Early scientific history of crystals and X-rays
Drawing of square (Figure A, above) and hexagonal (Figure B, below) packing from
work, Strena seu de Nive Sexangula
Crystals, though long admired for their regularity and symmetry, were not investigated scientifically until the 17th century.
Johannes Kepler hypothesized in his work Strena seu de Nive Sexangula (A New Year's Gift of Hexagonal Snow) (1611) that the hexagonal symmetry of
snowflake crystals was due to a regular packing of spherical water particles.
As shown by X-ray crystallography, the hexagonal symmetry of snowflakes results from the
about each water molecule. The water molecules are arranged similarly to the
atoms in the
. The resulting crystal structure has hexagonal symmetry when viewed along a principal axis.
The Danish scientist
Nicolas Steno (1669) pioneered experimental investigations of crystal symmetry. Steno showed that the angles between the faces are the same in every exemplar of a particular type of crystal,
René Just Haüy (1784) discovered that every face of a crystal can be described by simple stacking patterns of blocks of the same shape and size. Hence,
William Hallowes Miller in 1839 was able to give each face a unique label of three small integers, the
Miller indices which remain in use today for identifying crystal faces. Haüy's study led to the correct idea that crystals are a regular three-dimensional array (a
Bravais lattice) of atoms and
molecules; a single
unit cell is repeated indefinitely along three principal directions that are not necessarily perpendicular. In the 19th century, a complete catalog of the possible symmetries of a crystal was worked out by
 and (belatedly)
William Barlow (1894). From the available data and physical reasoning, Barlow proposed several crystal structures in the 1880s that were validated later by X-ray crystallography;
 however, the available data were too scarce in the 1880s to accept his models as conclusive.
X-ray crystallography shows the arrangement of water molecules in ice, revealing the
(1) that hold the solid together. Few other methods can determine the structure of matter with such precision (resolution
Wilhelm Röntgen discovered X-rays in 1895, just as the studies of crystal symmetry were being concluded. Physicists were initially uncertain of the nature of X-rays, but soon suspected (correctly) that they were waves of
electromagnetic radiation, in other words, another form of
light. At that time, the wave model of
Maxwell theory of
electromagnetic radiation—was well accepted among scientists, and experiments by
Charles Glover Barkla showed that X-rays exhibited phenomena associated with electromagnetic waves, including transverse
spectral lines akin to those observed in the visible wavelengths. Single-slit experiments in the laboratory of
Arnold Sommerfeld suggested that X-rays had a
wavelength of about 1
angstrom. However, X-rays are composed of
photons, and thus are not only waves of electromagnetic radiation but also exhibit particle-like properties.
Albert Einstein introduced the photon concept in 1905,
 but it was not broadly accepted until 1922,
Arthur Compton confirmed it by the scattering of X-rays from electrons.
 Therefore, these particle-like properties of X-rays, such as their ionization of gases, caused
William Henry Bragg to argue in 1907 that X-rays were not electromagnetic radiation.
 Nevertheless, Bragg's view was not broadly accepted and the observation of
X-ray diffraction by
Max von Laue in 1912
 confirmed for most scientists that X-rays were a form of electromagnetic radiation.
The incoming beam (coming from upper left) causes each scatterer to re-radiate a small portion of its intensity as a spherical wave. If scatterers are arranged symmetrically with a separation d
, these spherical waves will be in sync (add constructively) only in directions where their path-length difference 2d
sin θ equals an integer multiple of the
λ. In that case, part of the incoming beam is deflected by an angle 2θ, producing a reflection
spot in the
Crystals are regular arrays of atoms, and X-rays can be considered waves of electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atoms' electrons. Just as an ocean wave striking a lighthouse produces secondary circular waves emanating from the lighthouse, so an X-ray striking an electron produces secondary spherical waves emanating from the electron. This phenomenon is known as
elastic scattering, and the electron (or lighthouse) is known as the scatterer. A regular array of scatterers produces a regular array of spherical waves. Although these waves cancel one another out in most directions through
destructive interference, they add constructively in a few specific directions, determined by
Here d is the spacing between diffracting planes, is the incident angle, n is any integer, and λ is the wavelength of the beam. These specific directions appear as spots on the
diffraction pattern called reflections. Thus, X-ray diffraction results from an electromagnetic wave (the X-ray) impinging on a regular array of scatterers (the repeating arrangement of atoms within the crystal).
X-rays are used to produce the diffraction pattern because their wavelength λ is typically the same order of magnitude (1–100 angstroms) as the spacing d between planes in the crystal. In principle, any wave impinging on a regular array of scatterers produces
diffraction, as predicted first by
Francesco Maria Grimaldi in 1665. To produce significant diffraction, the spacing between the scatterers and the wavelength of the impinging wave should be similar in size. For illustration, the diffraction of sunlight through a bird's feather was first reported by
James Gregory in the later 17th century. The first artificial
diffraction gratings for visible light were constructed by
David Rittenhouse in 1787, and
Joseph von Fraunhofer in 1821. However, visible light has too long a wavelength (typically, 5500 angstroms) to observe diffraction from crystals. Prior to the first X-ray diffraction experiments, the spacings between lattice planes in a crystal were not known with certainty.
