Early Days of X-ray Crystallography

The role of crystallography has been recognized by the United Nations by declaring 2014 “Year of Crystallography” and the International Union of Crystallography (www.iycr2014.org), together with UNESCO, has launched an intense program of events and initiatives all over the world to promote the knowledge and image of this important science, not only within the scientific community, but also to the general public. The publication of Early Days of X-ray Crystallography comes at the most appropriate moment to give a lively historical account of crystallography and its foundations.

The book is the result of a monumental work of historical documentation , for which we must be grateful to the author, who has been able to elaborate this large mass of information into a pleasant book full of relevant and interesting stories. While maintaining its main goal of describing the historical and scientific context in which X-ray crystallography originated, the book is a well conceived excursion through the history of science in the 20^th century. The two main characters of the story, the crystal and X-rays, are described through the historical progress in the understanding of their nature. Brilliant and appropriate quotations, as subtitles of the chapters, and many figures, pictures, and short biographies of the main characters embellish the text. Interrelations among scientists, their collaborations and controversies over the proper interpretation of X-ray diffraction by crystals are presented in a fetching way: most famous and less famous scientists come in and out of the story at the appropriate moment and their contribution is clearly explained.

Chapter 1, on the “Significance of the discovery of X-ray diffraction” gives a short introduction to the discovery of X-ray diffraction and its impact on many branches of science: chemistry, physics, mineralogy, biology, materials science, and cultural heritage.

Chapter 2, on “The various approaches to the concept of space lattice” gives an illustration of the space lattice and symmetry concepts and their historical developments. It is an introductory, but fairly complete account, with cross references to the last two chapters (11 & 12) forming two appendices with more details on the various steps that led to the present concept of space lattice and to the derivation of crystal symmetry properties; they are full of interesting and amusing accounts of the ingenuity of our predecessors. Space-filling and close-packing approaches to space lattice are presented. The last paragraph tells of the contrasting views of physicists to explain the elastic and optical properties of crystals, which, in the early 20^th century, just prior to the discovery of X-ray diffraction by crystals, had seen a strong support for the continuum theory against the space-lattice theory.

Chapter 3, on “The dual nature of light” illustrates the early theories of light from Ptolemy to Hooke: Grimaldi and light diffraction; Newton and his corpuscular emission theory of light, opposed by Hooke and Huygens, who favoured the wave nature of light; Huygens’ explanation of double refraction or “strange refraction of Iceland crystal” and fate of his wave theory and its resuscitation in the early 19^th century by Young interference experiments; other evidences by Malus and Fresnel who developed the theory of diffraction. The most recent developments from Maxwell electromagnetic theory to the quantum theory of light and its dual nature are mentioned in the last paragraph.

Chapter 4, on “Roentgen and the discovery of X-rays” is a fascinating account of Roentgen’s discovery and of its impact and consequences.

In Chapter 5, on “The nature of X-rays: waves or particles” there is a wonderful quotation by Sir W.H. Bragg, “On Mondays, Wednesdays, and Fridays we use the wave theory; on Tuesdays, Thursdays, and Saturdays we think in streams of flying energy quanta or corpuscles,” describing the developments from the study of cathode rays, to the discovery of electrons, to X-rays and g-rays due to Hertz, Lenard, Thomson and many others. Secondary X-rays and Barkla’s investigations on the characteristic radiation of the elements, W.H. Bragg’s early corpuscular theory of X-rays and the estimation of X-ray wavelength by Sommerfeld (diffraction), Wien and Stark (energy of elements), are illustrated.

