
Figure 4
Tetragonal 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 5 twinrelated domain II (small crosses, lefthanded blue unit cell a_{2}, b_{2}, c_{2}) and the 5 coincidence lattice (large black points, righthanded red unit cell a_{T}, b_{T}, c_{T}). The four alternative twin reflection planes m'(120), m'(20), m'(310) and m'(30) 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 = 2 arctan (1/2) = 53.13° around the tetragonal c axis (cf. Appendix C). This cell is commonly used in structure determinations. 