The invention: Technique for using X rays to determine the crystal structures of many substances.
The people behind the invention:
Sir William Lawrence Bragg (1890-1971), the son of Sir William
Henry Bragg and cowinner of the 1915 Nobel Prize in Physics Sir William Henry Bragg (1862-1942), an English mathematician
and physicist and cowinner of the 1915 Nobel Prize in
Physics
Max von Laue (1879-1960), a German physicist who won the
1914 Nobel Prize in Physics Wilhelm Conrad Rontgen (1845-1923), a German physicist who
won the 1901 Nobel Prize in Physics Rene-Just Haiiy (1743-1822), a French mathematician and
mineralogist Auguste Bravais (1811-1863), a French physicist
The Elusive Crystal
A crystal is a body that is formed once a chemical substance has solidified. It is uniformly shaped, with angles and flat surfaces that form a network based on the internal structure of the crystal’s atoms. Determining what these internal crystal structures look like is the goal of the science of X-ray crystallography. To do this, it studies the precise arrangements into which the atoms are assembled.
Central to this study is the principle of X-ray diffraction. This technique involves the deliberate scattering of X rays as they are shot through a crystal, an act that interferes with their normal path of movement. The way in which the atoms are spaced and arranged in the crystal determines how these X rays are reflected off them while passing through the material. The light waves thus reflected form a telltale interference pattern. By studying this pattern, scientists can discover variations in the crystal structure.
The development of X-ray crystallography in the early twentieth century helped to answer two major scientific questions: What are Xrays? and What are crystals? It gave birth to a new technology for the identification and classification of crystalline substances.
From studies of large, natural crystals, chemists and geologists had established the elements of symmetry through which one could classify, describe, and distinguish various crystal shapes. Rene-Just Haiiy, about a century before, had demonstrated that diverse shapes of crystals could be produced by the repetitive stacking of tiny solid cubes.
Auguste Bravais later showed, through mathematics, that all crystal forms could be built from a repetitive stacking of three-dimensional arrangements of points (lattice points) into “space lattices,” but no one had ever been able to prove that matter really was arranged in space lattices. Scientists did not know if the tiny building blocks modeled by space lattices actually were solid matter throughout, like Haiiy’s cubes, or if they were mostly empty space, with solid matter located only at the lattice points described by Bravais.
With the disclosure of the atomic model of Danish physicist Niels Bohr in 1913, determining the nature of the building blocks of crystals took on a special importance. If crystal structure could be shown to consist of atoms at lattice points, then the Bohr model would be supported, and science then could abandon the theory that matter was totally solid.
X Rays Explain Crystal Structure
In 1912, Max von Laue first used X rays to study crystalline matter. Laue had the idea that irradiating a crystal with X rays might cause diffraction. He tested this idea and found that X rays were scattered by the crystals in various directions, revealing on a photographic plate a pattern of spots that depended on the orientation and the symmetry of the crystal.
The experiment confirmed in one stroke that crystals were not solid and that their matter consisted of atoms occupying lattice sites with substantial space in between. Further, the atomic arrangements of crystals could serve to diffract light rays. Laue received the 1914 Nobel Prize in Physics for his discovery of the diffraction of X rays in crystals.
Sir William Henry Bragg and Sir William Lawrence Bragg
William Henry Bragg, senior member of one of the most illustrious father-son scientific teams in history, was born in Cumber-
land, England, in 1862. Talented at mathematics, he studied that field at Trinity College, Cambridge, and physics at the Cavendish Laboratory, then moved into a professorship at the University of Adelaide in Australia. Despite an underequipped laboratory, he proved that the atom is not a solid body, and his work with X rays attracted the attention of Ernest Rutherford in England, who helped him win a professorship at the University of Leeds in 1908.
By then his eldest son, William Lawrence Bragg, was showing considerable scientific abilities of his own. Born in Adelaide in 1890, he also attended Trinity College, Cambridge, and performed research at the Cavendish. It was while there that father and son worked together to establish the specialty of X-ray crystallography. When they shared the 1915 Nobel Prize in Physics for their work, the son was only twenty-five years old—the youngest person ever to receive a Nobel Prize in any field.
