Comparison of results using synchrotron radiation and X-ray tubes requires a knowledge of the fundamentally different profile shapes inherent in the methods. The varying asymmetric shapes and peak shifts in focusing geometry limit the accuracy and applications of the method and their origins are reviewed. Most o f the focusing aberrations such as specimen displacement, flat specimen and θ-2θ mis-setting do not occur in the parallel beam geometry. The X-ray optics used in synchrotron parallel beam methods produces narrow, symmetrical profiles which can be accurately fit with a pseudo-Voigt function, They have the same shape in the entire pattern. Only the width increases as tanθ due to wavelength dispersion but with higher resolution systems dispersion can be eliminated. The constant instrument function contribution to the experimental profile shape is an important advantage in studies involving profile shapes, e.g., small particle sizes and microstrains, and accurate integrated intensities. The absence of systematic errors leads to more precise lattice parameter determinations.

Quantitative X-ray powder diffraction using the complete digitized diffraction pattern has proved to be an effective approach to improving the accuracy of the analysis of complex mineral mixtures, provided representative reference patterns and accurate Reference Intensity Ratio (RIR) factors arc available for each component phase. However, chemical and structural variability of common rock-forming minerals may complicate the pattern fitting approach. A method has been developed which utilizes X-ray fluorescence chemistry of an unknown and realistic compositional ranges for component phases as constraints on the quantitative XRD analysis without significant compromise of the pattern fit. This unique approach no only yields accurate weight fractions, but also provides indications of the specific compositions of each phase present in the mixture.

The discovery of the superconducting material Y1Ba2Cu3O6+δ ( “123” material) resulted in a world wide interest in the pseudo-ternary system BaO·YO·CuO. A complete study of the phases present in this system was initiated to develop a better understanding and processing of the superconducting 123 material. The crystal structures were established for two of the three ternary compounds in this system immediately after the discovery of superconductivity. One such phase was a green insulating compound Y2Ba1Cu1O5 (”211”) which has the space group. The superconducting 123 compound was found to have the space group Pmmm and an ordered triple-celled perovskite structure.

The synthesis of a new family of aluminophosphate-based molecular sieves containing pores defined by 18 tetrahedrally linked atoms has been described by Davis et al at Virginia Polytechnic Institute (VPI). This development reprsents the first reposted synthesis of a moleoular sieve with pores larger than those defined by 12 tetrahedrally linked atoms. This material has been shown to readily absorb triisopropyl benzene and exhibits a pore size distribution, determined from, its argon adsorption isotherm, which suggests the existence of molecular sieve pores larger than those found in known molecular sieves. Confirmation of a pore defined by 18 tetrahedrally linked aluminum and phosphorus atoms was accomplished by crystallographic structure determination using X-ray powder diffraction data. Single crystal methods were impossible due to an inability to synthesize crystals of suitable size. Details of the structure determination are reported in this work.

A method for quantitative phase analysis without standards (QPAWS) has been published in 1977 and has gained considerable interest, as the calibration for quantitative XRD may sometimes be difficult. Standards for quantitatative XRD are not generally available, and in many cases even the pure phases cannot be obtained.

The QPAWS method is based on (i) analysis of all phases present in the samples, (ii) foreknowledge of mass absorption coefficients (MAC)(iii) measuring samples which contain all phases in varying concentrations. In this method the number of samples used is equal to the number of phases to be analysed.

The Shadow system consists of a set of programs and files, the relation of which is illustrated in Figures 1 and 2. Of the three programs in the system, Shadow and XRDPLT are used routinely. The third, INSCAL, is used in the determination of various parameters characterizing the instrument. Two files, Shadow.DFT and XRDPLT.DFT, contain default values for parameters controlling the reduction and display of data. A third file, WSGDAT, contains the instrument parameters.

A Search/Match program using digitized X-ray powder diffraction Scans as inputs was presented in 1987 at the Denver Conference. This program has been rewritten to take full advantage of ttie newest capabilities of high end personal Computers. Software improveraents combined with the faster processing speed make it possible to search the 53,791 pattern data base in less than 16 seoonds typically (Compaq Deskpro 386/30 with 9 MB of RAM and 80387-20 numerioal processor), This can be compared to the 90 seconds one year ago. A specific program allows the user to create and, if desired, customize the searoh data base directly from the CD-ROM PDF-2, making Updates much easier. Praotiaal examples are given to illustrate the Operation of the proprietary pattern-recognition algorithm used in the search program.

