The Rhombohedral Lattice

In order for the (ABC) layered lattice to be closest packed, the interlayer spacing must be exactly equal to CPIS^.r with c:a = 1.5^.CPIS. Thus, the standard reduced cell is then the special rhombohedron found in the face centered cubic lattice.

If the c:a ratio differs from the closest packed value, then the standard reduced unit cell is still a rhombohedron (a = b = c and a = b = g), but the cell edges need not be of length 2r and/or the inter edge angles need not be 60^o. In these quasi-closest packed structures, either the hexagonal
(Z = 3) cell or the primitive (Z = 1) unit cell may be used to describe this lattice, which is called, by convention, rhombohedral.

(crossed stereo pair)

Note that quasi-closest packed (AB) lattices are also possible if c:a differs from CPIS.

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