The Annual James L. Waters Symposium at Pittcon (2000) - Journal of

Waters Symposium: X-ray Diffraction of Powders and Thin Films

Landmarks in the Development of Powder Diffraction Instrumentation Ron Jenkins International Centre for Diffraction Data, Newtown Square, PA 19073-3273

Following the discovery of X-rays by Röntgen in 1895, three major fields of materials investigation and analysis developed. Röntgen already demonstrated the first of these— radiography—in his first publication. The second, X-ray emission spectroscopy, has its roots in the 1910s, but was rediscovered as X-ray fluorescence spectrometry in the 1940s. The third technique is X-ray diffractometry. While X-ray powder diffractometry has its roots in the earlier half of the 20th century, it was not until the 1960s that the technique became widely accepted (1, 2). In the 1920s and 1930s the instruments employed for phase identification were nonfocusing cameras. In the mid1930s, the establishment of a file of reference powder patterns did much to popularize the use of powder diffractometry in industry. The growth in the application of X-ray methods for materials analysis grew rapidly between 1960 and 1970, then made another major leap forward in the early 1970s with the introduction of minicomputers. The usefulness of the technique was further enhanced by the advent of attachments to allow the recording of diffraction data under nonambient conditions. In the 1990s use of synchrotron radiation allowed

the use of intense parallel beams of monochromatic Xradiation, providing sensitivity and resolution hitherto undreamed of. The Early Years Wilhelm Conrad Röntgen was born on March 27, 1845, in Lennep-Remscheid in Westphalen, Germany. After a somewhat checkered education in the Netherlands, he settled at the Technische Hochschule in Zurich, where he was awarded a doctorate on Thermodynamics of Gases. History records that he was a brilliant student with a wide variety of interests (3). In 1888 he became Head of the Physics Department at the University of Wurtzburg. Like many other scientists of his day, he was studying the visible light emitted in a closed glass (Crookes) tube under reduced pressure when high voltage is applied. On November 8, 1895, Röntgen wrapped his glass tube in paper to prevent seeing the light, and darkened the room. He used fluorescence screens in his experiments, and to his great surprise, when voltage was applied across his glass tube the fluorescence screens showed flashes of light.

The Annual James L. Waters Symposium at Pittcon The objectives of the annual James L. Waters Symposium at Pittcon are different from those of other symposia at either Pittcon or other conferences. Waters, founder of the wellknown Waters Associates, Inc., and currently president of Waters Business Systems, Inc., arranged with the Society for Analytical Chemists of Pittsburgh (SACP) in 1989 to offer an annual symposium at Pittcon to explore the origins, development, and commercialization of scientific instrumentation of established and major significance. The main goals were and still are to ensure that the early history of this cooperative process be preserved, to stress the importance of contributions of workers with diverse backgrounds, objectives and perspectives, and to recognize some of the pioneers and leaders in the field. Important benefits of these symposia are creation of awareness of the way in which important new instruments and, through them, new fields are created, and promotion of interchange among inventor, development engineer, entrepreneur, and marketing organization. The topics of the first Waters Symposia, beginning in 1990, were gas chromatography, atomic absorption spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry, high-performance liquid chromatography, ion-selective electrodes, lasers in chemistry, immunoassay, and atomic emission spectroscopy. Publication of the papers presented at the Waters Symposia is a high priority of the SACP. The papers of the first symposium were published in LC.GC Magazine and those of the next four symposia ap-

peared in Analytical Chemistry. The next Waters Symposia were published in this Journal: the sixth, on high-performance liquid chromatography, appeared in the January 1997 issue (pages 37–48); the seventh, on ion selective electrodes, appeared in the February 1997 issue (pages 159–182); the eighth, on lasers in chemistry, was featured in the May 1998 issue (pages 555–570); the ninth, on immunoassay, appeared in the June 1999 issue (pages 767–792); and the tenth, on atomic emission spectroscopy, was featured in the May 2000 issue (pages 573–607). The topic of the eleventh Waters Symposium, held in March 2000, was X-ray diffraction of powders and thin films, and three of the papers are featured in this issue of the Journal. In the introductory paper, Ron Jenkins (International Centre for Diffraction Data) presents an overview of the development of powder diffraction instrumentation. This is followed by a description of the development of the rotating anode X-ray generator and of imaging plates by Jimpei Harada (Rigaku Corporation). In the third paper, Tom Ryan (Philips Analytical) discusses the special requirements for instrumentation suitable for diffraction of thin films such as those that are important in semiconductors, recording media, and other technologies. J. F. Coetzee University of Pittsburgh Waters Symposium Coordinator

JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education

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Being the great experimenter that he was, he immediately investigated further. He tried to block the fluorescence screen with his hand, but then saw the shadow of his hand on the screen. He soon discovered that the radiation had great penetrating power, was absorbed by lead, and could blacken a photographic plate. On the 28th of December 1895 he published his first paper, in which he included a radiograph of his wife’s hand (4). The impact of his publication was enormous. In 1901 Röntgen was awarded the first Nobel Prize in Physics. He refrained from taking any patent rights on his new Xradiation, maintaining that his discovery should be used for the benefit of mankind. The main application of X-radiation at this time was in radiography, and several German organizations, notably Siemens, started the commercial development of X-ray tubes and highvoltage generators. In 1912, there was much discussion at the University of München about whether X-radiation could best be described as wave or particle radiation. Following a theory by Ewald, Max von Laue wagered a bet that X-rays were electromagnetic waves and they could be diffracted by crystals. Two of von Laue’s students, Friedrich and Knipping, obtained a diffraction pattern from a single crystal. Friedrich, one of Sommerfeld’s assistants, started the experiment. After many trials he still did not get a diffraction pattern because he was misled by previous scattering experiments, in which the film was placed perpendicular to the X-ray beam. Another assistant, Knipping, finally set up the correct geometry—source, crystal, and plate—and they were able to demonstrate diffraction spots on the plate using white X-radiation (5). Soon after, von Laue proposed a theory to describe the conditions for diffraction; and in England, the Braggs, father and son, began their work on crystal structure analysis. Using a much improved X-ray tube, which gave a significant amount of characteristic radiation, and an ionization chamber as a detector, they showed that X-rays are diffracted only in specific directions. Sir William Bragg derived a simple equation relating the diffraction angle with the diffracted wavelength and the interplanar spacing. After establishment of the basic theory many application methods were established. Table 1 gives a short list of the methods, which were developed in the years following Röntgen’s initial discovery. Camera Methods Just before the outbreak of World War I, Peter Debye in Zurich was mainly interested in diffraction from liquids. During the course of his work he found that crystalline powders give diffraction rings at discrete diffraction angles (6 ). Using Bragg’s law he was able to calculate a set of interplanar spacings (d-values) for lithium fluoride. Hull did similar studies in the USA at General Electric (7 ). It was thus shown that powder diffraction could be used for qualitative phase identification. The Debye–Scherrer–Hull camera was the instrument of choice in the 1920s and 1930s. Later, the introduction of the focusing Guinier camera (8), as modified by de Wolff (9), was found to be a relatively simple and inexpensive instrument, without need for expensive stabilized generators and electronics. Even today, the resolving power of the Guinier camera is, in general, better than that of the conventional diffractometer. 602

