Neutron Scattering

e performed in an efficient manner .
Some easy examples may illustrate the significance of the terras in (13 .12) . Let us
consider a longitudinal acoustic mode polarized in the [100]-direction ;
for this mode all
e d are parallel to [100] . 'T'he polarization term becomes maximum for Q=(h+~ 0 0) and
can be extracted from thé sum .
'The remaining interférence sum corresponds for small
~ to thé elastic structure-factor of (h00), 'T'he dynamic structure becomes hence strong
for strong Bragg-reflections .
thé strong Bragg-reflections .
In general acoustic phonons have to be measured close to
Since for optical modes thé atours may be displaced in
opposite directions, thé polarization term can change sign yielding a rather distinct sum
of thé interférence terra .
In a simple two-atomic structure thé optical mode has only a
weak dynamical structure factor close to thé strong Bragg-reflections (this argument is
no longer valid in case of négative scattering lengths) .
Why does inelastic neutron scattering play such a dominant role in thé study of lattice
dynamics? The central point is certainly due to similar masses of atours and thé neutron .
This yields thé possibility of elastic as well as that of inelastic scattering and renders
thé wave-vectors of thermal neutrons comparable to thé wave-vectors of thé phonons .
Inelastic neutron scattering allows to determine thé phonon dispersion over thé whole
Brillouin zone, whereas optical techniques (Raman and Infra-Red-scattering yields only
thé analvsis of modes at thé zone conter .
Recently there are serions efforts to perform lattice dynamical studies using synchrotron
radiation .
In case of thermal neutrons thé energies amount to 1-100 meV and
correspond to thé typical phonon energies .
For comparison, CuK,-radiation has an energy
of 8 - l0'meV . In order to determine phonon frequencies, one needs a relative energy
resolution of 10 -s - 10 - ', which may be achieved only by extreme expérimental effort .
The concomitant lors of intensity permits these measurements only at thé mort powerful
synchrotron sources, and even then thé measurements remain slow .
Me have seen that
thé one-phonon process ,yields an intensity proportional to Q2, this may be used in case of
neutron scattering due to thé nuclear interaction .
In case of thé x-rays thé large Q-range
is not at disposition since thé form-factor strongly reduces thé interaction .
In thé Gloser
future one may not expect, that inelastic x-ray scattering will supply results comparable
to neutron studies concerning thé eficiency .
The x-ray measurements, however, may become
valuable in cases where thé neutron scattering is hampered either by sample size or
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