International
Tables for Crystallography Volume H Powder diffraction Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk © International Union of Crystallography 2018 |
International Tables for Crystallography (2018). Vol. H, ch. 3.2, pp. 253-254
Section 3.2.2.2.1. X-rays^{a}Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794–3800, USA |
Consider a polycrystalline sample of randomly oriented grains of a single crystalline phase, containing a total number of atoms N, illuminated by an X-ray beam of wavelength λ. Each reciprocal-lattice vector with magnitude |G| < 2/λ will produce a cone of diffracted radiation with opening half-angle 2θ = 2 sin^{−1}(λ|G|/2). Regarding the diffraction lines as perfectly sharp delta-functions, the differential cross section for one powder Bragg peak is (Marshall & Lovesey, 1971)In this equation, r_{e} = 2.82 × 10^{−15} m is the classical electron radius and V is the unit-cell volume; the polarization factor P, multiplicity m_{hkl} and structure factor A_{hkl} are discussed below. Finite crystallite size, strain and other factors discussed below will broaden the peak, but the integrated intensity of each peak will be given by the factors before the delta-function.
Before continuing, note that this result is correct within the approximation that the entire sample is uniformly bathed in radiation (i.e., there is no absorption) and the kinematic approximation that the scattering power is very weak, so that further interaction of the scattered radiation with the sample may be neglected. The latter is equivalent to the Born approximation of elementary quantum mechanics, and its failure is referred to as extinction; it is discussed below in Section 3.2.3.6. It should also be noted here that equation (3.2.2) refers only to diffraction in Bragg peaks, and not to thermal diffuse scattering or scattered radiation due to static distortions from an ideal crystal lattice (Huang scattering; Krivoglaz, 1969).
Returning to the terms in equation (3.2.2), for X-rays the polarization factor P accounts for the polarization dependence of the Thomson scattering cross section from a free electron. For a completely polarized incident beam: P = 1 for S polarization (polarization perpendicular to the scattering plane); P = cos^{2} 2θ for P polarization (in the scattering plane). For unpolarized X-rays (no monochromator), P = (1 + cos^{2} 2θ)/2. For horizontally polarized X-rays from a synchrotron-radiation source, diffracted vertically, the polarization factor is very close to unity, and so P can generally be taken as 1. If there is a crystal monochromator (deflection angle 2θ_{m}) coplanar with the diffractometer and an unpolarized source, P = (1 + cos^{2} 2θ_{m} cos^{2} 2θ)/(1 + cos^{2} 2θ_{m}). If there is a polarization-sensitive analyser crystal following the sample, its relative transmission of X-rays polarized within or perpendicular to its plane of scattering must also be taken into account.
The multiplicity factor m_{hkl} accounts for the number of symmetry-equivalent reflections in the crystal structure, i.e., the number of distinct reflections that have the same structure factor. Note that the equivalence of Bragg reflections depends on the Laue group of the sample. For example, in the cubic space group , reflections (hkl) and (khl) are equivalent, whereas they are not in space group . If the atomic scattering factors are purely real or the structure is centrosymmetric, the intensities of Friedel pairs hkl and are equal. However, in an acentric structure with significant imaginary component of any of the atomic scattering factors (see below), Friedel pairs cannot be treated as symmetry equivalent.
The (dimensionless) X-ray structure factor is given bywhere the sum runs over all atoms in the unit cell. In the first form of this equation, r_{n} is the vector from the origin of the unit cell to the nth atom; in the second form, x_{n}, y_{n} and z_{n} are fractional coordinates of the nth atom in the unit cell such that r_{n} = x_{n}a + y_{n}b + z_{n}c.
The term exp(−W_{n}) is called the Debye–Waller factor, and it accounts for random dynamical motion of the nth atom away from its equilibrium position. This may be due to thermal fluctuations or (at low temperature) the quantum zero-point motion. If the atomic motion is isotropic, this term takes the formwhere is the mean-square fluctuation of the atom's distance from its equilibrium position (in three dimensions). Observed values of B are generally in the range of 0.5 to 1 Å^{2} for inorganic crystals, and as large as 5 Å^{2} for organic crystals.
A more general treatment of the Debye–Waller factor considers the probability density of the centre of the atom to be a three-dimensional ellipsoidal Gaussian function. Then for a given diffraction peak, it takes the formThe parameters U_{ij} that define the displacement ellipsoid are constrained to match the point symmetry of the site. For example, an atom that lies on a site of mmm symmetry will have the cross terms U_{12} = U_{23} = U_{13} = 0.
In equation (3.2.3), f_{n} is the atomic scattering factor, which arises because X-rays interact with the electrons in the sample, and the strength of that interaction depends both on the magnitude of the scattering vector and the X-ray frequency (wavelength). The electrons are distributed around the nucleus, and so their scattering power decreases with increasing magnitude of the scattering vector. There is also a dependence of the phase and amplitude of the scattering factor on the X-ray energy, which is especially significant near resonance with atomic transition energies. These factors are gathered into the atomic scattering factor f_{n}, which is commonly written asf ^{0} is the Fourier transform of the electron number density, so that f ^{0}(0) is equal to the number of electrons in the atom or ion. These factors are tabulated in International Tables for Crystallography, Volume C, Chapter 6.1 and https://physics.nist.gov/PhysRefData/FFast/html/form.html and http://henke.lbl.gov/optical_constants/asf.html . Values of atomic scattering factors are normally incorporated into data-analysis programs that require them, so the user does not often have to worry about them.
Equation (3.2.3) above is premised on the assumption that the distribution of electrons in the sample is confined to spherically symmetric atoms, i.e., ignoring charge density in bonds and the deformation of core electron density. For the purposes of phase identification and structure refinement, this is generally a good approximation. X-ray determination of non-spherical charge densities goes beyond the scope of this introduction; for further information, see e.g. Coppens (1997) and Bindzus et al. (2014).
In energy-dispersive X-ray measurements, the detector sits at a fixed 2θ and collects diffracted radiation from a continuum source. The independent variable of the measurement can be taken as the X-ray energy, E = hc/λ. With the appropriate change of variables, equation (3.2.2) can be written as
Recall that hc = 12.398 KeV Å in convenient units. Again, instrument resolution and sample effects will broaden the peak, but its integrated area is given by the terms preceding the delta-function. Of course, the diffracted intensity must be normalized to the incident spectrum as a function of energy; this may be a rather complicated undertaking as it involves energy-dependent corrections for such factors as detector sensitivity, absorption in the sample etc.
References
Bindzus, N., Straasø, T., Wahlberg, N., Becker, J., Bjerg, L., Lock, N., Dippel, A.-C. & Iversen, B. B. (2014). Experimental determination of core electron deformation in diamond. Acta Cryst. A70, 39–48.Google ScholarCoppens, P. (1997). X-ray charge densities and chemical bonding. Oxford: International Union of Crystallography/Oxford University Press.Google Scholar
Krivoglaz, M. A. (1969). Theory of X-ray and thermal-neutron scattering by real crystals. New York: Plenum Press. (Translated from the Russian by S. Moss.)Google Scholar
Marshall, W. & Lovesey, S. W. (1971). Theory of Thermal Neutron Scattering. Oxford: Clarendon.Google Scholar