
Figure 9
Hexagonal lattices (ab planes, common c axis pointing upwards) of twin domain I (start domain, lattice points small circles, righthanded green unit cell a_{1}, b_{1}, c_{1}), of the 7 twinrelated domain II (small crosses, lefthanded blue unit cell a_{2}, b_{2}, c_{2}) and of the 7 coincidence lattice (large black points, righthanded red unit cell a_{T}, b_{T}, c_{T}). The six alternative twin reflection planes m'(120), m'(120), m'(210), m'(410), m'(140) and m'(410) are indicated by dashed lines. The coordinate axes a_{2}, b_{2}, c_{2} of domain II (blue) are defined by the reflection plane m'(120). The righthanded yellow unit cell a_{3}, b_{3}, c_{3} of domain II is obtained from a_{1}, b_{1}, c_{1} by a clockwise rotation of = 120° + 2 arcsin [(1/2)(3/7)^{1/2}] = 120° + 38.2° = 158.2° around the hexagonal axis (cf. Appendix C). This cell is commonly used in structure determinations. 