# Difference between revisions of "Laue equations"

### From Online Dictionary of Crystallography

Equations de Laue (Fr). Laue-Gleichungen (Ge). Equazioni di Laue (It). ラウエ方程式 (Ja). уравнения Лауэ (Ru). Ecuaciones de Laue (Sp).

## Definition

The three Laue equations give the conditions to be satisfied by an incident wave to be diffracted by a crystal. Consider the three basis vectors, OA = a, OB = b , OC = c of the crystal and let so and sh be unit vectors along the incident and reflected directions, respectively. The conditions that the waves scattered by O and A, O and B, O and C, respectively, be in phase are that

a . (sh - so) = h λ

b . (sh - so) = k λ

c . (sh - so) = l λ.

If these three conditions are simultaneously satisfied, the incoming wave is reflected on the set of lattice planes of Miller indices h/n, k/n, l/n. (h, k, l are the indices of the reflection.)

The three Laue equations can be generalized by saying that the diffraction condition is satisfied if the scalar product r . (sh/λ - so/λ) is an integer for any vector r = u a + v b + w c (u, v, w integers) of the direct lattice. This is the case if

(sh/λ - so/λ) = h a* + k b* + l c*,

where h, k, l are integers, namely if the diffraction vector OH = sh/λ - so/λ is a vector of the reciprocal lattice. This is the diffraction condition in reciprocal space.

## History

The three Laue conditions for diffraction were first given by Laue, M. [(1912). Eine quantitative Prüfung der Theorie für die Interferenz-Erscheinungen bei Röntgenstrahlen. Sitzungsberichte der Kgl. Bayer. Akad. der Wiss. 363-373, reprinted in Ann. Phys. (1913), 41, 989-1002], where he interpreted and indexed the first diffraction diagram [Friedrich, W., Knipping, P. and Laue, M. (1912). Interferenz-Erscheinungen bei Röntgenstrahlen, Sitzungsberichte der Kgl. Bayer. Akad. der Wiss., 303-322, reprinted in Ann. Phys., (1913), 41, 971-988], taken with zinc-blende, ZnS. For details, see P. P. Ewald (1962), IUCr, 50 Years of X-ray Diffraction, Utrecht: IUCr/Oosthoek, Section 4, p. 52.