Topics in..
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Chemical Instrumentation
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Edited by GALEN W. W I N G , Seton Hall University, So. Orange, N. J. 07079 These articles, ,mxt of wliich are lo be contributed bg guest authors, are intended to serve the readers o j this JOURNAL by calling attention to new developments i n the theory, design, or availability of chemical laboratory instrumenlation, 07 by presenting useful insights and explanations qf topics that are of prmtical importance lo tliose who use, or teach the use of, modern instrumentation and instrnmental techniques.
XXXII. X-Ray Diffraction Analysis
(concluded)*
Part Six-Single Crystal Methods (continued) and Miscellaneous Methods
as goniostata, unless the manufacturer's term diffen. One major difference between these two t , y p s of diffractometers is that, under similar conditions (same crystal, same radiation, and ,,sing only one mounting of the crystal), prnctically the whole of the reflecting sphere (up t,o the maximum 28 value) can be explored with the goniostat, while a. smaller volume can be explored with the counter Weissenberg. The range of the counter Weissenberg is limited by the maximum attainable value of the eqni-inclination angle. A minor difference between these two instruments is that data processing (e.g., Lorentz-polarization corrections) is simpler for eoniontat data than for eounter Weis-
REUBEN RUDMAN, Deparfment of Chemisfry Brookhaven Nationol Laboratory, Upton, N.Y. 1 1973 Single-Crystal Counter M e t h o d s ' The development of sensitive counters, reliable electronic panels, and stabilised X-ray generators over tho past fifteen yeam has been accompanied by the development of sophisl.icated X-ray diffraction apparatus. The apparatus used in roujunatio~ln i t h a counter detector is, basically, a crystal rxienting device or goniometer. While i t is troe that film devices must be constructed so that the crystal remains in the center of the X-ray beam ~s it is rotated or precessed about some axis, greater accuracy is required of counter devices. For this reason, eo~mterinstruments are elshorate, rather expensive items. Two basic geometries have been ntiliaed in the construction of counter instruments. The first is that of t,he Weissenherg goniometer, with the film replaced by a counter mounted on a moveable arm. This instrument is known as the Counter Weissenherg and data is collected exactly as on a Weissenberg camera. The second differs from the first in the follon.ing manner: Upper-level reflections are measured b y tilting the crystal rather than the detector. The detector remains in one plane (e.g., the horizontal plane, when a horizontal diffractometer is used) and the crystal is carried in R. two-circle assembly which is mounted on the omega ( w ) axis (the w axis lies parallel to the 28 axis). The two-circle assembly consists *Prepared under the auspices of the Atomic Energy Commission. 'The List of Manufacturers is in the first installment, THIS JOURNAL, 44, A42 (1967). Price Range: A = under 510; B = $10-50; C = 550-100; D = 5100-500; E = $500-1000; F = 5100(t3000; G = $3000-7000; H = $700&12,000; I = $12,00W25,000; J = above$25,000.
of a x-circle, whieh rotates about a horizontal axis and a 4 - r i d e , mounted on the x-circle so that the +%xis is parallel to s. radius of the x-circle. The crystal is mounted on a goniometer head carried on the + axis (Fig. 100). A reflection is obtained by setting the 28, x and 4 angles so t,hat the reflection is in the zero-level plane (i.e., the plane perpendicular to the w axis which contains thedirect beam). The tm.o-circle assembly must be designed so that a properly aligned crystal will remain centered in the X-ray beam regtudle.9~of the angular settings. These devices, known as Eulerian cradles, have been built with a +-circle capable of rotating 360' and a 90, 180 or 360" x-circlo. Some of the more popular terms used to describe thc3e instruments are goniostat and full-circle or quarter-eiircle diffractometer; they are also described as having Eulerian or aero-level geometry. The term diffractometer has been applied to all counter units (e.g., powder diffraetometer, U'eissenherg diffractometer). I n this article we shall refer to those instruments employing zero-level geometry
put& programs far dat,a-processing, this objection to the use of counter Weissenberg instnments is not very serious. Goniostnts have been designed for m e with horizontally or vertically mounted X-ray tubes; because of t,he splitting of the focal spot discussed in a previous article, the omega axis must be perpendirular to the tube mounting (e.g., a vertical omega axis is used with a horizontally mounted tube). Once n elystnl of known unit cell dimensions is mount,ed, i t is a routine, and rather tedious, matter to adjust the crystal t,o the angles necessary to bring a given set of planes into reflect,ingposition and to measure the intensity. The most rceent advance in X-ray instrumentxtian has been the automation of counter units. The angles are sutomat,ieally adjusted ior a given reflection, the intensity is measured and the instrument moves on t,o the next reflection. Computer, punched-card, and punched-tape operated units are on the market. These instrument6 are presently the most sophisticated and costly X-ray ditlractiou unite available. M a n u a l l y O p e r a t e d Counter Diffractometers
Figure 100. Schematic di~gromshowing relationrhip of 28, w, 6,and x axes.
Canter Weissenberg Instn~menls. Bath manually m d automatically operated counter Weissenberg goniometers are available from Supper, Stoe, and Crystal Structures Lmtd. and a manual device is marketed by Rigaku-Denki. The Rigakn-Denki apparatus (Cat. No. 2522, Price Range G ) allows one to investigate Bragg angles up to a value of 164.5' 20 (Fig. 101). An added feature is that the instrument has provision for the mounting of a cylindrical camera so (Continued on page ASOO) 'olume 44, Number 6, June 1967
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Figure 105.
Figure 101. berg.
Rigoku-Denki Counter Weirren.
that. an oscillation photograph can be taken. Although it is entirely possible to align a crystal on the diffractometer, the rough alignment can usually be carried out more efficiently with film methods. Furthermore, crystal anomalies (e.g., twinning) e m be easily recognized by examining the film. The m d s b i l i t y of a film cassette negates the need for transferring the crystal from one unit to another. Gmioslale. G. E . manufactures a quarter-circle goniostat (52700) designed for use with 8. horizontally mounted tnbe; angles of -25 to +160° in 28, a full 360" in 8, and -10 to +90° in can be set on this apparatus (Fig. 102).
,l~-"w,kw,
Figure 103.
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E 8. A Full-Circle Goniometer.
absorption effects. Cumulative angular errors are claimed to be less than 1 min of are; cumulative linear errors to 0.001 in. These units can also be completely automated and pre-programmed. The Picker Full-Circle Goniostat (Cat. No. 6239C, Price Range I, sold only with diflractometer) has an inside diameter of 10 in.; the manual controls are interchangeable wit.h m&mrttic high-speed motors and angle enooders. The x and 9 drives are disengageable to fachtrtte alignment and rapid preliminary examina, tions (Fig. 104). The instrument accepts either the 63.96 or 49.00 mm goniometer heads with the use of an auxiliary base
01
siemens ~ m w i e a n ~, n c . )
Siemens Full-Circle Goniometer.
an a special goniometer head (the Stoe microgoniometer head) whieh can be m d e to comply with international standards (ACA-thread) by the use of an adapter. Stoe has developed a large fulbcircle Eulerian cradle E C 63 of outer diameter 330 mm (Fig 106); Stoe eueentric goniometer beads can be used. The angular accuracy is better than &O.0l0. The large Eulerian cradle can be operated either manually or automatically (with encoders).
. {(,'",'~LP~,, Figure 106.
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