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, CdCl_{2} |

Pearson symbol | hR9 |

Space group | 166 R-3m D_{3d}^{5} |

Chemical formula | AX2 |

Bonding | metallic, ionic |

Atomic positions | ```
A (3a) 0 0 0 (marked by C)
``` |

Coordination | AX – 6 (6o), X – 3A+3X+6X (12aco) |

Sublattices | A, X – fcc |

PDB files | C19 (as fcc with unoccupied voids) |

Parameters | 2*sqrt(6)<c/a<3*sqrt(6), 2/9<ζ<1/4, for metals the parameters are close to ideal packing |

Special values | fcc 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 |

Substances | metals and alloys: α-Sm (c/a=7.22, ζ≈2/9), low-temperature Li and Na ionic compounds: CdCl _{2} (c/a=4.53, ζ=.2435) |