Table 1. Methods Following from Röntgen's Discovery Year

Method

Developer

1896

X-ray radiography

Röntgen

1912

X-ray diffraction

von Laue

1913

X-ray emission

Moseley

1914

Crystal structure analysis

Bragg and Bragg

1915

Amorphous scattering

Debye

1916

Powder diffraction

Debye, Scherrer, and Hull Glokker and Schreiber

1928

X-ray fluorescence

1931

X-ray reflection

Keissig

1937

Small angle scattering

Warren

However, the precision of quantitative phase analysis is poor, owing to the fact that the specimen is measured as a thin film in transmission. Further complications are due to film grain size and nonlinearity of the film response. Sources and Beam Conditioning The output from the original cold-cathode X-ray tube design was considerably enhanced with Coolidge’s development of the hot-cathode tube (10). Previously, a simple selfrectifying generator–tube system had been sufficient; but now, the generator and tube output had to be stabilized both in kV and mA. The need for higher intensities led to full-wave rectification and later to constant potential units. The maximum voltage increased over the years from 40 kV, which was ample for a Cu-anode tube, to 60 kV for a Mo-anode tube. The tubes themselves improved as well; they had higher purity, a longer lifetime, thinner windows, better-defined foci, and four windows, including both spot and line foci. For many years almost all X-ray powder diffraction measurements were carried out using a sealed or demountable fixed-anode X-ray tube. The sealed tube offered the advantages of good stability, reasonably high photon yield, and reliability. One disadvantage of the sealed tube as it was employed in the 1950s and 1960s was contamination of the tube, due mainly to deposition of tungsten from the filament on the surface of the anode. This, plus some degree of anode pitting, decreased the tube’s usefulness. For this reason, some laboratories preferred to use demountable fixed-anode tubes, which could be disassembled and cleaned. As the power loading of X-ray tubes increased over the years, the tubes would fail long before contamination became a serious problem. Thus the popularity of demountable tubes waned in the 1970s. Today, the sealed X-ray tube is the most popular source for probably 90% of all diffraction installations. However, other sources have gained in popularity during the last two decades. Another method to increase the intensity of an Xray tube is to increase the amperage; to avoid melting the target with the higher current, a much larger anode mass is used, and the anode is rotated at high speed. Two advantages accrue for the rotating anode: first, the irradiation area is larger by about a factor of 60, and second, the rotation continuously brings cooler metal into the path of the focused electron beam. Because of the more efficient cooling, rotating anode systems can be run at loadings around an order of magnitude higher than an equivalent fixed anode system. Thus, rotating

Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu

Waters Symposium: X-ray Diffraction of Powders and Thin Films Table 2. Evolution of Beam-Conditioning Methods

Table 3. Evolution of the Powder Diffractometer

Period

Innovation

Year

Development

1920s

Use of β filters

1935

First diffractometer developed by Le Galley

1950s

Proportional detectors plus pulse-height selection

1940

Trost develops a Geiger counter diffractometer but this is not a commercial success

1945

Parrish and Gordon describe the first parafocusing diffractometer based on the quartz crystal orientation unit

1948

NV Philips produce a commercial version of the Parrish–Gordon diffractometer

1966

Rex describes the first automated powder diffractometer

1970

Norelco introduces the first automated powder diffractometer

1985

Use of the synchrotron and development of parallel beam optics

1989

Development of grazing incidence diffractometry

1996

Schuster and Göbel introduce optics based on graded multilayers

1960s

Use of (graphite) diffracted-beam monochromators

1980s

Use of the germanium primary-beam monochromator

1990s

Development of Göbel mirrors

anode tubes produce very intense X-ray beams. Recently, the growth in the availability and application of synchrotron sources has provided diffractionists with a very bright and continuous wavelength source, allowing a new generation of experiments heretofore impossible with sealed tubes. All sealed X-ray tube sources give radiation beyond the simple K α emission required for diffraction measurements. This additional radiation includes other emission lines from the anode, including a strong K β emission, and emission lines from tube impurities such as tungsten, arising from the tube cathode. White radiation also comes from the tube and this can contribute to the background in a pattern. As shown in Table 2, various techniques of beam conditioning have been employed over the years to reduce the influence of scatter and diffraction of these additional wavelengths on the experimental pattern. In the early days, it was common to use a β filter to reduce Kβ radiation from the source as well as the effects of white radiation. Later on, the advent of proportional detectors and pulse height selection helped to further reduce the effects of white radiation from the source and to minimize the effects of fluorescence from the specimen. The use of diffracted beam monochromators started in the late 1950s, but the LiF crystal available at that time was an inefficient diffractor of X-rays, and not until the development of the much more effective graphite analyzing crystal did the use of diffracted beam monochromators become widespread. In the 1980s, Siemens promoted the use of the germanium primary beam monochromator, which, though more difficult to align, gave a very clean primary beam. Finally, the development of so-called Göbel mirrors in the 1990s has brought another new dimension to beam conditioning. Table 2 gives a short list of the developments in X-ray beam conditioning. Development of the Powder Diffractometer Table 3 lists the milestones in the development of the powder diffractometer. The first X-ray powder diffractometer was developed in 1935 by Le Galley (11) but, owing mainly to the lack of parafocusing conditions, the instrument gave relatively poor intensities. During World War II, Trost in Germany described the development of a diffractometer system employing a Geiger–Müller detector. A German firm tried to commercialize this idea, but without success. In 1945, Buerger and two of his students at MIT, Parrish and Gordon, developed a Geiger counter for the precision cutting of quartz oscillator plates used in frequency control for military communication equipment (12). In 1947, North American Philips introduced the first commercial modern parafocusing X-ray powder diffractometer based on these ideas. The system consisted of an X-ray generator with a sealed-off tube, mechanical

