Structures of the ζ and ζ′ phases in the Ag-Ga system

The crystal structures of the ζ-phase and ζ′-phase have been determined by X-ray powder diffraction and refined by Rietveld method. The space group of the ζ′-phase is P6̄2m with the lattice parameters a = 7.7710(1) and c = 2.8788(1) Å. The ideal formula is Ag₂Ga (real composition Ag_2+xGa_1-x, x = 0.16) and the chemical formula per unit cell is Z = 3. Six Ag atoms occupy the 3g positions with x = 0.63970(8) and the 3f positions with x = 0.30347(9), respectively. Three Ga atoms occupy the 2c and the 1b sites. The ζ-phase is obtained by quenching the sample containing 20 at.% Ga from 500 °C to room temperature. Its space group is close-packed hexagonal, P6₃/mmc, and the lattice parameters are a = 2.8818(1) and c = 4.6956(1) Å. There are 2 atoms (1.5Ag + 0.5Ga) per unit cell, which occupy the 2c site statistically. The relation between the structures of the ζ- and ζ′-phase is discussed.