First-principles structure optimization of scandium and lutetium trihydrides

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 P63/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 P63. Further lowering of symmetry does not lead to further distortions.