goniometer, flat powder specimen, Bragg–Brentano focusing, Geiger counter, electronic registration on a strip chart recorder, and counting tubes. The scanning goniometer had a primary drive on the counter axis, with a 2:1 reduction on the specimen arm. The diffractograms were taken on a strip chart recorder. Scanning could be done with variable time constants, depending on scanning speed and accuracy required. Step scanning with fixed time measurements enabled the determination of the shape of a peak and its maximum intensity. Scanning over a preset angular range gave the area under a peak. With lowintensity peaks, correction for background intensity is necessary; therefore the peak and background intensities had to be determined on two specific angles. One could collect counts on these angles either by counting a prefixed number of counts or by collecting the counts for a preset number of seconds. It was soon realized that only a limited number of particles in the specimen actually contribute to the diffracted peak using a stationary sample. Rotating sample holders were therefore introduced; but even so, large errors were still possible. Other useful attachments included the diffracted beam monochromator (13) and the automated divergence (θcompensating) slit (14 ). Detection and Measurement of X-radiation The latest versions of the conventional powder diffractometer have changed little in their construction and geometry, but considerable advances have been made in detection and counting systems and in automation. While the development of the Geiger counter in the 1920s (15) was a great advance over the ionization chamber, it, too, had disadvantages. The discharge after ionization by an entering X-ray photon takes place throughout the total volume of the counter; this produced a large electric impulse, and no further amplification of the signal was needed. However, the large discharge involved in the pulse formation led to a significant dead-time, so that high count rates could not be handled. The higher diffracted beam intensities coming from the development of higher-power X-ray tubes in the late 1950s made the situation increasingly untenable. The scintillation counter was introduced in 1950 (16 ) and is still in common use. A NaI crystal scintillator is optically coupled to a photomultiplier tube. Because the discharge


603


takes place over a small volume, the outgoing impulse is proportional to the energy of the incoming X-rays photons. The recovery is fast, resulting in a very short dead-time, and high intensities could be handled. An added advantage is that the proportionality of the outgoing pulse and the energy of the incoming X-ray photons permit the use of a pulse-height discriminator. Because the window can be set for the characteristic radiation, the background can be reduced considerably and spurious radiation can be reduced. A further development was the lithium drifted silicon detector, Si(Li), introduced in 1966 (17). This detector offers very-high-energy resolution and in some instruments is used to replace the function of the crystal monochromator. An especially novel development was the position sensitive detector (PSD). By arranging a pulse-development network at each end of the cathode wire and using a high-resistance cathode wire to slow the passage of the current, it is possible to correlate the rise time of the pulses at each end of the detector with the distance along the wire at which the ionization process occurred. The use of electronic PSDs in scanning mode has been developed by Göbel (18). In this mode the active window of perhaps 10° (2θ) is passed over the full 2θ range to be measured, the full pattern being summed into a multichannel analyzer (MCA). The attachment of such a scanning PSD to a Seemann–Bohlin transmission diffractometer can produce a digital powder pattern equivalent in resolution to a Guinier camera in less than five minutes. The development of fast PSDs has also led to the ability to record the diffraction peaks in a region of 2θ in as little as a few microseconds. This permits a complex temperature profile to be followed, collecting diffraction patterns every few minutes. High-temperature reactions can be mapped over hundreds of temperatures in an overnight run. Hardware control methods, too, were changing by the 1970s (see Table 4). The servomotors of the 1950s and early 1960s were now replaced with the stepper motor, offering a greater level of feedback control. The cumbersome switching and mechanical step scanning had already been displaced by microprocessors. The use of electronic control boxes soon followed the introduction of the stepper motor. While computer control of hardware was used for a couple of years in the early 1970s, it soon became apparent that much was to be gained by separating the tasks of hardware control and data processing. In response to this need, several universal electronic interface modules were developed. As an example, CAMAC (19) was designed as a universal interface to provide a bridge between high-level instructions from a computer program and various instrumental controllers. By the end of the 1960s the control of the powder diffractometer was completely automated. Table 3 lists the landmarks in the development of the powder diffractometer. Most of the early use of computers to control diffractometers came from the single-crystal rather than the powder field. But in 1966, Rex’s classic paper describing his IBM-1401-controlled powder diffractometer (20) spurred manufacturers to greater efforts. In 1970 the first commercial automated powder diffractometer was introduced by Philips (21). Much of the development work for this product was a spin-off from work at the Lunar Receiving Laboratory. The APD-3500 was controlled, not with a computer in the modern sense of the word, but with a program logic controller—the Ferroxcube FX-3000. 604

