Neutron Scattering

1s. -
1
tUMl
[cc01
~ 1 ~
10
517
if
0
0 0 .2 0 .4 0 .6 0 .8 1 .0 0 .8 0 .6 0 .4 0 .2 0 0 0 .2 0 .4
Reduced wave vector coordinate
(c)
Figure 8 Dispersion curves and crystal structure of F'eO (from reference [2]) .
Figure 7 further shows how to measure the A-branches .
One may observe the longitudinal
brauch in the (400)-zone passing to (4+x 0 0) (the (400)-zone has to be preferred to
(200) due to the langer value of Q2), whereas one may determine the transverse frequencies
in the (022)-zone at (x 2 2) .
- NaCl-structure - The NaCI-structure represents one of the mort simple possible
crystal structures with two atours per primitive tell .
It consists of two fcc-lattices shifted
against each other by (0 .5 0 .5 0 .5) . 'The entire crystal structure possesses fcc-symmetry
too . Figure 8 shows the dispersion curves of FeO .
T'he six branches expected for the two-atomic structure are observed only in [xx0]-
direction .
Like in case of the fcc Ne-lattice the transverse A- and A-branches are doubly
degenerate . Also in other aspects, there is some resemblance with the Ne-phonon dispersion
; in both cases LA-A and TA-S branches coincide at the zone boundarv .
A more detailed discussion is needed in order to understand the optical modes at F .
The polarization pattern of an optical mode corresponds to an anti-phase axial movement
of the ion pairs connected in [100], [010] or [001] direction . The three vibrations polarized
in the crystal directions should, however, be degenerate due to the cubic symmetry ;
one might expect only one optical frequency.
The optical vibration in the ionic lattice
possesses a polar character, i .e . there is a local polarization due to the opposite shifts
of cations and anions . For large wave-length, i .e . close to the zone-tenter, the local
polarization adds to a macroscopic polarization only in case of the longitudinal mode .
13-13