First-principles structure optimization of scandium and lutetium trihydrides

Abstract A group–subgroup family tree for the structure of stoichiometric scandium and lutetium trihydrides with an essentially hexagonal close-packed (hcp) metallic matrix is processed. The underlying symmetry reduction procedure starts from the full hcp symmetry (space group type P 6 3 / mmc ) and leads to triclinic symmetry via point-symmetry reduction and unit-cell enlargement. It is shown that for both trihydrides the energetic and vibrational stability is reached for the space group P 6 3 . Further lowering of symmetry does not lead to further distortions.