Table 4. Evolution of Hardware Control Methods Year

System

Control

1953

Servo-motors

Individually switched

1955

Servo-motors

Mechanical step-scanning

1962

Servo-motors

Logic switching (pegboards)

1968

Stepper-motors

Electronic logic boxes

1970

Stepper-motors

Mainframe computers Electronic module (NIM, CAMAC)

1974

Stepper-motors

1975

Stepper-motors

Microprocessor

1985

Stepper-motors

Personal computer

This instrument is somewhat typical of the controllers of the 1970s and was also a forerunner of what we were later to call a microprocessor. The “computer” itself had 4K of memory with 18-bit words, and the assembler language program had just 11 basic instructions. The 18-bit word length was to prove invaluable in that one could pack 3 ASCII characters into a single word. A savings indeed, when one considers that in 4K/18-bit words we controlled all functions of the diffractometer, including data collection and processing! Even with skilled programming, not everything would fit into the memory. Therefore overlays were loaded with paper tape to modify the basic program for various modes of operation. Thus much of the precious memory (about one-third) was taken up with arithmetic routines. During the 1970s, instrument automation became established as the computer began to reveal its potential power. As would be expected, the sequence of events in the automation of data collection and data processing closely followed developments in computer hardware and peripherals. Today, when the use of computers 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 drawing board. Developments in Search–Match Methods The X-ray powder diffractometer has always been used in phase identification, a process based on the comparison of experimental data with a file of reference patterns. The Powder Diffraction File (PDF) is a collection of single-phase X-ray powder diffraction patterns in the form of tables of interplanar spacings (d) and relative intensities (Irel) characteristic of the compound. The PDF has been used for almost five decades (22). It is maintained by the ICDD, which continually adds new and updated diffraction patterns. In the early 1980s, the ICDD editorial system was automated to permit detailed review of all new patterns entering the PDF. Currently 2,500 such patterns, comprising 1,900 inorganic and 600 organic patterns, are added each year. The ICDD makes a continuing effort to ensure that new patterns added to the PDF contain a significant proportion of phases that represent current needs and trends in industry and research. The effort is implemented, in part, by sponsoring grants-inaid for the production of new patterns of phases of current interest or preparation of the phases themselves. Although the master database of powder patterns undergoes continual revision and updating, to ensure that all database users can work with the same version, a frozen version of the master database is produced each year and supplied as the PDF-2.



There are a number of X-ray diffraction databases. Three of the more important ones are the Powder Diffraction File, the Inorganic Crystal Structure Database (ICSD) maintained by Fachsinformationzentrum [FIZ] Karlsruhe, and the Cambridge Crystallographic Data Centre (CCDC). While these databases have proved their usefulness in a wide range of applications, until recently there has been little attempt to exploit a combination of them. Following an initiative by the ICDD, an agreement was made between the ICDD and FIZ of Karlsruhe, which allows mutual use of the PDF and the ICSD databases (23). Three major benefits have accrued from this cooperation. First, by use of cross-reference hooks for each database entry, the user has access to experimental powder data from the PDF and structural information from the ICSD, permitting the full modeling of the experimental pattern. Second, the PDF can be supplemented using powder patterns calculated from structural data in the ICSD. Third, the combined efforts of the two editorial groups will certainly help to improve the overall content and quality of diffraction data. The early use of the computer for data processing depended on IBM mainframes, initially designed mainly for accounting purposes, like the IBM-1800. Later, instruments such as the IBM-1130 were designed specifically for laboratory use. The introduction of Digital Equipment Corporation’s PDP-8 was as dramatic 30 years ago as was IBM’s introduction of the PC 15 years later. Great patience and skill were required for the programming of these early computers. Only 4K (16 bits) of memory was available for hardware control, data collection, and data analysis. All code was written in assembler language at the average rate of about one instruction per hour. By the mid-1960s sophisticated programs were available for X-ray diffraction; the work of Johnson and Vand in powder diffraction file searching was particularly noteworthy (24). The movement proceeded from punch cards to paper tape (1968), then to tape cassette (1970), floppy disk (1972), Phoenix hard drive (1974), Winchester disk technology (1975), and the personal computer (1980), finally arriving at the implementation of the CD-ROM (1987) and the DVD (1998). Most contemporary methods for qualitative analysis of multiphase materials are still based on the classic search– match–identify process developed by Hanawalt, Rinn, and Frevel in the 1930s (25). During the past 10 years or so the personal computer, with associated CD-ROM storage, has had a dramatic impact on the ways in which classical procedures are implemented. Until recently, most commercial mainframeand PC-based software packages for qualitative phase identification were designed to implement a fully automatic search– match sequence. All 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 and reference data. Until the problems arising from comparing experimental and reference data of variable quality are completely understood, it appears that there will continue to be an interest in userinteractive (computer-aided) manual methods of search– matching. The traditional methods of searching the PDF are based on the use of combinations of d-spacings, intensities, and chemistry. While the PDF database contains much more information than simply d’s, I’s, and chemistry, limitations

