Static Structure and Thermodynamics

Abstract

The most basic characteristic of a liquid is that it possesses short-range order, as opposed to the long-range periodicity of a crystalline solid. Since the structure of a crystal is determined experimentally by observing the Bragg reflections of X-rays, it is natural to seek a quantitative description of the liquid structure via the intensity I of X-rays scattered through an angle 2ϑ say, from the liquid. If we introduce the usual variable k = 4π sin ϑ/λ, λ being the X-ray wavelength, then the liquid structure factor S(k) is defined by

$$$$

(1.1)

where N is the total number of atoms in the liquid, assumed monatomic. In eqn (1.1), f(k) is the atomic scattering factor, i.e. the Fourier transform of the electron density in the atom. f(k) falls from a value Z, the atomic number, at k = 0 to zero at large k, the asymptotic behaviour being proportional to k⁻⁴.