31.3: The Spacing Between Lattice Planes Can Be Determined from X-Ray Diffraction Measurements

The structures of crystals and molecules are often being identified using x-ray diffraction studies, which are explained by Bragg’s Law. The law explains the relationship between an x-ray light shooting into and its reflection off from crystal surface.

Introduction

Bragg’s Law was introduced by Sir W.H. Bragg and his son Sir W.L. Bragg. The law states that when the x-ray is incident onto a crystal surface, its angle of incidence, $$\theta$$, will reflect back with a same angle of scattering, $$\theta$$. And, when the path difference, $$d$$ is equal to a whole number, $$n$$, of wavelength, a constructive interference will occur.

Consider a single crystal with aligned planes of lattice points separated by a distance d. Monochromatic X-rays A, B, and C are incident upon the crystal at an angle θ. They reflect off atoms X, Y, or Z.

The path difference between the ray reflected at atom X and the ray reflected at atom Y can be seen to be 2YX. From the Law of Sines we can express this distance YX in terms of the lattice distance and the X-ray incident angle:

If the path difference is equal to an integer multiple of the wavelength, then X-rays A and B (and by extension C) will arrive at atom X in the same phase. In other words, given the following conditions:

then the scattered radiation will undergo constructive interference and thus the crystal will appear to have reflected the X-radiation. If, however, this condition is not satisfied, then destructive interference will occur.

Bragg’s Law

$n\lambda = 2d\sin\theta$

where:

• $$\lambda$$ is the wavelength of the x-ray,
• $$d$$ is the spacing of the crystal layers (path difference),
• $$\theta$$ is the incident angle (the angle between incident ray and the scatter plane), and
• $$n$$ is an integer

The principle of Bragg’s law is applied in the construction of instruments such as Bragg spectrometer, which is often used to study the structure of crystals and molecules.

References

1. Bragg, W.L. The Crystalline State: Volume I. New York: The Macmillan Company, 1934.
2. McQuarrie, Donald A. Physical Chemistry: A molecular Approach. Sausalito: University Science Books, 1997.