Crystal structure of the new ternary aluminide ErAg0.77Al2.23

Abstract

Сrystal structure of the ternary erbium and silver aluminide ErAg 0 . 77 Al 2 . 23 has been studied using powder X-ray diffraction data: space group R-3m , Pearson symbol h R36 , lattice parameters a = 0.55049(1) nm, c = 2.61415(6) nm, final residual values are R I = 0,0450; R P = 0,0314; wR P = 0,0430. Samples were prepared by an arc melting under purified argon atmosphere with nonconsumable tungsten electrode and water-cooled copper hearth of the elemental components with certified purities of 99.999 wt.% Er, 99.95 wt.% Ag and 99.99 wt.% Al. All samples were re-melted twice to ensure homogeneity, and then heat treated in evacuated silica tubes at 600 o C during 700 hours. Annealed alloys were quenched in cold water without breaking the tubes. Phase analysis of the prepared samples and crystal structure determination of new compound were carried out using X-ray powder diffraction patterns recorded on a automatic powder diffractometer Huber Imaging Plate Guinier Camera G670 (Cu K α1 -radiation, l = 0.154056 nm; 3.503 ≤ 2 θ ≤ 99.99°; step 0,01° of 2 θ , scan time is 250 sec per step). Refinement of the atomic positional and displacement parameters in the crystal structure of the compounds was carried out by the full profile Rietveld method in the range 2 θ =3.50­–99.99 o (Cu K α1 -radiation) using the WinCSD program package. Existence of the new ternary aluminide ErAg 0 . 77 Al 2 . 23 with the rhombohedral structure of the PuNi 3 -type was found in the two three-component samples of the starting compositions Er 0 . 25 Ag 0 . 20 Al 0 . 55 and Er 0 . 25 Ag 0 . 17 Al 0 . 58 . In both samples new aluminide was in thermodynamic equilibrium with the small admixtures of the earlier known ternary phases ErAg 2 . 5 Al 2 . 5 (DyAg 2 . 4 Al 2 . 6 -type structure) and Er 8 Ag 17 Al 49 (Yb 8 Cu 17 Al 49 -type structure). Partially ordered distribution of the smaller atoms Ag and Al was observed in the crystal structure of new intermetallide, in particular Wyckoff site 6 с is fully occupied by Ag atoms, whereas Wyckoff sites 3 b and 18 h are occupied by the statistical mixtures of Ag and Al atoms with a predominant content of aluminum atoms leading to the next formulas ErAg 0 . 77(1) Al 2 . 23(1) or Er(Ag 0 . 26 Al 0 . 74 ) 3 . Interatomic distances in the ErAg 0 . 77 Al 2 . 23 crystal structure are in good correlation with the respective sums of the atomic radii of the components indicating predominance of the metallic bonding. Structure type PuNi 3 (or NbBe 3 ) can be considered as formed from the structural units of the CaCu 5 and MgZn 2 structure types. Keywords: crystal structure, X-ray powder diffraction, aluminide, erbium, silver.