The object of the Rietveld method is to produce refined values of crystal structural parameters from powder diffraction data. Many materials of great interest can not be made available for study in single crystal form. This may be because it is not possible to prepare a single-crystal form at all (e.g., human tooth enamel) or because the single-crystal form differs from the polycrystalline form with the properties of interest (e.g., catalysts). Thus, our basic understanding of the atomic scale mechanisms is limited on the structural side by the information that can be deduced from powder diffraction patterns. (Only diffraction and EXAFS are direct probes of the spatial arrangements of atoms.) The Rietveld method has greatly extended the amount of structural detail that we can obtain routinely from powder diffraction patterns. In this method, structural parameters such as atom coordinate, thermal motion, and site occupancy parameters are adjusted in a least-squares refinement procedure until the best fit is obtained between entire calculated and observed powder diffraction patterns, as a whole.

The X-ray diffraction patterns from fibrous polymers are generally characterized by a small number of relatively broad overlapping peaks often overlaid with a diffuse halo. Single fibres give the same pattern as a parallel bundle of fibres, so that the pattern is equivalent to the rotation pattern of a single crystal. Despite the inherent difficulty of insufficient well-defined reflections, all commercially useful fibres have been indexed, starting with cellulose, through nylon 66 and polyester, to the more recent fibres, such as Nomex and Kevlar. Only the structural complexities of the natural fibres of keratin (wool and hair) remain to be fully determined.

Many sophisticated computations have been carried out to refine unit-cell constants; unfortunately, much of this work is of doubtful value to those interested in fibre characterisation. Fibres are semicrystalline materials and their molecules are insufficiently well ordered for precision calculations of atomic positions. Indeed, there can be significant error involved in evaluating unit-cell constants from overlapping peaks, unless precise profile resolution is carried out.

The goal of the occupational health professions is ”… to assure so far as possible every working man and woman in the Nation safe and healthful working conditions and to preserve our human resources” (Occupational Safety and Health Act, PL91-596). When the health of workers is threatened by chemical agents in the air, occupational health chemistry has a role to play. Analyses of workplace air are performed by the thousands each year by industry and by state and federal agencies. In the federal government, these analyses, as well as occupational health work in general, are performed by three agencies: the National Institute for Occupational Safety and Health (NIOSH, DHHS), which does occupational health research; the Occupational Safety and Health Administration (OSHA, DOL), which enforces safety and health standards in the general workplace; and the Mine Safety and Health Administration (MSHA, DOL), which enforces similar standards in mining operations.

Porotectosilicates are a class of siliceous crystalline materials which includes both zeolites and materials which resemble zeolites in crystal structure, but may or may not have ion exchange capability. The framework structures of these porotectosilicates are comprised of “T” atoms tetrahedrally coordinated to oxygen, where “T” can be Al, Si or any other element capable of isomorphous substitution for silicon. The occurrence of small crystals and the additional problems introduced by the presence of stacking faults and crystal twinning make structure determination of porotectosilicates by conventional approaches difficult.

The industrial significance of these materials has led to the development of a technique which permits the determination of their structure. The method involves the construction of appropriate hypothetical models, a DLS refinement followed by computation of a Smith plot for comparison with the experimental powder pattern. Model crystal structures may now be refined using the Rietveld technique. It is expected that this technique will contribute significantly to the solution of porotectosilicate structures which are difficult--if not impossible--to establish by other techniques.

Amorphous materials give rise to rather diffuse diffraction patterns. In contrast to diffraction patterns from polycrystalline materials which are characterized by a large number of sharp diffraction rings, patterns from amorphous materials are composed of relatively few broadened features. Such a diffuse pattern, however, contains much structural information in the form of an interatomic distance distribution that may be computed directly from the measured diffraction pattern. This radial distribution function (RDF) provides information concerning bonded distances, the types of atomic groupings and the extent of ordering in the sample. Special care, however, must be taken during the analysis to avoid introducing spurious details into the RDF that may be confused with, or mask real structural features. Such false detail may arise both from data collection and data reduction procedures. The availability of modern computers has greatly facilitated the calculation of accurate RDF's by readily permitting the introduction of physical criteria into the data reduction procedure that must be satisfied by the RDF.

