In these modern times, where the use of the computer in the analytical laboratory is taken for granted, it is perhaps difficult to realize that, less than one generation ago, computers were little more than an idea on an engineer's desk. It is interesting to note the sequence in which the automation of data collection and data processing developed. As would be expected, the time sequence followed closely the developments in computer hardware and peripherals. An important factor in the development of most commercial automated systems was the “20%” rule. This rule required that the total cost of any computer package should not exceed 20% of the sale price of the final automated product. Rex's “Numerical Control Powder Diffractometer” was described in the 1966 Denver Conference and this machine was to be the forerunner of a whole host of automated diffractometers which appeared in the early 1970s. Typical systems used either a 4K minicomputer or a time-sharing system with a large main-frame computer. It is interesting to observe that, as we come into the 1990s, the argument as to whether the main-frame will survive as a viable alternative to the rapidly developing PC still goes on.

PC software for XRD is strongly influenced by the cost of the hardware. Contrasted to mainframe and mini platforms, PC's are essentially free, or at least much less expensive than maintenance contracts for the hardware we've used in the past. But when we get our own computer, we discover that much more time goes into tasks others did before such as system maintenance. PC software needs to be simple to use yet very powerful, so we can recover some of that time. Changes have been made. Much more remains to be done and that change is coming fast.

As in other fields of spectroscopy, software for x-ray fluorescent analysis has to assist the user in instrument control, raw data refinement, qualitative interpretation of spectral data, and computations for obtaining quantitative results. From a historical point of view, instrument automation and data evaluation routines tor large numbers of samples have been a most important incentive for the introduction of computers into x-ray analysis.

The determination ab initio of crystal structures from powder diffraction data has been the most striking advance of modern powder crystallography. It is a consequence of the major developments occurred in instrument resolution, powder pattern indexing and Fitting techniques, e.g. the problem of peak overlap resulting from the collapse of the three dimensional pattern into one dimensional powder diffraction data has been circumvented by the advent of the Rietveld method. A structure analysis starting from scratch involves successive stages from the collection of high quality powder diffraction data to the refinement of the atomic coordinates by the Rietveld method. Since the pioneering work by Werner and co-workers a number of crystal structures solved from powder diffraction data, collected with synchrotron and conventional sources have been reported. With the growing development of this important application of the powder method, integrated softwares for solving crystal structures are now of interest, and a number of programs are available for the analysis of the different stages of a structural study. These programs combine computer routines for the treatment of powder diffraction data and routines used in conventional structure determination from single crystal data. Most of these programs have been listed in the Powder Diffraction Program Information 1990 Program List. Owing to the efficiency of modern personal computers, solving a crystal structure ab initio from powder diffraction data can now be carried out with a desk computer.

The characterization of finely divided and structurally imperfect solids is required in the study of many classes of materials, e.g. ceramics, catalysts, decomposition products. Diffraction line broadening is influenced by the microstructure of the solid and is a valuable technique for a unique characterization of a material in terms of size and morphology of crystallites (a region over which diffraction is coherent) and imperfections (microstrains, d- spacing fluctuations, stacking faults, etc). The extraction of these features has traditionally been based on the study of individual diffraction lines, which restricts the analysis to a limited number of directions of diffraction vectors. The complexity of most powder patterns has been a serious impediment to the widespread use of the procedure for detailed microstructural characterization. The advent of fitting techniques, with the use of structural information (Rietveld method) and without structure model (pattern decomposition method) have extended the frontiers of diffraction line profile analyses. In the Rietveld method it has been recommended to have a prior knowledge of the origin and lattice direction dependences of the imperfection effect before embarking in the modelling of line broadening through the pattern. This modelling can be carried out with pattern decomposition methods, which provide individual diffraction lines for a subsequent study of microstructural properties. Several aspects of this technique, performances, limitations and practical problems are reviewed and discussed. Applications to oxides with high specific surface-area are used to illustrate the detailed microstructural information contained in a powder diffraction pattern.

The development of powder-pattern-fitting techniques greatly changed the methodological aspect of materials characterization using powder diffraction data. Pencil and ruler are going out of use in modern powder data analysis, and present-day materials scientists hit a keyboard and manipulate a mouse of computer for deducing the final parameters of the model, which gives the best fit of calculated pattern to the observed one on a CRT screen. Computer software is now widespread, and the success in materials characterization becomes much dependent on their availability.

