantitative X-ray powder diffraction analysis

Modern powder diffraction employing computer-controlled diffractometers now allows quantitative analytical methods to use the whole diffraction trace rather than only individual peaks. Two such methods are in common use: the Rietveld method, which refines the crystal structures of the component phases as part of the matching calculation, and the pattern-fitting method, which uses reference patterns from a database. Potential accuracies of these methods seems to be around 1% absolute. The most severe limitation on the potential accuracy of these methods is particle statistics, which has been reviewed in considerable detail.

The Powder Diffraction File: Past, Present, and Future

The Powder Diffraction file has been the primary reference for Powder Diffraction Data for more than half a century. The file is a collection of about 65 000 reduced powder patterns stored as sets of d/I data along with the appropriate crystallographic, physical and experimental information. This paper reviews the development and growth of the PDF and discusses the role of the ICDD in the maintenance and dissemination of the file.