Bragg's law


In physics, Bragg's law, or Wulff–Bragg's condition, a special case of Laue diffraction, gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move, as does any electromagnetic wave. The movement of these charges re-radiates waves with the same frequency, blurred slightly due to a variety of effects; this phenomenon is known as Rayleigh scattering. The scattered waves can themselves be scattered but this secondary scattering is assumed to be negligible.
A similar process occurs upon scattering neutron waves from the nuclei or by a coherent spin interaction with an unpaired electron. These re-emitted wave fields interfere with each other either constructively or destructively, producing a diffraction pattern on a detector or film. The resulting wave interference pattern is the basis of diffraction analysis. This analysis is called Bragg diffraction.

History

Bragg diffraction was first proposed by Lawrence Bragg and his father William Henry Bragg in 1913 in response to their discovery that crystalline solids produced surprising patterns of reflected X-rays. They found that these crystals, at certain specific wavelengths and incident angles, produced intense peaks of reflected radiation. The concept of Bragg diffraction applies equally to neutron diffraction and electron diffraction processes. Both neutron and X-ray wavelengths are comparable with inter-atomic distances and thus are an excellent probe for this length scale.
Lawrence Bragg explained this result by modeling the crystal as a set of discrete parallel planes separated by a constant parameter d. It was proposed that the incident X-ray radiation would produce a Bragg peak if their reflections off the various planes interfered constructively. The interference is constructive when the phase shift is a multiple of 2; this condition can be expressed by Bragg's law and was first presented by Lawrence Bragg on 11 November 1912 to the Cambridge Philosophical Society. Although simple, Bragg's law confirmed the existence of real particles at the atomic scale, as well as providing a powerful new tool for studying crystals in the form of X-ray and neutron diffraction. Lawrence Bragg and his father, William Henry Bragg, were awarded the Nobel Prize in physics in 1915 for their work in determining crystal structures beginning with NaCl, ZnS, and diamond. They are the only father-son team to jointly win. Lawrence Bragg was 25 years old, making him the youngest physics Nobel laureate.

Bragg condition

Bragg diffraction occurs when radiation, with a wavelength comparable to atomic spacings, is scattered in a specular fashion by the atoms of a crystalline system, and undergoes constructive interference. For a crystalline solid, the waves are scattered from lattice planes separated by the interplanar distance d. When the scattered waves interfere constructively, they remain in phase since the difference between the path lengths of the two waves is equal to an integer multiple of the wavelength. The path difference between two waves undergoing interference is given by 2dsinθ, where θ is the glancing angle. The effect of the constructive or destructive interference intensifies because of the cumulative effect of reflection in successive crystallographic planes of the crystalline lattice. This leads to Bragg's law, which describes the condition on θ for the constructive interference to be at its strongest:
where n is a positive integer and λ is the wavelength of the incident wave. Note that moving particles, including electrons, protons and neutrons, have an associated wavelength called de Broglie wavelength. A diffraction pattern is obtained by measuring the intensity of scattered waves as a function of scattering angle. Very strong intensities known as Bragg peaks are obtained in the diffraction pattern at the points where the scattering angles satisfy Bragg condition. As mentioned in the introduction, this condition is a special case of the more general Laue equations, and the Laue equations can be shown to reduce to the Bragg condition under additional assumptions.
The phenomena of Bragg diffraction by a crystal lattice shares similar characteristics with that of thin film interference, which has an identical condition in the limit where the refractive indices of the surrounding medium and the interfering medium are equal.

Heuristic derivation

Suppose that a single monochromatic wave is incident on aligned planes of lattice points, with separation, at angle. Points A and C are on one plane, and B is on the plane below. Points ABCC' form a quadrilateral.

There will be a path difference between the ray that gets reflected along AC' and the ray that gets transmitted along AB, then reflected along BC. This path difference is
The two separate waves will arrive at a point with the same phase, and hence undergo constructive interference, if and only if this path difference is equal to any integer value of the wavelength, i.e.
where the same definition of and apply as above.
Therefore,
from which it follows that
Putting everything together,
which simplifies to which is Bragg's law.
If only two planes of atoms were diffracting, as shown in the pictures, then the transition from constructive to destructive interference would be gradual as a function of angle, with gentle maxima at the Bragg angles. However, since many atomic planes are interfering in real materials, very sharp peaks surrounded by mostly destructive interference result.
A rigorous derivation from the more general Laue equations is available.

Bragg scattering of visible light by colloids

A colloidal crystal is a highly ordered array of particles that forms over a long range ; colloidal crystals have appearance and properties roughly analogous to their atomic or molecular counterparts. It has been known for many years that, due to repulsive Coulombic interactions, electrically charged macromolecules in an aqueous environment can exhibit long-range crystal-like correlations, with interparticle separation distances often being considerably greater than the individual particle diameter. Periodic arrays of spherical particles give rise to interstitial voids, which act as a natural diffraction grating for visible light waves, when the interstitial spacing is of the same order of magnitude as the incident lightwave. In these cases in nature, brilliant iridescence is attributed to the diffraction and constructive interference of visible lightwaves according to Bragg's law, in a matter analogous to the scattering of X-rays in crystalline solid. The effects occur at visible wavelengths because the separation parameter d is much larger than for true crystals.

Volume Bragg gratings

Volume Bragg gratings or volume holographic gratings consist of a volume where there is a periodic change in the refractive index. Depending on the orientation of the modulation of the refractive index, VBG can be used either to transmit or reflect a small bandwidth of wavelengths. Bragg's law dictates which wavelength will be diffracted:
where m is the Bragg order, λB the diffracted wavelength, Λ the fringe spacing of the grating, θ the angle between the incident beam and the normal of the entrance surface and φ the angle between the normal and the grating vector. Radiation that does not match Bragg's law will pass through the VBG undiffracted. The output wavelength can be tuned over a few hundred nanometers by changing the incident angle. VBG are being used to produce widely tunable laser source or perform global hyperspectral imagery.

Selection rules and practical crystallography

Bragg's law, as stated above, can be used to obtain the lattice spacing of a particular cubic system through the following relation:
where is the lattice spacing of the cubic crystal, and h, k, and ℓ are the Miller indices of the Bragg plane. Combining this relation with Bragg's law gives:
One can derive selection rules for the Miller indices for different cubic Bravais lattices; here, selection rules for several will be given as is.

Bravais latticesExample compoundsAllowed reflectionsForbidden reflections
Simple cubicPoAny h, k, ℓ-
Body-centered cubicFe, W, Ta, Crh + k + ℓ = evenh + k + ℓ = odd
Face-centered cubic Cu, Al, Ni, NaCl, LiH, PbSh, k, ℓ all odd or all evenh, k, ℓ mixed odd and even
Diamond FCCSi, GeAll odd, or all even with h + k + ℓ = 4nh, k, ℓ mixed odd and even, or all even with h + k + ℓ ≠ 4n
Triangular latticeTi, Zr, Cd, Beℓ even, h + 2k ≠ 3nh + 2k = 3n for odd ℓ

These selection rules can be used for any crystal with the given crystal structure. KCl has a face-centered cubic Bravais lattice. However, the K+ and the Cl− ion have the same number of electrons and are quite close in size, so that the diffraction pattern becomes essentially the same as for a simple cubic structure with half the lattice parameter. Selection rules for other structures can be referenced elsewhere, or derived. Lattice spacing for the other crystal systems can be found here.