Structure determination of structurally complex Ag₃₆Li₆₄ gamma-brass

The crystal structure of the Ag₃₆Li₆₄ gamma-brass was determined by analyzing the powder diffraction pattern taken using a synchrotron radiation beam with wavelength 0.50226 Å. It turned out that the compound contained 52 atoms in its unit cell with the space group and that the Li atom enters exclusively into inner tetrahedral (IT) and cubo-octahedral (CO) sites, whereas the Ag atom enters into those on outer tetrahedral (OT) and octahedral (OH) sites in the 26-atom cluster. Small amounts of Li also exist in OT and OH sites, resulting in chemical disorder. We discovered that the volumes of the IT and CO polyhedra shrink, while those of the OT and OH polyhedra expand relative to those of the corresponding polyhedra in the original b.c.c. (body-centered cubic) structure. This feature is universal and is found in other gamma-brasses such as Cu₅Zn₈ and Al₈V₅, for which the structure data are available. Among these gamma-brasses, we revealed the unique bond-length distribution for pairs connecting the atom on OH sites and that on CO sites, depending on the degree of d-p orbital hybridization between the transition metal elements such as Ag, Cu and V on OH sites, and the non-transition metal elements such as Li, Zn and Al on CO sites. It is suggested that this may hold a clue to resolving why some gamma-brasses such as the present Ag-Li and Cu-Zn possess a finite solid solution, but others such as Al₈V₅ and Mn₃In exist as line compounds.

Computing details

Cell refinement: RIETAN-2000 (Izumi and Ikeda, 2000); program(s) used to refine structure: RIETAN-2000 (Izumi and Ikeda, 2000); molecular graphics: ATOMS (Eric Dowty 1999).

silver-lithium gamma phase

Crystal data

Ag18.56Li33.44	F(000) = 972.64
Mr = 2234.14	Dx = 4.185 Mg m^−3
Cubic, I43m	Synchrotron radiation, λ = 0.50226 Å
a = 9.6066 (8) Å	µ = 3.85 mm^−1
V = 886.56 (13) Å^3	T = 300 K
Z = 1

Data collection

BL02B2 diffractometer	k = →
Si monochromator	l = →
h = →

Refinement

S = 1.67

52 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Biso*/Beq	Occ. (<1)
Ag1	0.82767 (8)	0.82767	0.82767		0.9311
Li1	0.82767	0.82767	0.82767		0.0689
Ag2	0.35410 (10)	0.0	0.0		0.9259 (9)
Li2	0.3541	0.0	0.0		0.0741
Li3	0.1168 (13)	0.1168	0.1168
Li4	0.3150 (14)	0.3150	0.0334 (17)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.178 (3)	0.178	0.178	0.033 (4)	0.033	0.033
Li1	0.178	0.178	0.178	0.033	0.033	0.033
Ag2	0.136 (6)	0.209 (4)	0.209	0.0	0.0	0.049 (8)
Li2	0.136	0.209	0.209	0.0	0.0	0.049
Li3	0.42 (9)	0.42	0.42	0.22 (8)	0.22	0.22
Li4	0.46 (7)	0.46	0.26 (14)	−0.07 (7)	−0.17 (6)	−0.17

Geometric parameters (Å, º)

Ag2—Li4^i	2.74 (2)	Ag2—Li4^v	3.070 (10)
Ag1—Li4^ii	2.77 (2)	Ag2—Ag2^vi	2.802 (2)
Ag2—Li3	2.7780 (10)	Ag1—Ag2^ii	2.9210 (10)
Ag1—Li4^iii	2.83 (2)	Li3—Li4	2.810 (2)
Ag1—Li3^ii	2.878 (7)	Li4—Li4^iv	2.843 (5)
Ag2—Li4^iv	3.05 (2)	Li3—Li3^vii	3.17 (3)
Ag2^vi—Ag2—Li4^i	66.7 (3)	Ag2^vi—Ag2—Li4	97.0 (3)
Ag2^vi—Ag2—Li4^iv	55.6 (4)

Symmetry codes: (i) −z+1/2, x−1/2, −y+1/2; (ii) −x+1, −y+1, z+1; (iii) x+1/2, y+1/2, z+1/2; (iv) z+1/2, −x+1/2, −y+1/2; (v) x, −y, −z; (vi) −x+1, −y, z; (vii) −x, −y, z.