Neutron Scattering - JUWEL - Forschungszentrum Jülich

14 M. Meven
Fig. 8
(a) orthorhombic distortion with twinning correspondint to a (1-10) mirroring
(b) corresponding reciprocal lattice
(c) Overlay of (110)- and (1-10)-mirroring in reciprocal space
In the HTT phase only reflections with h, k, l of equal parity (g for even, u for uneven) are
allowed - (uuu) and (ggg). They are called in the following main structrure reflections.
In the LTO phase additional reflections occur, called super structure reflections: In the Abma-
Structure (ugg), l�0 and (guu), in the Bmab structure (gug), l�0 and(ugu).
Forbidden in both the HTT and the LTO phase are (uug), (ggu), (ug0) and (gu0).
These extinction rules will become important later.
In the real structure of the crystal there exist four domain types in total. They are separated
into two pairs with the couple Abma1/Bmab1 (I/II) with the (1-10) mirror plane as grain
boundary and the couple Abma2/Bmab2 (III/IV) with the (110) mirror plane as grain boundary
(fig. 8).
The following overlaps of reflections result from this twinning:
- No splitting of the (00l) reflections,
- triple splitting of the (hh0) reflections
- fourfold splitting of the (h00) reflections.
An equal distribution of the volumetric portion of each single domain yields a ratio of
intensities of 1:2:1 for the triple splitting. The distance �� between the centre and the side