International
Tables for Crystallography Volume B Reciprocal space Edited by U. Shmueli © International Union of Crystallography 2010 
International Tables for Crystallography (2010). Vol. B, ch. 5.1, pp. 630633
Section 5.1.3. Solutions of planewave dynamical theory^{a}Institut de Minéralogie et de la Physique des Milieux Condensés, Bâtiment 7, 140 rue de Lourmel, 75015 Paris, France 
The wavefields excited in the crystal by the incident wave are determined by applying the boundary condition mentioned above for the continuity of the tangential component of the wavevectors (Section 5.1.2.3). Waves propagating in a vacuum have wavenumber . Depending on whether they propagate in the incident or in the reflected direction, the common extremity, M, of their wavevectorslies on spheres of radius k and centred at O and H, respectively. The intersections of these spheres with the plane of incidence are two circles which can be approximated by their tangents and at their intersection point, , or Laue point (Fig. 5.1.3.1).
Bragg's condition is exactly satisfied according to the geometrical theory of diffraction when M lies at . The departure Δθ from Bragg's incidence of an incident wave is defined as the angle between the corresponding wavevectors OM and . As Δθ is very small compared to the Bragg angle in the general case of Xrays or neutrons, one may write
The tangent is oriented in such a way that Δθ is negative when the angle of incidence is smaller than the Bragg angle.
The boundary condition for the continuity of the tangential component of the wavevectors is applied by drawing from M a line, Mz, parallel to the normal n to the crystal surface. The tie points of the wavefields excited in the crystal by the incident wave are at the intersections of this line with the dispersion surface. Two different situations may occur:
It will be apparent from the equations given later that the incident wavevector corresponding to the middle of the reflection domain is, in both cases, OI, where I is the intersection of the normal to the crystal surface drawn from the Lorentz point, , with (Figs. 5.1.3.4 and 5.1.3.5), while, according to Bragg's law, it should be . The angle Δθ between the incident wavevectors and OI, corresponding to the middle of the reflecting domain according to the geometrical and dynamical theories, respectively, is

Boundary conditions at the entrance surface for reflection geometry. (a) Reciprocal space; (b) direct space. 
In the Bragg case, the asymmetry ratio γ is negative and is never equal to zero. This difference in Bragg angle between the two theories is due to the refraction effect, which is neglected in geometrical theory. In the Laue case, is equal to zero for symmetric reflections .
The solutions of dynamical theory are best described by introducing a reduced parameter called the deviation parameter,wherewhose real part is equal to the half width of the rocking curve (Sections 5.1.6 and 5.1.7). The width 2δ of the rocking curve is sometimes called the Darwin width.
The definition (5.1.3.5) of the deviation parameter is independent of the geometrical situation (reflection or transmission case); this is not followed by some authors. The present convention has the advantage of being quite general.
In an absorbing crystal, and δ are complex, and it is the real part, , of which has the geometrical interpretation given in Section 5.1.3.3. One obtainswhere β is the phase angle of [or that of ].
This length plays a very important role in the dynamical theory of diffraction by both perfect and deformed crystals. For example, it is 15.3 µm for the 220 reflection of silicon, with Mo Kα radiation and a symmetric reflection.
In transmission geometry, it gives the period of the interference between the two excited wavefields which constitutes the Pendellösung effect first described by Ewald (1917) (see Section 5.1.6.3); in this case is called the Pendellösung distance, denoted hereafter. Its geometrical interpretation, in the zeroabsorption case, is the inverse of the diameter of the dispersion surface in a direction defined by the cosines and with respect to the reflected and incident directions, respectively (Fig. 5.1.3.4). It reduces to the inverse of (5.1.2.23) in the symmetric case.
In reflection geometry, it gives the absorption distance in the totalreflection domain and is called the extinction distance, denoted (see Section 5.1.7.1). Its geometrical interpretation in the zeroabsorption case is the inverse of the length , Fig. 5.1.3.5.