imposed by the use of printed search manuals make it almost impossible to use this additional information. However, the CD-ROM sparked a major change in the late 1970s, offering storage of and access to the full database. Thus by use of suitable index files and Boolean search logic, a whole new dimension of search–matching was opened up. Between the two extremes of fully automated and manual search–matching is a third area, which has been gaining increased attention. It is best described as “computer-aided” search– matching. With the availability of the full file on CD-ROM, much more data is now available for data retrieval. This has spawned a new generation of programs, good examples of which are PCPDFWIN (26 ) and PCSIWIN (27), developed by the International Centre for Diffraction Data. The first of these programs utilizes Boolean logic to search on a variety of parameters such as d-spacings, chemistry, name, formula, CAS number, color, density, and subfile type. The second program has the functionality of the traditional but cumbersome paper search manual. The recent introduction of the Rietveld method (28) has done much to improve the overall methodology of pattern fitting. The Recent Years Angular resolution can only be achieved by sacrificing intensity. For this reason the highest resolution diffractometers have been built at high-intensity synchrotron facilities. The simplest of these was built at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory by D. Cox and colleagues (29). It is nothing more than two germanium crystals, one placed in the incident beam in front of the sample, the other in the diffracted beam after the sample. This two-monochromator system uses a parallel beam geometry rather than focusing and eliminates most of the instrumental effects that cause diffraction profiles to broaden, allowing measurement of the profile shape effected only by the sample. A second design for a high-resolution diffractometer uses parallel-beam geometry rather than focusing or parafocusing. In addition, it uses a type of monochromator in which two diffracting surfaces, cut exactly at the Bragg angle, are carved into a single crystal of silicon. This channel-cut monochromator introduces no instrumental aberrations into the diffraction experiment. In another use of a parallel beam diffractometer the incident beam is directed at the sample at a very shallow glancing angle to meet the condition of total reflection. The enhanced diffraction intensity resulting from this geometry allows a full diffraction pattern to be obtained from 20 ng of sample. The incident beam is passed through a manmade multilayer monochromator. These procedures are particularly suitable for thin film analysis. Grazing angle experiments have also been used to examine structural variations via diffraction from films as a function of depths as little as 50 Å. The analysis of crystalline organic phases by X-ray powder diffraction presents special problems beyond those typically associated with inorganic materials (30). The large unit cells often characteristic of organic compounds, combined with the low symmetry of the structures, give rather complicated diffraction patterns that contain many low-angle lines. The Bragg–Brentano geometric arrangement employed in most commercial diffractometers gives maximum d-spacing error at low diffraction angles. Among the procedures useful in the


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preparation of high-quality organic powder patterns are the control of beam penetration by use of a thin-film specimen support (31), reduction of air scatter by use of partial or full vacuum, avoidance of specimen fluorescence by choice of wavelength and detector, and careful selection of the divergence beam aperture. While careful alignment procedures and optimum choice of experimental conditions can do much to yield patterns of reasonable quality, several factors that present difficulties with conventional instrumentation remain. Probably the most important of these is the choice of the experimental wavelength. A potential solution to these problems lies in the design of a long-wavelength diffractometer. In designing a typical long-wavelength diffractometer, Philips (32) gave special attention to the problems outlined above. Use of a full vacuum path is desirable to reduce both beam absorption and air scatter, and use of a longer wavelength increases both the diffraction angle and the dispersion of the diffractometer. Use of a primary-beam monochromator is desirable to reduce scatter from radiation from the source. But of all the experimental parameters, the value of the experimental wavelength is probably the most critical choice. Too long a wavelength would give too much pattern dispersion and increase absorption problems. The choice of wavelength is also predicated on what anode materials are available and suitable for this application. Anode elements need to be very good heat conductors, stable under intense electron flux, and mechanically workable. The Philips unit was made up of three major parts: the X-ray tube and housing, the primary-beam crystal monochromator chamber, and the vacuum diffractometer. A primarybeam monochromator was used to minimize sample irradiation and fluorescence background. The monochromator was a logarithmically bent germanium (111) crystal; entrance and exit slits were designed to take radiation from a specimen surface area of several square centimeters and focus to a sharp line at a flow counter window. A fine-focus X-ray diffraction tube was placed at the primary focus of the monochromator, causing the diffracted beam to be focused through a fine slit onto the specimen. Literature Cited 1. Jenkins, R. Adv. X-ray Anal. 1995, 39, 13–18. 2. de Vries, J. L. Adv. X-ray Anal. 1980, 24, 73–81. 3. Nitske, R. W. The Life of Wilhelm Conrad Roentgen; Academic: New York, 1971.