We have recently described the use of the Compact Disk Read Only Memory disk for the storage of data in the Powder Diffraction File (PDF). This work has now been extended to include the NBS (1987) Crystal Data File (CDF). The CDF contains 115,753 entries of which 59,613 are Inorganic materials and 56,140 organic. The data base can be accessed either by means of bit maps built on chemistry and subfile restrictions, or by means of a Boolean search system allowing combinations of search parameters including: chemistry; space group; cell volume; density and unit cell data.

A common analytical problem in a X-ray diffraction laboratory is the phase identification of an unknown sample. Presently, most phase identification is either performed manually or through computer search-match programs, as found on automated powder diffraction systems. Whether phase identification is performed manually or with computer search-match, often, the diffractionist wishes or needs to be able to make use of more sample information in the phase identification process than just d-I data. Additional information may be in the diffractionist's possession or can be readily obtained. The types of additional information which can be used in computer search-match programs are currently limited to information in the Powder Diffraction File-1 (PDF- 1) (1), formerly Level I, database (ie., JCPDS sub-files, elemental and functional groups). The use of other information is not possible. Users of manual search procedures are not able to make optimum use of additional information.

With the increasing use of automated powder diffractometers, more and more reliance is being placed on peak searching and α2 stripping software to allocate a wavelength for the calculation of the d-value from the observed peak maxima. The complications in the correct allocation of the wavelength are well known and include problems due to the polychromatic nature of the diffracted X-ray beam, changes in the angular dispersion of the diffractometer over the angular range of the experimental data, and effects of instrument geometric aberrations on the profile shape. Further difficulties may accrue because of idiosyncrasy of the peak searching arid α2 stripping software.

Last year at this conference we submitted a preliminary report on an X-ray powder diffraction round robin sponsored by the JCPDS-ICDD. This round robin was designed primarily to study the intensities of diffraction lines found by users in routine work. At that time only a portion of the data had been analyzed, and we reported initial findings on the α-Al2O3, and Zn0 /CaCO3 samples. These included studies on counting statistics, resolution, and the effect of software on intensity precision. Since that time, all the data from the round robin has been entered into Lotus 1-2-3 (*) spread-sheets and numerous additional tests have been carried out. This paper discusses some of the more interesting findings. A complete paper on all of the tests performed is in preparation for submission to the "Methods and Practices Manual" published by the JCPDS-ICDD.

For several years we have prepared and submitted a variety of quality powder x-ray diffraction patterns to the International Centre for Diffraction Data (ICDD) for inclusion as reference standards in their Powder Diffraction File (PDF). Patterns submitted and/or currently under development include metal hydrides (inorganics), flavanoids and related compounds (organics), organic compounds involved in pollution (e.g., dioxins), explosives (organics and metal organics) and glass-ceramic phases (inorganics).

XRD analysis of fly ash was quantitated using the Reference Intensity Ratio (RIR) method and rutile (TiO2) as an internal standard. Rutile RIR's for 15 of the crystalline phases commonly observed in North American fly ash were determined. Error analysis on the various steps in quantitation indicated that precision ranged from ±10% of the amount present for phases that diffract x-rays strongly to ±21% for weakly diffracting phases. Limit of detection in the mostly glassy fly ashes ranged from 0.2% for lime, the most strongly diffracting phase, to 3.5% for weakly diffracting mullite. Accuracy evaluated with a simulated fly ash was within the limits established by precision, but in actual fly ash samples, accuracy will be a function of the match between the crystallinity and composition of the analyte and the analyte standard. Overlaps among peaks of some of the important phases require intensity proportioning; for this reason, the method is best described as semi-quantitative.

In this article, we show that mechanically-induced phase transformations can be readily achieved in two Pu-alloy systems. We have observed mechanically-induced phase transformations in both Ti-stabilized β-Pu and Ga-stabilized δ-Pu, In both of these alloys, the parent phase has been largely transformed to α-Pu, and the cause of these transformations was mechanical strain introduced by the metallographic sample preparation. For the Ga alloys, x-ray diffraction (XRD) patterns were taken at about 1.5-μm steps down to the undisturbed material in order to develop depth profiles of the surface damage. The total depth of the disturbed material in these alloys is estimated to be about 7.6μm, but this depth was not measured for the Ti alloys. The proportions of α-Pu and δ-Pu in the Ga alloys have been estimated using (a) a new quantitative phase analysis program (SPECQUAN) that uses multiple peaks of each phase in order to minimize the effects of preferred orientation and (b) an older manual technique (i.e., hand calculations). The results from these techniques are compared. SPECQUAN was developed to use the Specplot data file structure directly, thus reducing our data processing. The program is written in Fortran 77 and employs an external intensity ratio quantification procedure to obtain its results. XRD calibration has been done independently by means of accurate density measurements on a reference Ga alloy.

