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Gontribhted by r m ~Resear& . Foundation -oiIllinois Institute of ‘fechnology .. A -_ ___._.
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HIS is the first in a series of monthly summaries of crystallographic data to be presented as a public service activity by Armour Research Foundation of Illinois Institute of Technology, Chicago, Ill. This project, under the direction of W. C. McCrone, will present structures from the field of industrially important or common compounds. The purpose of this project is to increase the number of dependable crystal structures, especially of organic compounds, available in the literature. It is hoped that in time these efforts will increase the use of crystallographic methods, both microscopic and x-ray, in research and analysis. The structures included will be chosen from those determined in the Armour Research Foundation laboratory and those submitted as a part of this project by cooperating laboratories. As announced in the February 16, 1948, issue of Chemical a n d Engineering News, page 472, any laboratory or individual may submit crystallographic data to Armour. It is, therefore, necessary to set up mechanisms a t once to ensure: (1) a continuing suppl’y of’crystal data; and (2) a reliable system for checking the data. This requires responsibility both with respect to quantity and quality of the data published. Further details are outlined below. DATA SUBMITTED
Any carefully determined data, however fragmentary, may be submitted. In general, only those compounds of interest and importance to research and analysis will be completed, checked, and published. Data may be submitted as photostats of notebook pages or in any other form representing little effort. However, any additional material such as well-formed crystals (either for single crystal x-ray work or optical crystallography), crystal drawings, or photomicrographs will be appreciated. The names of all persons contributing different portions of the data should be included, so that acknowledgment can be made. In general, the over-all description will be credited to the laboratories submitting and checking the work and the data itself t o the individuals who performed the viork. This is desirable because the different portions will, in general, have been determined by different individuals. .In outline of the final description is as follows:
drawn up based partly on the literature and partly on a meeting held a t the invitation of Armour to a number of crystallographers attending the 1947 fall meeting of the AMERICANCHEMICAL SOCIETY in New York. This tentative set of conventions was then submitted for criticism to the entire membership of the Crystallographic Society and the American Society for X-Ray and Electron Diffraction. As a result, nearly 100 outstanding crystallographers (eight European crystallographers) representing many diverse backgrounds have sent detailed comments, criticisms, and suggestions. It is, perhaps, unfortunate that these letters cannot be published in their entirety, as they represent the ideas and opinions of some of the best crystallographers in this country and Europe. These letters show a wide variation in practice on almost every point under discussion. This has made the problem of setting u p uniform conventions very difficult, since each crystallographer has good reasons for his point of view. The conventions, shown below, were chosen after careful consideration of these letters and are believed to be the best compromise possible; they will be used for the publication of data by the Armour Research Foundation. Most of the conventions chosen agree with established usage. In every case an effort has been made to choose nomenclature that will be readily understood by all crystallographers, whether their background is x-ray diffraction, chemistry, or mineralogy. Although every effort has been made to eliminate any cause for cciticism of this project in any of its aspects, comments or suggestions are most welcome. In all cases not specifically covered below, the conventions used will be those published by Donnay (4)of Johns Hopkins University or by the Committee of the Division of Analytical and Micro Chemistry (10) (C. IT. Mason, Chairman; W. PIX. D. Biyant, RIam L. JTiIlard, E. F. \Villjams). CONVEVTIONS USED IN CRYSTAL DESCRIPTIOKS
Axial Ratios. a : b : c - 0.61:1:1.10. Bisectrices. If not writren out, the acute and obtuse bisectrices 1%ill be designated as Bra and Bz,, respectively. Beta Angle. The beta crystallographic angle will ah-ays be expressed as the obtuse angle, in keeping with existing x-ray crystsllographic conventions. I n the definition o€ extinction apgles the apparently redundant word “obtuse” will be retained to avoid confusion. Thus, for a monoclinic crystal: the view normal to (010) shows oblique extinction; Xhc = 11’ in obtuse beta. Acute beta need not be used, because the extinction can always be expressed as an angle smaller than 45 by picking the vibration direction and crystallographic axis to give the smallest angle. Occasionally, the term “acute” may be necessary in the interest of brevity. There is, for example, no other simple way of defining the location of an optic axial plane that is perpendicular to (010) and in the acute angle beta. Crystallographic Axes. a, b, and c (lower case italics) will be used. The choice of a, b and c is based on the dimensions of the unit cell with c