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4. Roentgen, W. C. Ann. Phys. 1898, 64, 1–11. 5. Friederich, W.; Knipping, P.; von Laue, M. Ann. Phys. 1912, 41, 971–988. 6. Debye, P.; Scherrer, P. Phys. Z. 1916, 17, 277–283. 7. Hull, A. W. Phys. Rev. 1917, 10, 661–696. 8. Guinier, A. C. R. Acad. Sci. Paris 1937, 204, 1115–1119. 9. de Wolff, P. M. Acta Crystallogr. 1948, 1, 207–212. 10. Coolidge, W. D. Phys. Rev. 1913, 2, 409–430. 11. Le Galley, D. P. Rev. Sci. Instrum. 1935, 6, 279–283. 12. Parrish, W.; Gordon, S. G. Am. Mineral. 1945, 30, 326–346. 13. Renninger, M. Z. Kristallogr. 1956, 107, 464–469. 14. Jenkins, R.; Paolini, F. R. Norelco Rep. 1974, 21, 9–16. 15. Geiger, H.; Muller, W. Phys. Z. 1928, 29, 839-841. 16. Hofstadder, R.; McIntyre, J. A. Nucleonics 1950, 7, 32–37. 17. Bowman, H. R.; Hyde, E. K.; Thompson, S. G.; Jared, R. C. Science 1966, 151, 562–568. 18. Göbel, H. E. Adv. X-ray Anal. 1982, 24, 123–138. 19. IEEE Standard Modular Instrumentation and Digital Interface (CAMAC) IEEE-Std. 583; Institute of Electrical and Electronic Engineers: New York, 1975. 20. Rex, R. W. Adv. X-ray Anal. 1966, 10, 366–373. 21. Jenkins, R.; Haas, D. J.; Paolini, F. R. Norelco Rep. 1971, 18, 1–16. 22. Jenkins, R.; Smith, D. K. In Crystallographic Databases; International Union of Crystallography: Chester, England, 1987; Section 3.3, pp 158–177. 23. Faber, J.; Jenkins, R.; Snyder, R. L. Proceedings of the International School on Powder Diffraction (ISPD ’98); IACS: Calcutta, India, 1998; pp 135–143. 24. Johnson, G. G. Jr.; Vand, V. Ind. Eng. Chem. 1967, 59, 18– 31. 25. Hanawalt, J. D. In Crystallography in North America—Apparatus & Methods; American Crystallographic Association: Buffalo, NY, 1983; Chapter 2, pp 215–219. 26. Needham, F. L.; et al. PCPDFWIN—Search/Retrieve Program for the ICDD Database on CD-ROM; IUCr Congress paper MS-18.01.08, 1993. 27. Jenkins, R. Adv. X-ray Anal. 1994, 37, 117–121. 28. The Rietveld Method; Young, R. A., Ed.; IUCr Monographs on Crystallography 5; Oxford University Press: Oxford, 1993. 29. Cox, D. E.; Hastings, J. B.; Cardoso, L. P.; Finger, L. W. Mater. Sci. Forum 1986, 9, 1–20. 30. Jenkins, R. Adv. X-ray Anal. 1991, 35, 653–660. 31. Post, B. Norelco Rep. 1973, 20, 8–11. 32. Jenkins, R.; Nicolosi, J. A. Long Wavelength X-ray Diffractometer; North American Philips Corp.; U.S. Patent Appl. 815349, Jul 15, 1987.


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