The idea that crystals could be used as a
diffraction grating for
X-rays arose in 1912 in a conversation between
Paul Peter Ewald and
Max von Laue in the
English Garden in
Munich. Ewald had proposed a resonator model of crystals for his thesis, but this model could not be validated using
visible light, since the wavelength was much larger than the spacing between the resonators. Von Laue realized that electromagnetic radiation of a shorter wavelength was needed to observe such small spacings, and suggested that X-rays might have a wavelength comparable to the unit-cell spacing in crystals. Von Laue worked with two technicians, Walter Friedrich and his assistant Paul Knipping, to shine a beam of X-rays through a
copper sulfate crystal and record its diffraction on a
photographic plate. After being developed, the plate showed a large number of well-defined spots arranged in a pattern of intersecting circles around the spot produced by the central beam.
 Von Laue developed a law that connects the scattering angles and the size and orientation of the unit-cell spacings in the crystal, for which he was awarded the
Nobel Prize in Physics in 1914.
As described in the
mathematical derivation below, the X-ray scattering is determined by the density of electrons within the crystal. Since the energy of an X-ray is much greater than that of a valence electron, the scattering may be modeled as
Thomson scattering, the interaction of an electromagnetic ray with a free electron. This model is generally adopted to describe the polarization of the scattered radiation.
The intensity of Thomson scattering for one particle with mass m and charge q is:
Hence the atomic nuclei, which are much heavier than an electron, contribute negligibly to the scattered X-rays.
Development from 1912 to 1920
(top left) and
(top right) are identical in chemical composition—being both pure
—X-ray crystallography revealed the arrangement of their atoms (bottom) accounts for their different properties. In diamond, the carbon atoms are arranged
and held together by single
, making it strong in all directions. By contrast, graphite is composed of stacked sheets. Within the sheet, the bonding is covalent and has hexagonal symmetry, but there are no covalent bonds between the sheets, making graphite easy to cleave into flakes.
After Von Laue's pioneering research, the field developed rapidly, most notably by physicists
William Lawrence Bragg and his father
William Henry Bragg. In 1912–1913, the younger Bragg developed
Bragg's law, which connects the observed scattering with reflections from evenly spaced planes within the crystal.
 The Braggs, father and son, shared the 1915 Nobel Prize in Physics for their work in crystallography. The earliest structures were generally simple and marked by one-dimensional symmetry. However, as computational and experimental methods improved over the next decades, it became feasible to deduce reliable atomic positions for more complicated two- and three-dimensional arrangements of atoms in the unit-cell.
The potential of X-ray crystallography for determining the structure of molecules and minerals—then only known vaguely from chemical and hydrodynamic experiments—was realized immediately. The earliest structures were simple inorganic crystals and minerals, but even these revealed fundamental laws of physics and chemistry. The first atomic-resolution structure to be "solved" (i.e., determined) in 1914 was that of
 The distribution of electrons in the table-salt structure showed that crystals are not necessarily composed of
covalently bonded molecules, and proved the existence of
 The structure of
diamond was solved in the same year,
 proving the tetrahedral arrangement of its chemical bonds and showing that the length of C–C single bond was 1.52 angstroms. Other early structures included
calcium fluoride (CaF2, also known as fluorite),
calcite (CaCO3) and
 in 1914;
spinel (MgAl2O4) in 1915;
anatase forms of
titanium dioxide (TiO2) in 1916;
pyrochroite Mn(OH)2 and, by extension,
brucite Mg(OH)2 in 1919;.
 Also in 1919
sodium nitrate (NaNO3) and caesium dichloroiodide (CsICl2) were determined by
Ralph Walter Graystone Wyckoff, and the
wurtzite (hexagonal ZnS) structure became known in 1920.
The structure of
graphite was solved in 1916
 by the related method of
 which was developed by
Peter Debye and
Paul Scherrer and, independently, by
Albert Hull in 1917.
 The structure of graphite was determined from single-crystal diffraction in 1924 by two groups independently.
 Hull also used the powder method to determine the structures of various metals, such as iron
 and magnesium.
Cultural and aesthetic importance
In what has been called his scientific autobiography, The Development of X-ray Analysis, Sir
William Lawrence Bragg mentioned that he believed the field of crystallography was particularly welcoming to women because the techno-aesthetics of the molecular structures resembled textiles and household objects. Bragg was known to compare crystal formation to "curtains, wallpapers, mosaics, and roses".
In 1951, the Festival Pattern Group at the
Festival of Britain hosted a collaborative group of textile manufacturers and experienced crystallographers to design lace and prints based on the X-ray crystallography of
china clay, and
hemoglobin. One of the leading scientists of the project was Dr.
Helen Megaw (1907–2002), the Assistant Director of Research at the Cavendish Laboratory in Cambridge at the time. Megaw is credited as one of the central figures who took inspiration from crystal diagrams and saw their potential in design.
 In 2008, the Wellcome Collection in London curated an exhibition on the Festival Pattern Group called "From Atom to Patterns".