Chapter 6, entitled “1912: the discovery of X-ray diffraction and the birth of X-ray analysis” deals with the exciting scientific and cultural environment in Munich in 1912. The scenario of the discovery in the three main scientific institutions: Institute for Mineralogy and Mineral Sites directed by Groth, Institute of Experimental Physics directed by Roentgen, and Institute of Theoretical Physics directed by Sommerfeld is vividly depicted. Ewald’s thesis under Sommerfeld, Laue formation, Ewald’s question to Laue leading to his intuition of diffraction by crystals, and Friedrich and Knipping’s first diffraction experiments are well documented. The propagation of and reaction to the news of the discovery and how it reached W.H. Bragg (father), who, together with Stark, favored a “corpuscular” interpretation and the role of W.L. Bragg (son) with his “wave” interpretation and his famous law are illustrated. The article by the science historian Paul Forman criticizing the “myths” over the discovery of X-ray diffraction is discussed in the last paragraph.

In Chapter 7, entitled “1913: the first steps” the reaction to the first and in some way contradictory experiments are well summarized by W.H. Bragg: “their [of reflected X-rays] energy is concentrated as if they were corpuscular” and they show “the contrary properties of extension over a wave-front and concentration in a point.” The equivalence of Laue’s diffraction relation and Bragg’s reflection law are described together with an account of the role of the French and Japanese schools. The importance of W.H. Bragg X-ray ionization spectrometer and the first experiments on the properties of X-rays are highlighted by W.L. Bragg’s quotation: “My father was supreme at handling X-ray tubes and ionization chambers.” The ionization spectrometer allowed the first structure determinations by the Braggs of alkali halides and diamond, followed by many other simple inorganic compounds. The following paragraphs deal with: the high frequency spectra of the elements, Mosley’s law, and the prediction of missing elements; Debye’s studies on the effect of thermal vibrations in crystals; the review and discussion on the one year results at three important meetings in Birmingham, Brussels (Solvay) and Vienna in autumn 1913; Friedel’s law and its derivation from the intensities and use of structure factor; Duane-Hunt law and the high frequency limit of the X-ray spectrum, allowing the derivation of the value of Plank’s constant. The Nobel Prize to Laue (1914) and the Braggs (1915) closes the chapter.

Chapter 8, on “The route to crystal structure determination” illustrates the relevant progress. From 1912 to 1920, only a few very simple inorganic structures were solved. From 1920 to 1925, more and somewhat more complex inorganic structures are solved by trial and error, with the aid of space-group symmetry in Great Britain, Germany, France, USA, Japan and some other countries. Besides Laue and rotating crystal photographs and spectrometer recordings, also diffraction by crystalline powders was introduced and used. in 1925-1930 a large number of rather complex inorganic structures is solved and Fourier methods became available. The structure of diamond had already shown the tetrahedral arrangement of the four carbon atoms around each carbon atom in the crystal, thus suggesting the same geometry for methane and all saturated aliphatic hydrocarbons, while the solution by K. Lonsdale of one of the first organic structures, hexamethylbenzene modified Kekulé’s picture of alternating single and double bonds in benzene. Structural information was stored by Wyckoff in The Structure of Crystals and in Strukturbericht. At the same time some insight into the intensity of diffracted X-rays, the factors affecting intensities, the integrated intensity, the atomic scattering factor, and the electron density was acquired. Improved powder diffraction cameras, rotating and oscillating crystal methods, and Weissenberg camera to explore the reciprocal lattice became available. A better understanding of the theory of absolute intensities, extinction, and the role of crystal perfection and crystal defects in the diffraction process was gained. Some landmark structures are illustrated: hexamethylenetetramine, graphite, hexamethylbenzene determined by Kathleen Lonsdale, first woman professor in Britain.

Chapter 9, on “X-rays as a branch of optics” is the most physical of all chapters describing Ewald’s dynamical theory and his wave expansion (similar to Bloch wave in solid state physics), the double-crystal spectrometer to record rocking curves and analyze crystal perfection, Compton effect explained using light quanta, Laue’s dynamical theory and optical properties of wavefields.

