text: The sigma (σ) phase is an intermetallic compound known as the one without definite stoichiometric composition and formed at the electron/atom ratio range of 6.2 to 7. It has a primitive tetragonal unit cell with 30 atoms. CrFe is a typical alloy crystallizing in the σ phase at the equiatomic composition. With physical properties adjustable based on its structural components, or its chemical composition provided a given structure.
  The μ phase has an ideal AB stoichiometry, with its prototype WFe, containing rhombohedral cell with 13 atoms. While many other Frank-Kasper alloy types have been identified, more continue to be found. The alloy NbNiAl is the prototype for the M phase. It has orthorhombic space group with 52 atoms per unit cell. The alloy CrMoNi is the prototype for the P-phase. It has a primitive orthorhombic cell with 56 atoms. The alloy CoCrMo is the prototype for the R-phase which belongs to the rhombohedral space group with 53 atoms per cell.