NON-FAMILY REFLECTIONS AND ORTHOGONAL PLANES

The number and features of the orthogonal planes (as defined above) depend both on the Class (lattice features) and on the subfamily (OD character). These are easily obtained by taking into account that polytypes in subfamily B and in subfamily A Series 0 never belong to Class a, whereas polytypes in subfamily A Series 0 always belong to 

Orthogonal polytypes. In case of subfamily A polytypes, only the three SX central planes are orthogonal, according to the above definition. For subfamily B and mixed-rotation polytypes, all the six central planes are orthogonal. 

Class b polytypes. None of the three SX central planes are orthogonal. In subfamily A polytypes Series 0) the SD central planes are non-orthogonal and thus none of the six densest central planes is orthogonal. In subfamily B, the three densest SD 

1) Subfamily A polytypes. As shown in the section dealing with the family structure, the family structure of subfamily A polytypes has symmetry //r(3)1/w, admitting a primitive rhombohedral cell. Within the Trigonal model the family reciprocal sublattice is rhombohedral both in its geometry and intensity distribution. In the real diffraction pattern the intensity distribution deviates from rhombohedral symmetry proportionally to the deviations of the layer from their archetypes described by the Trigonal model, but the geometry remains rhombohedral. 

2) Class b polytypes. Successive lattice planes parallel to (001) are shifted by 1/3 of the short Class a) or the long Class b) diagonal of the two-dimensional pseudo-hexagonal mesh built on A, A2) axes. For Class b polytypes a pseudo-rhombohedral primitive cell can be chosen, having (almost) the same volume of the reduced cell (Fig. 19). The primitive cell is closer to rhombohedral when the layers are closer to 

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