Chemical Crystallography - an Introduction to Optical and X Ray Methods

Text extracted from opening pages of book: CHEMICAL CBYSTALLOGRAPHY AN. INTRODUCTION TO OPTICAL AND X-RAY METHODS BY C. W. BUNN OXFORD AT THE CLARENDON PRESS Oxford University Press, Amen House, London E. G. 4 GLASGOW NEW YORK TORONTO MELBOURNE WELLINGTON BOMBAY CALCUTTA MADRAS CAPETOWN Geoffrey Cumberlege, Publisher to the University FIRST PUBLISHED 1045 REPRINTED WITH CORRECTIONS 1946 Reprinted lithographically in Great Britain at the UNIVERSITY PRESS, OXFORD, 1948, 1949, 1952, from corrected sheets of the second impression PREFACE CEYSTALLOGRAPHIC methods are used in chemistry for two main pur poses the identification of solid substances, and the determination of atomic configurations ; there are also other applications, most of which, as far as technique is concerned, may* be said to lie between the two main subjects. This baok is intended to be a guide to these methods. I have tried to explain the elementary principles involved, and to give as much practical information as will enable the reader to start using the methods described. I have not attempted to give a rigorous treat ment of the physical principles: the f approach is consistently from the chemist's point of view, and physical theory is included only in so far as it is necessary for the general comprehension of the principles and methods described. Nor have I attempted to give an exhaustive account of any subject ; the aim throughout has been to lay the founda tions, and to give sufficient references ( either to larger works or to original papers) to enable the reader to follow up any subject in greater detail if he so desires. The treatment of certain subjects is perhaps somewhat unorthodox. Crystal morphology, for instance, is described in terms of the concept of the unit cell ( rather than in terms of the axial ratios of the earlier morphologists), and is approached by way of the phenomena of crystal growth. The optical properties of crystals are described solely in terms of the phenomena observed in the polarizing microscope. X-ray diffrac tion is considered first in connexion with powder photographs; it is moj* e usual to start with the interpretation of the diffraction effects of single crystals. These methods of treatment are dictated by the form and scope of the book ; they also reflect the course of the writer's own experience in applying crystallographic methods to chemical problems. It is therefore hoped that they may at any rate seem natural to those to whom the book is addressed - students of chemistry who wish to acquire some knowledge of crystallographic methods, and research workers who wish to make practical use of such methods. If the book should come to the notice of a more philosophical reader, I can only hope that any qualms such a reader may feel about its avoidance of formal physical or mathematical treatment may be somewhat offset by the interest of a novel, if rather severely practical, viewpoint. The difficulties of three-dimensional thinking have, I hope, been lightened as much as possible by the provision of a large number of vi PREFACE diagrams ; but crystallography is emphatically not a subject which can be learnt solely from books: solid models should be used freely models of crystal shapes, of atomic . nd molecular configurations, of reciprocal lattices and of vectorial representations of optical and other physical properties. Most of the diagrams are original, but a few have been reproduced, by permission, from published books and journals: Figs. 197, 207-9, 215, and 222 from the Journal of the Chemical Society ; Figs. 199, 203, and 217 from the Proceedings of the Royal Society, Figs. 102-4 from the Journal of Scientific Instruments ; Fig. 229 from the Journal of the American Chemical Society, Fig. 161 from Inter nationale Tabetten ftir Bestimmung von Kristallstrukturen ( Berlin: Born traeger); Fig. 192 from the * Strnkturbericht ' of the Zeitschrift fttr Kristallographie\ and Figs. 212 and 216 from Bragg's The Crystalline State