Talking about X-Ray Diffraction in Europe, I'll limit myself to personal experiences. I started with XRD in Amsterdam University, namely structure analysis, but as well powder diffraction for identification of minerals, determination of crystallinity of stretched gelatine and many more such applications. Exclusively, cameras with photographic recording were used, and they are still used in Europe, to a great extent; maybe in 30-40% of all cases. We used a mechanical rectifier controlled by an adjustable spark gap. This system worked fine, till our laboratory cat decided to sniff at the sparks! That was the end of our generator and the cat. The powder samples in our D.S. cameras, mostly glued onto glass fibers were rotated during exposure. As we could not afford small motors, the rotation was done by blowing air against a windmill system mounted on the sample holder. A typical Dutch solution.

The use of computers to aid in the identification of phases from their powder diffraction patterns was pioneered in the mid 1960's by Frevel, Nichols and Johnson (1-3). Today's most widely used Johnson algorithm conducts a reverse sequential search by comparing each reference pattern in the JCPDS powder diffraction file (PDF) to the unknown pattern. A figure of merit is computed for each match and the patterns with the best figures of merit are listed at the end of the search. The Nichols approach is a reverse search of a singly inverted reference file. An inverted file is one which stores the reference patterns according to the d value of the lowest angle 100% intensity line (d1) . This type of file is analogous to the Hanawalt search books distributed by the JCPDS for manual searching. When an inverted file is stored in a random format, along with suitable disk directory files, only reference patterns containing d1 values of interest need be read in the search.

During the past three years we have undertaken the development of a complete X-Ray Powder Diffraction, facility with the goal of fully integrating experimental and analytical procedures. Such an approach potentially offers substantially improved performance over previously existing systems by virtue of its internal self-consistency and it opens the possibility of significantly extending analytic procedures for both qualitative and quantitative analyses. Our work to date has resulted in improved performance and significant extensions in both areas, and today I will report on those advances in the area of qualitative analysis.

Techniques for the computer-assisted evaluation of crystallographic data have been developed to improve the data compilations of the NBS Crystal Data Center and the JCPDS—International Centre for Diffraction Data. The resulting computer program, NBS*ATDS80, can be used for the analysis of unit-cell and powder data by the general scientific community as well. NBS*AIDS80 is written in FORTRAN to permit implementation on a wide variety of computers, and input may be from cards or from a terminal. The research and analysis components that will be of use to the individual scientist include the following:

• 1) Calculation of the Crystal Data cell, determinative ratios, and space group for the comparison and reporting of unit cell parameters in a standard setting and for the identification of unknowns.

• 2) Determination of the reduced cell, reduced form number, and the unit cell with the highest metric symmetry.

• 3) Calculation of the molecular weight from the formula using the most recent atomic weights, and comparison of the density calculated by the program with the measured density.

• 4) Generation of d-spacings and indices for any input cell and crystal system.

• 5) Comparison of input powder data with calculated d-spacings, indexing of lines based on known unit cell parameters, and calculation of figures of merit.

Calibrated powder patterns of a number of high symmetry crystalline materials were recorded in three different Guinier-type focusing cameras. The films were then measured by three different techniques, one visual and two instrumental. Cell parameters were calculated by three different data reduction procedures. The aim of the work was to establish the level of reproducibility for cell parameters obtained in routine work with these instruments.

A variety of x-ray diffractometers supporting stress measurements, routine phase identification, precision lattice parameter determinations, crystallite size estimates, and high temperature studies have been interfaced to a central minicomputer. The unique attributes of each instrument have been retained while the central minicomputer provides a common control format, data storage area, and analysis program library. Two movable desk top computers provide limited backup for the minicomputer, and serve as test beds for technique development.

By collecting the diffracted X-rays in a focussing powder diffractometer with a linear position-sensitive detector (PSD), the data accumulation time can be widely reduced. Due to the focussing properties of the geometrical arrangement good resolution can be simultaneously achieved over a few degrees of the diffraction angle 2 Theta. The full pattern is collected by scanning the PSD along the entire 2 Theta arc. Scanning speeds of several hundred degrees per minute are possible and still yield well plotted diagrams.

This work demonstrates the performance of the system in different tasks of X-ray powder diffraction such as identification of unknown materials, quantitative analysis, determination of lattice constants and microcrystalline properties. The results, evaluated by parts of the program system DIFFRAC 11, show that the accuracy and resolution of the continuously scanning PSD technique match these tasks without difficulty and reach the precision limits of powder diffractometry itself. It is demonstrated that the data collection velocity, which matches well the data evaluation times, is not the only advantage compared to conventional diffractometets.