The Rietveld profile refinement method is probably the most popular technique used for the crystallographic characterization of materials including crystal structures and phase analysis, but it has been used mostly with ideal powder sample, not with textured polycrystals, because effects of strong and complex textures. Most technological materials are fabricated by using thermo-mechanical forming processes, which inevitably produce strong and complex preferential orientations of the crystallites. Consequently, the diffraction patterns of a given technological material are not unique but vary considerably with the measuring direction, with intensity variations as large as factors of hundreds, depending on the degree of texture. The texture effect on the diffraction pattern of a certain sample direction is directly proportional to the pole density of the corresponding inverse pole figure, which can be obtained from the three-dimensional orientation distribution function (ODF) of the material. The ODFs of materials with high crystal symmetry, such as cubic, hexagonal, tetragonal, and orthorhombic, can be determined quite precisely, using modern texture analysis techniques (for example, Bungel, Wenk, and Kallend et al.). The pole density distributions of the inverse pole figures can be used in the diffraction profile calculation of a highly textured sample.

The advantages of using the full trace as input for search/match rather than the traditional list of d/I values are emphasized. These advantages stem from the availability and the logical employment of the whole information of the diffractogram.

The two key features which enable successful identification of minor phases are discussed: (1) the reliable discrimination between background and weak lines and (2) by regarding the actual full widths of major lines as regions where weak lines can, potentially, he obscured.

The user friendliness of the. package will be presented: the graphical user interface, the high level of interactivity, and the speed of the search/match - now less than 8 seconds for searching the current whole ICDD database of 61,993 reference patterns when using an i486/66-based PC.

The affect of several aberrations on the figures of merit, FOM, used in the identification analysis of diffraction patterns has been examined by generating simulated diffraction traces with mathematically induced errors. Patterns of mixtures with random 20 shifts, intensity variations, and profile broadening were generated for analysis by MATCHDB, and the behavior of three FOMs during the search step was followed. The results have shown that the angle errors are the most serious in degrading the potential for identification of the phases in the mixtures. Intensity errors cause some degradation on the results. Broadening has little affect when using whole-pattern matching methods.

Microwave dielectric ceramics with high quality have received attention due to the rapid progress in microwave telecommunication and satellite broadcasting. The desirable properties in microwave dielectric resonators are a high dielectric constant (εr), low dielectric loss (tanδ) (i.e. high Q (Q =1/tanδ)), and low temperature coefficient of the resonant frequency(τf). Properties of dielectric ceramics in the TiO2-rich region of the BaO-Nd2O3-TiO2 ternary system (Fig. 1) were reported to have excellent dielectric constants, low dielectric losses and low temperature coefficients of capacitance by Kolar et al. (1978).

Computational methods for determining precision lattice parameters based on extrapolation, lattice refinement, least squares and trial and error indexing, with intermediate corrections for peak positions using external and internal standards, have been evaluated with respect to the ICDD published values for zincite. Lattice parameters can be routinely determined to an accuracy of one part in ten thousand without the use of external or internal standards, by using computational extrapolation on data with a well aligned and maintained diffractometer. Lattice refinement determination of lattice parameters was only effective, with respect to the ICDD values, when used in conjunction with external and internal Standards. Least squares yielded results with low standard deviations, but the use of standards did not reduce the refined sample displacement or zero angle errors, and gave an increased error in lattice parameters with respect to the ICDD values. The trial and error indexing, which can only be used with peak positions corrected with respect to both external and internal standards, gave relatively low de Wolff and Smith-Snyder figures of merit, but nevertheless yielded lattice parameters of the highest accuracy of one part in twelve thousand, with respect to the ICDD values.