The younger Bragg was also an artillery officer in France during World War I. Meanwhile, his father worked for the Royal Admiralty. The hydrophone he invented to detect submarines underwater earned him a knighthood in 1920. The father moved to University College, London, and became director of the Royal Institution. His popular lectures about the latest scientific developments made him famous among the public, while his elevation to president of the Royal Society in 1935 placed him among the most influential scientists in the world. He died in 1942.
The son taught at the University of Manchester in 1919 and then in 1938 became director of the National Physics Laboratory and professor of physics at the Cavendish. Following the father’s example, he became an administrator and professor at the Royal Institution, where he also distinguished himself with his popular lectures. He encouraged research using X-ray crystallography, including the work that unlocked the structure of deoxyribonucleic acid (DNA). Knighted in 1941, he became a royal Companion of Honor in 1967. He died in 1971.
Still, the diffraction of X rays was not yet a proved scientific fact. Sir William Henry Bragg contributed the final proof by passing one of the diffracted beams through a gas and achieving ionization of the gas, the same effect that true X rays would have caused. He also used the spectrometer he built for this purpose to detect and measure specific wavelengths of X rays and to note which orientations of crystals produced the strongest reflections. He noted that X rays, like visible light, occupy a definite part of the electromagnetic spectrum. Yet most of Bragg’s work focused on actually using X rays to deduce crystal structures.
Sir Lawrence Bragg was also deeply interested in this new phenomenon. In 1912, he had the idea that the pattern of spots was an indication that the X rays were being reflected from the planes of atoms in the crystal. If that were true, Laue pictures could be used to obtain information about the structures of crystals. Bragg developed an equation that described the angles at which X rays would be most effectively diffracted by a crystal. This was the start of the X-ray analysis of crystals.
Henry Bragg had at first used his spectrometer to try to determine whether X rays had a particulate nature. It soon became evident, however, that the device was a far more powerful way of analyzing crystals than the Laue photograph method had been. Not long afterward, father and son joined forces and founded the new science of X-ray crystallography. By experimenting with this technique, Lawrence Bragg came to believe that if the lattice models of Bravais applied to actual crystals, a crystal structure could be viewed as being composed of atoms arranged in a pattern consisting of a few sets of flat, regularly spaced, parallel planes.
Diffraction became the means by which the Braggs deduced the detailed structures of many crystals. Based on these findings, they built three-dimensional scale models out of wire and spheres that made it possible for the nature of crystal structures to be visualized clearly even by nonscientists. Their results were published in the topic X-Rays and Crystal Structure (1915).
Impact
The Braggs founded an entirely new discipline, X-ray crystallography, which continues to grow in scope and application. Of particular importance was the early discovery that atoms, rather than molecules, determine the nature of crystals. X-ray spectrometers of the type developed by the Braggs were used by other scientists to gain insights into the nature of the atom, particularly the innermost electron shells. The tool made possible the timely validation of some of Bohr’s major concepts about the atom.
X-ray diffraction became a cornerstone of the science of mineralogy. The Braggs, chemists such as Linus Pauling, and a number of mineralogists used the tool to do pioneering work in deducing the structures of all major mineral groups. X-ray diffraction became the definitive method of identifying crystalline materials.
Metallurgy progressed from a technology to a science as metallurgists became able, for the first time, to deduce the structural order of various alloys at the atomic level. Diffracted X rays were applied in the field of biology, particularly at the Cavendish Laboratory under the direction of Lawrence Bragg. The tool proved to be essential for deducing the structures of hemoglobin, proteins, viruses, and eventually the double-helix structure of deoxyribonucleic acid (DNA).
See also Field ion microscope; Geiger counter; Holography; Mass spectrograph; Neutrino detector; Scanning tunneling microscope; Thermal cracking process; Ultramicroscope.