Since 1973 X-ray position sensitive detectors (P.S.D.) have been employed for stress analysis. The first models, based on the gas flow of Ar-CH4, were composed of a quartz fiber anode (20 um diameter), coated by a thin carbon layer. From 1975 the gas filling has become permanent. Hence the use of fiber P.S.D. (F-P.S.D.) was made easier; nevertheless, two important inconveniences remained. On the one hand, an intensive X-ray beam could destroy rapidly and locally the carbon film, which makes the detector malfunction; on the other hand, the pulse height analysis (P.H.A.), carried out in 1976, has proved to be ineffectual because of the design of the F-P.S.D. itself: its fiber impedance does not remain constant. Thus the appearnace, in 1979, of a new P.S.D. with a metallic wire (W-P.S.D.) presented a significant advantage (see Fig. 1). The anode—made of tungsten or molybdenum Gilded--can resist intensive X-ray beams perfectly. Hence the life of the detector is only limited by a possible ionization of the filling gas, which approximately corresponds to 1012 counts. A gas refilling--which will be provided in the standard equipment--allows us to use the detector again.

Our goniometer used is a classical one with step scanning motors both on θ and 2θ movements. The theoretical radius is 250 mm but it can be changed easily.

Two kinds of PSD have been used, the older one of a resistive type with a carbon coated quartz fiber and, for this year, a capacitive one with a gold-plated molybdenum wire anode, which is more reliable, with a very good pulse height analyser. It is connected to a 48K calculator (IN 90) with a floppy disk unit. A special interface has been made which is able to drive separately the 2θ and θ movements of the goniometer, using the output signals of the calculator. After the different settings of the goniometer and the initialization of the positions in 9 and 26, every movement can be ordered either by a very simple software from the keyboard or inside a more complex program.

Instrumentation for X-ray stress analysis has been advanced rapidly in the last few years. Especially, the time required for data accumulation has been remarkably reduced without motion of the detector by using a new X-ray detector called a position sensitive detector (PSD). Applications of PSD to the field of X-ray stress analysis were carried out by James and Cohen, Ruud and Barrett, and the authors. In our laboratory, several position sensitive proportional counters (PSPCs) were designed and manufactured for residual stress measurements. Results show that the PSPC method is a powerful alternative to the conventional counter method or the film method.

This paper reports a design of a versatile PSPC X-ray stress analyzer for use in industry and the laboratory.

Recently intense X-ray sources have been used in diffraction experiments on stress analysis, dynamical structure analysis, and so on. Since in these experiments one-dimensional position-sensitive X-ray detectors are very useful to reduce the measuring time, various kinds of X-ray detectors and processing systems have been developed. These detectors may be divided into two groups, the pulse type and the integral type. The pulse type employing either the charge division method, or the delay line method, processes each signal produced by an incident X-ray photon. To achieve a precise measurement, a sufficient gas gain and a relatively long processing time are needed. Therefore, the maximum counting rate depends on both detector characteristics and a signal processing time. On the other hand, the integral type, such as self-scanning photodiode array detectors (1, 2) periodically processes the charges produced by X-rays in each pixel. The total maximum counting rate of the detector is the sum of the maximum counting rate of each pixel and is higher than that of the pulse type.

Slow data acquisition rates have, in the past, limited the use of X-ray diffraction for characterization of polymeric materials. Photon counting techniques yield quantitative data in digital form for computer analysis. However, a great deal of data acquisition time is required when data is taken sequentially; i.e., when each intensity determination (at a particular goniometer setting) requires a separate time interval, during which intensity at other angle values is ignored. The problem is particularly acute for oriented polymers since two or more diffraction angles are involved: The Bragg angle along with at least one orientation angle. For this reason, an area-imaging (two-dimensional) proportional counter has been developed for use with a four-circle X-ray diffraction system. Although basically a single-crystal unit, this goniometer has been used in this laboratory and others for studies of oriented polymers. The receiving pinhole collimator and aperture have been removed, and the area-imaging counter has been mounted on the detector arm track. The original receiving aperture is still used for alignment, and the area detector Is positioned with its center at the receiving aperture center position.

X-ray stress analysis in austenltic stainless steel is generally carried out on the ϒFe(311) diffraction line produced by Cr- Kβ X-rays. However, it is often pointed out that not much reliance can be placed on the precision of the stress because the contrast between a diffraction peak and its background is poor. In addition, to measure the stress is sometimes impossible on a specimen which has martensite structure produced by thes train induced transformation, because the ϒFe(211) line appears in the neighborhood of the desired ϒFe(311) line, since Cr-K6 X-rays accompany Cr-Ka from a conventional X-ray source. If Cr-Ka X-rays can be eliminated and only Cr-KB X-rays directed on the specimen, only the λe(311) line with high contrast will be obtained and one can expect to measure the residual stress with high precision.