Structural typeC19
Definitionmetals – hexagonal close-packed lattice in ABABCBCAC=chh stacking,
ionic crystals – fcc lattice of large ions (X) with the smaller ions (A) occupying every second octahedral void along the main diagonal, the resulting lattice is compressed along this diagonal towards close packing
Prototypeα-Sm, CdCl2
Pearson symbolhR9
Space group166 R-3m D3d5
Chemical formulaAX2
Bondingmetallic, ionic
Atomic positions  A (3a)  0  0  0    (marked by C)
 X (6c)  0  0 zeta  (marked by S)
 V (3b)  0  0 1/2   (unoccupied void marked by N)
CoordinationAX – 6 (6o), X – 3A+3X+6X (12aco)
SublatticesA, X – fcc
PDB filesC19 (as fcc with unoccupied voids)
Parameters2*sqrt(6)<c/a<3*sqrt(6), 2/9<ζ<1/4, for metals the parameters are close to ideal packing
Special valuesfcc with voids for c/a=2*sqrt(6)≈4.90 and ζ=1/4,
ideal packing for c/a=3*sqrt(6)≈7.35 and ζ=2/9≈.22
Substancesmetals and alloys: α-Sm (c/a=7.22, ζ≈2/9), low-temperature Li and Na
ionic compounds: CdCl2 (c/a=4.53, ζ=.2435)