| Structural type | A3 |
| Common name | hexagonal close-packed (hcp) |
| Definition | AB close-packed structure |
| Prototype | Mg |
| Pearson symbol | hP2 |
| Space group | 194 P63/mmc D6h4 |
| Chemical formula | A |
| Bonding | metallic |
| Atomic positions |
A (2c) 1/3 2/3 1/4 |
| Coordination | 6+6 (12aco) |
| PDB files | A3 (in ideal packing), voids (6o at 2a labeled by O, 4t/5by at 4f/2d by H) |
| Parameters | c/a≈1.63 |
| Special cases | ideal packing for c/a=sqrt(8/3)≈1.63 |
| Substances | late rare-earth and other metals (see the table below) |
| A | a | c/a |
|---|---|---|
| Be | 2.2826 | 1.5700 |
| Co | 2.507 | 1.623 |
| Zn | 2.6649 | 1.8563 |
| Ru | 2.706 | 1.582 |
| Os | 2.7345 | 1.5789 |
| Tc | 2.740 | 1.605 |
| Re | 2.7615 | 1.6139 |
| Ti | 2.9512 | 1.5867 |
| Cd | 2.9791 | 1.8859 |
| Hf | 3.1880 | 1.5816 |
| Mg | 3.2088 | 1.6235 |
| Zr | 3.2317 | 1.5928 |
| Sc | 3.3088 | 1.592 |
| Lu | 3.5052 | 1.5832 |
| Tm | 3.5375 | 1.5700 |
| Er | 3.5592 | 1.5692 |
| Ho | 3.5778 | 1.5702 |
| Dy | 3.5915 | 1.5732 |
| Tb | 3.6055 | 1.5800 |
| Gd | 3.6336 | 1.5910 |
| Y | 3.6451 | 1.5721 |