C19

Structural type	C19
Definition	metals – 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
Prototype	α-Sm, CdCl₂
Pearson symbol	hR9
Space group	166 R-3m D_3d⁵
Chemical formula	AX2
Bonding	metallic, 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)`
Coordination	AX – 6 (6o), X – 3A+3X+6X (12aco)
Sublattices	A, X – fcc
PDB files	C19 (as fcc with unoccupied voids)
Parameters	2sqrt(6)<c/a<3sqrt(6), 2/9<ζ<1/4, for metals the parameters are close to ideal packing
Special values	fcc with voids for c/a=2sqrt(6)≈4.90 and ζ=1/4, ideal packing for c/a=3sqrt(6)≈7.35 and ζ=2/9≈.22
Substances	metals 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)