Stacking Sequences in LDH Polytypes

Note that in the case of the 2Hi polytype, the cations all occupy b positions, i.e. are aligned on a normal to the layers (this is also the case for the IH polytype), whereas for the other two polytypes the cations alternate between b and c sites. Note also that the interlayers in the 2Hi poly type are all prismatic and those in the 2H2 polytype are all octahedral, whilst in the 2H3 polytype both types of interlayers are present. 

There are nine possible three-layer polytypes. Two of these have rhombo-hedral symmetry (3R)  

Note that the cations are homogenously distributed over a, b and c sites in the case of the 3Ri, 3R2 and 3H2 polytypes. For the 3Ri polytype, the interlayers are all prismatic and in the case of 3R2, 3Hi and 3H2 they are all octahedral other polytypes involve both types of interlayers. 

Bookin and Drits [98-100] have also described the large number of possible six-layer poly types, some of which have rhombohedral symmetry (6R) and the remainder hexagonal symmetry (6H). 

The identity of the polytype present in a given LDH sample may, in principle at least, be determined from the powder XRD pattern, although as we shall see for many LDHs this is not possible, as the amoimt of useful information therein is limited. By convention, the indexing of powder patterns for rhombohedral polytypes is based on a triple hexagonal unit cell (see Fig. 3). 

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