One of the earliest occupational diseases to be recognised was silicosis, a pathological condition of the lungs caused by the inhalation of dust containing free crystalline silica. During the industrial, revolution in the UK the incidence of silicosis rose dramatically; for example in the Sheffield cutlery industry, where dry grinding of knives and forks was practised, the life expectancy of workers was reduced on average by twenty five years. At the present day there is a wide spectrum of industry in which exposure to crystalline silica, may occur; in the UK alone it is estimated that more than 100,000 workers are potentially at risk. The great majority of this exposure is to quarts but cristobalite is also commonly encountered where silica rich materials are subject to very high temperature e.g. ceramics, refractories, brick manufacture. Cristobalite is also found in certain natural deposits including bentonite clay and chert.

Conventional selected area diffraction patterns as obtained in the TEM present difficulties for identification of materials such as asbestifonn minerals, although diffraction data is considered to be one of the preferred methods for making this identification. The preferred orientation of the fibers in each field of measurement, and the spotty patterns that are obtained, do not readily lend themselves to measurement of the integrated intensity values for each dspacing, and even the d-spacings may be hard to determine precisely because the true center location for the broken rings requires estimation. To overcome these problems, we have implemented an automatic method for diffraction pattern measurement. It automatically locates the center of patterns with high precision, measures the radius of each ring of spots in the pattern, and integrates the density of spots in that ring.

High peak to background ratios are especially important in powder diffractometry when attempting to identify minor phases in a sample or improving the limit of detection in quantitative determinations. Instrumental techniques to improve peak to background generally involve the employment of monochromatic or partially monochromatic radiation through the use of filters, crystal monochrometers, or pulse height discriminators.

In this study, a digital pulse height discriminator, configured as a card in a personal computer (Apple IIe) with appropriate software, is used in conjunction with a scintillation detector to improve peak to background ratios. The software allows the pulse height distribution to be scanned and the optimum pulse height window to be set for a given set of sample and instrumental conditions. Results obtained by this technique are directly compared with results obtained using a pyrolytic graphite monochrometer and beta filters. Examples cited include qualitative phase identification in both fluorescent and non-fluorescent samples and semi-micro quantitative analysis (determination of airborne silica).

An array of molecules that is anisotropic in the extreme has been discovered in certain thermal-energy storage materials and is reported here: neopentylglycol (NPG) and NPG-rich solid solutions with pentaglycerine (PG) have a crystal structure, stable at room temperature, that consists of bimolecular chains of molecules that are all unidirectionally aligned throughout a crystal. There are hydrogen bonds between every molecule in one chain and its neighbors in that chain, but none between molecules of one chain and any molecules of the neighboring parallel chains. Thus there are strong intermolecular bonds along each chain and only weaker bonds between the chains. The structure has been determined by using modern single crystal techniques with 529 independent reflections from a crystal of NPG (C5H12O2). The structure is monoclinic with space group P21/c - C2h5. This anisotropic structure transforms to a cubic structure at higher temperatures.

Differential scanning calorimetry (DSC) and variable temperature x-ray diffraction (XRD) are two complementary techniques which provide thermal and structural information. By using DSC and XRD simultaneously, one can directly correlate the results from the two techniques. The simultaneous measurements eliminate problems which might arise due to sample inhomogeneity and instrumental differences. In addition, the DSC provides precise control over temperature, and heating and cooling rates.

Quantitative phase analysis by XRD requires the attainment of (1) random crystallite orientation and (2) homogeneous and intimate mixing of the constituent phases in the samples. These two requirements must also be met in the preparation of random composite materials, particularly those containing randomly-oriented fibers or whiskers.

The two most common methods for producing random crystallite orientation are spray drying and the air suspension method. In the latter, an air suspension of the crystallites is rapidly collected onto a glass fiber filter. The crystallites then assume the random orientation of the filter fibers on which they are deposited.