In the past the powder diffraction data where presented as d-I sets as obtained experimentally and systematic errors were utilized only for the derivation of the unit cell parameters. This attitude was justified by the fact that the major work of XRPD made by Debye-Scherrer camera and it was assumed that most users would obtain the same systematic errors. Nowadays, diffractometry has taken over, and the diffractometers have lower systematic errors, which can minimized by calibration. Thus, they are now preferred. There are many phases which can be used as standards, but only four were selected, namely, Si, Ag, W, and mica (FP), which can easily be obtained as pure substances, have a limited number of diffraction lines and the distribution of intensities along 2θ is good. The calibration is made by fitting a polynomial which correlates the standard experimental peak positions versus the expected (calculated) values. However, while on the one hand, the more peaks which are used, the better the fit which can be achieved; on the other hand using a standard with many peaks enhances the probability for interference with the examined specimen peaks. Thus, it was decided to determine each line position by line profile fitting as was recommended elsewhere. In order to derive real observed data we do not link the diffraction lines between themselves by global structural or tine shape parameters. Thus, local variations in line profile parameters are treated. In the line-profile-fitting method suggested here the final structural details (atomic positions) are not required. Similar method have been used in the past for structure analysis, unit-cell refinement, and broadening analysis of pure substances. It was found that using the suggested method yields accurate unit cell parameters for each individual phase in the polyphase mixture. Several systems will be demonstrated in this work.

The graphics capability and speed available in modern personal computers has encouraged an increase in the use of a direct pattern comparison approach to the analysis of x-ray and electron diffraction patterns. Several researchers over the past 30 years have presented programs and algorithms which calculate and display powder patterns for xray diffraction. These programs originally required a main frame computer which was expensive and generally not available to all researchers. With the recent advances in the speed of personal computers, language compilers, and high resoultion graphics, expecially within the past 5 years, real time calculations and display of calculated patterns is becoming widely available. The power of this approach will be demonstrated through the use of an IBM compatable personal computer code developed by the authors.

The International Centre for Diffraction Data (1CDD), formally the Joint Committee for Powder Diffraction Standards (JCPDS), recently published a compilation of metal and alloy indexes of powder diffraction and structural data. This compilation is designed to make the identification and characterization of metallic and related materials easier and far more accurate. Systematic searches for chemical analogs and greater use of partial chemical knowledge are now possible.

While most contemporary methods of qualitative analysis of multi-phase materials are still based on the classic Search/Match/Identify process developed by Hanawalt, Rinn and Frevel in the 1930s, during the past 10 years or so the personal computer, with associated CD-ROM storage, has made a dramatic impact on the ways in which classical procedures are being implemented. Until recently, most of the commercial mainframe and PC based software packages for qualitative phase identification have been designed to implement a fully automatic search/matching sequence. All of the major instrument suppliers now offer such programs as part of their Automated Powder Diffractometer (APD) packages. While these programs are extremely useful, the success of their application to a specific problem is critically dependent on the quality of both experimental data and reference data. Until the problems arising from comparing variable quality experimental and reference data are completely understood, it appears that there will continue to be an interest in user-inter active (computer-aided) manual methods of search/matching. This paper explores the use of the personal computer in the area of computer-aided search/matching

Two new methods of comparison of lattice parameters are described. The first method uses a double crystal diffractometer, fitted with a specimen rotation stage. Bragg peak positions of a silicon reference and the specimen crystal are recorded for the crystals at 0 and rotated 180° about the diffraction vector. From the mean Bragg positions the lattice parameters can, in principle, be compared to parts in 106. The second method is a variant on the above, using a triple axis diffractometer. It uses a beam conditioner and monochromator to define the incident wavevector ko, and an analyser crystal to define the diffracted wave vector kh. Only the analyser motion must be accurate and stable to the arc second level. We present a detailed error analysis for the case where reference and specimen Bragg angles differ significantly. Peak location to a confidence level of 1 arc second permits absolute, traceable lattice parameter determination to several parts in 106.

We report the implementation of two new methods of accurate comparison of lattice parameters against a silicon standard using a high resolution X-ray diffractometer. The double axis method uses a specimen rotation stage which set the limit of reproducibility (at 3 sigma) to 3 parts in 105. An application of the technique is illustrated in measurements of the zinc concentration in Cd1-xZnx Te to an accuracy of 0.1%. The triple axis technique uses beam conditioner and analyser crystals to define the incident and diffracted wave vectors. In measurement of the lattice parameters of InAs, we found a precision of 1 part in 105 and traceable accuracy of a several parts in 105.