# Laue equations

(Difference between revisions)
 Revision as of 12:10, 25 January 2006 (view source)← Older edit Revision as of 08:58, 27 February 2006 (view source)Newer edit → Line 10: Line 10: 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 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 waves scattered by ''O'' and ''A'', ''O'' and ''B'', ''O'' and ''C'', respectively, be in phase are that +
'''a''' . ('''sh''' - '''so''') = ''h'' λ '''a''' . ('''sh''' - '''so''') = ''h'' λ Line 16: Line 17: '''c''' . ('''sh''' - '''so''') = ''l'' 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. 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. Line 21: Line 23: 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 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*''', ('''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]]. where ''h'', ''k'', ''l'' are integers, namely if the diffraction vector '''OH''' =  '''sh,'''/λ - '''so'''/λ is a vector of the [[reciprocal lattice]]. Line 30: Line 33: - === See also === + == See also == [[reciprocal lattice]]
[[reciprocal lattice]]
Line 38: Line 41: ---- ---- - [[Category:Fundamental crystallography]]
+ [[Category:X-rays]]

# Laue equations

### Other languages

Equations de Laue (Fr). 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.

## History

The three Laue conditions for diffraction were first given in Laue, M. von (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 where he interpreted and indexed the first diffraction diagram (Friedrich, W., Knipping, P., and Laue, M. von (1912). Interferenz-Erscheinungen bei Röntgenstrahlen, Sitzungsberichte der Kgl. Bayer. Akad. der Wiss, 303--322, reprinted in Ann. Phys., (1913), 41, 971, taken with zinc-blende, ZnS. For details, see P. P. Ewald, 1962, IUCr, 50 Years of X-ray Diffraction, Section 4, page 52.