As shown in Chapter 10,“Early application of X-ray Crystallography,” X-ray diffraction allowed the investigation of the structure of matter in the solid state and gave a major contribution to the understanding of the nature of chemical bonds, allowing Pauling to open the new science of structural chemistry. Polar and non polar bonds in inorganic crystals, ionic and atomic radii, solid solutions and Vegard law, Goldschmidt classification and the coordination number, Pauling rules and coordination polyhedral are mentioned. Organic crystals showed the presence of molecular crystals and gave insight on the nature of covalent bonds and of intermolecular interactions and on the stereochemical properties of molecules. More recently the information on the large number of organic and metallo-organic crystal structures was collected in the Cambridge Crystallographic Database.

The study of metals and alloys not only allowed for the definition of the way in which atoms are held together, but also for more clear and complete phase diagrams, insight on order-disorder transitions, the properties of intermetallic compounds, and the study of the effects of imperfections and mutual orientation of the crystalline grains on the properties of metallic compounds.

Up to 1912, crystallography was a branch of mineralogy, but the study of minerals by X-ray diffraction opened the new reach field of structural mineralogy. An example of the major outcome of these studies is the structural classification of silicates.

Since many physical properties of crystals depend on their structure, diffraction studies are essential in materials science, where the possibility of appropriate atom exchanges in the structure can be used to modulate the properties of the materials.

The structural interpretation of physical properties is an important goal of crystal physics. Three early examples are the zero-point energy, piezoelectricity, and phase transformations. Crystal imperfections and their effects were also investigated.

The first structural studies of biological substances were carried out on natural fibres such as silk, wool, cellulose, etc. especially by Atsbury, who is considered the father of molecular biology. The study of proteins only took off later in the 1930s.

The last paragraph deals with the role of X-ray spectroscopy in the elucidation of the structure of the atom and the relevant experimental and theoretical studies.

As already mentioned, Chapters 11 and 12 contain developments on the evolution of the space lattice and symmetry concepts. The first, on “Unravelling the mystery of crystals: the forerunners,” with an illuminating quotation by von Laue as subtitle, is a more detailed history of the studies of crystals from ancient times to the end of the 18^th century, with details on the contributions of the main characters. The second, on “The birth and rise of the space-lattice concept”illustrates the contribution to the crystal lattice and symmetry ideas by several scientists such as Haüy, Mohs, Neumann, Miller, Seeber, Bravais, Sohncke, Barlow, Fiedorov and Schoenflies.

The book is written by a well-known physicist with a long and important scientific career in crystal physics and gives the reader a clear understanding of the basic physical processes underlying X-ray interaction with matter. As a counterpart to this important feature some relevant omissions should be pointed out. The essential role of the phase problem in the solution of crystal structures is only mentioned as a general statement and the use of the Patterson function and the heavy atom method are not mentioned. Direct methods, that triggered a rapid increase of the entries in the Cambridge Database in the 1960’s, are indeed more recent, but the same applies to the Rietveld method and to many other more physical developments, which have been mentioned. The most relevant omission is represented by the three cursory lines over the “prodigious development of the applications of X-ray diffraction,” which deserved some more attention, considering the number of Nobel awards given for such studies.

The book tells that W.L. Bragg in 1919 was advised by his scientific friends to drop the study of crystals, pointing out that all crystals would soon be worked out. The same advice was also given to me by some chemist friends in the mid 1960’s and, fortunately, I dropped the advice and not crystallography. Crystallography is an acquired taste and is often seen just as a routine useful tool by those who have not seen the beauty and richness of our science. This book, giving a vivid insight into its early developments, is certainly a valid tool to promote our science and to show our chemist, physicist, mineralogist, biologist, and material scientist “users” that we are constantly improving our tools to obtain a clearer picture of the structure of matter. Crystallography is ready to face the coming 100 years of progress to continue its essential role in science and life!

Davide Viterbo <davide.viterbo@mfn.unipmn.it> is retired Professor of Physical-Chemistry by the Università del Piemonte Orientale and Chair of the IUCr Book Series Committee, Italy.