In a deformed crystal, if distortions are of the order of the width of the rocking curve over a distance , the crystal is considered to be slightly deformed, and ray theory (Penning & Polder, 1961; Kato, 1963, 1964a,b) can be used to describe the propagation of wavefields. If the distortions are larger, new wavefields may be generated by interbranch scattering (Authier & Balibar, 1970) and generalized dynamical diffraction theory such as that developed by Takagi (1962, 1969) should be used.
Using (5.1.3.8), expressions (5.1.3.5) and (5.1.3.6) can be rewritten in the very useful form:
The order of magnitude of the Darwin width 2δ ranges from a fraction of a second of an arc to ten or more seconds, and increases with increasing wavelength and increasing structure factor. For example, for the 220 reflection of silicon and Cu Kα radiation, it is 5.2 seconds.
The coordinates of the tie points excited by the incident wave are obtained by looking for the intersection of the dispersion surface, (5.1.2.22), with the normal Mz to the crystal surface (Figs. 5.1.3.4 and 5.1.3.5). The ratio ξ of the amplitudes of the waves of the corresponding wavefields is related to these coordinates by (5.1.2.24) and is found to bewhere the plus sign corresponds to a tie point on branch 1 (j = 1) and the minus sign to a tie point on branch 2 (j = 2), and is the sign of (+1 in transmission geometry, −1 in reflection geometry).
In this case (Fig. 5.1.3.4) is +1 and (5.1.3.10) may be written
Let and be the intersections of the normal to the crystal surface drawn from the Lorentz point with the two branches of the dispersion surface (Fig. 5.1.3.4). From Sections 5.1.3.3 and 5.1.3.4, they are the tie points excited for and correspond to the middle of the reflection domain. Let us further consider the tangents to the dispersion surface at and and let , and , be their intersections with and , respectively. It can be shown that and intersect the dispersion surface at the tie points excited for and , respectively, and that the Pendellösung distance , the width of the rocking curve and the deviation parameter , where and are the intersections of the normal to the crystal surface drawn from the extremity of any incident wavevector OM with and , respectively.
In this case (Fig. 5.1.3.5) is now −1 and (5.1.3.10) may be written
Now, let and be the points of the dispersion surface where the tangent is parallel to the normal to the crystal surface, and further let and be the intersections of these two tangents with and , respectively. For an incident wave of wavevector OM where M lies between I′ and I″, the normal to the crystal surface drawn from M has no real intersection with the dispersion surface and defines the totalreflection domain. The tie points and correspond to and , respectively, the extinction distance , the width of the totalreflection domain and the deviation parameter , where and are the intersections with and of the normal to the crystal surface drawn from the extremity of any incident wavevector OM.
References
Authier, A. & Balibar, F. (1970). Création de nouveaux champs d'onde généralisés dus à la présence d'un objet diffractant. II. Cas d'un défaut isolé. Acta Cryst. A26, 647–654.Ewald, P. P. (1917). Zur Begründung der Kristalloptik. III. Röntgenstrahlen. Ann. Phys. (Leipzig), 54, 519–597.
Kato, N. (1963). Pendellösung fringes in distorted crystals. I. Fermat's principle for Bloch waves. J. Phys. Soc. Jpn, 18, 1785–1791.
Kato, N. (1964a). Pendellösung fringes in distorted crystals. II. Application to twobeam cases. J. Phys. Soc. Jpn, 19, 67–77.
Kato, N. (1964b). Pendellösung fringes in distorted crystals. III. Application to homogeneously bent crystals. J. Phys. Soc. Jpn, 19, 971–985.
Penning, P. & Polder, D. (1961). Anomalous transmission of Xrays in elastically deformed crystals. Philips Res. Rep. 16, 419–440.
Takagi, S. (1962). Dynamical theory of diffraction applicable to crystals with any kind of small distortion. Acta Cryst. 15, 1311–1312.
Takagi, S. (1969). A dynamical theory of diffraction for a distorted crystal. J. Phys. Soc. Jpn, 26, 1239–1253.