Neutron Scattering

5
100s1 [OSl ] IOSSI [SSS
y i
4
0
r
1 .0 1.0 0
X W X E r
Reduced wave vector coordinate
(s)
0 .5
l
Figure 9 Phonon dispersion of NaCI (from reference [2]) .
This macroscopic polarization requires an additional energy, the longitudinal optic (LO)
mode is always higher in frequency than the trnaversal optic ('T'O) mode (note, that
there modes are only defined as the limit for q tending to zero .
The splitting between
LO and TO-mode frequencies is called Lydane-Sachs-'T'eller (LST) splitting and is related
by the LST-relation to the dielectrical constant .
Polar mode frequencies may be easily
determined by Infra-Red techniques ;
due to the form of the resolution ellipsoid neutron
scattering on this topic is frequently difficult . In metallic materials electric fields are
screened due to the free charge carriers at least for macroscopic distances .
As consequence
the LST-splitting disappears .
The phonon dispersion may thon give information on the
efflciency of the metallic screening.
Figure 9 shows the phonon dispersion in NaCI, which is isostructural to FeO . Compared
to the latter one recognizes the perturbation of the curves arising mainly from the lower
optical frequencies . Within the Brillouin-zone it is no longer possible to separate optical
and acoustic modes, the branches tend to cross each other but there is always a small gap
between branches of the came symmetry. 'This represents a general property : branches
of the same symmetry may not cross .
By mixing the polarization patterns it is always
possible to yield a gap reducing the total energy of the system . Whether this gap is small
or large depends on the similarity between the polarization patterns of the two branches .
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