Mg_1 + xIr_1 - x (x = 0, 0.037 and 0.054), a binary intermetallic compound with a new orthorhombic structure type determined from powder and single-crystal X-ray diffraction

The new binary compound Mg_1 + xIr_1 - x (x = 0-0.054) was prepared by melting the elements in the Mg:Ir ratio 2:3 in a sealed tantalum tube under an argon atmosphere in an induction furnace (single crystals) or by annealing cold-pressed pellets of the starting composition Mg:Ir 1:1 in an autoclave under an argon atmosphere (powder sample). The structure was independently solved from high-resolution synchrotron powder and single-crystal X-ray data: Pearson symbol oC304, space group Cmca, lattice parameters from synchrotron powder data a = 18.46948 (6), b = 16.17450 (5), c = 16.82131 (5) Å. Mg_1 + xIr_1 - x is a topologically close-packed phase, containing 13 Ir and 12 Mg atoms in the asymmetric unit, and has a narrow homogeneity range. Nearly all the atoms have Frank-Kasper-related coordination polyhedra, with the exception of two Ir atoms, and this compound contains the shortest Ir-Ir distances ever observed. The solution of a rather complex crystal structure from powder diffraction, which was fully confirmed by the single-crystal method, shows the power of powder diffraction in combination with the high-resolution data and the global optimization method.

Computing details

Cell refinement: FullProf.2k (Rodríguez-Carvajal, 2002); data reduction: FullProf.2k; program(s) used to refine structure: FullProf.2k; software used to prepare material for publication: WinPLOTR, (Roisnel & Rodríguez-Carvajal, 1998), Atoms, (Dowty, 1993).

Figures

magnesium iridium

Crystal data

IrMg	Z = 152
Mr = 216.51	Dx = 10.95 Mg m^−3
Orthorhombic, CMCA	Synchrotron radiation, λ = 0.500120 Å
Hall symbol: -C 2bc 2	µ = 3.55 mm^−1
a = 18.46948 (6) Å	T = 293 K
b = 16.17450 (5) Å	Particle morphology: spherical
c = 16.82131 (5) Å	black
V = 5025.11 (3) Å^3	cylinder, 50 × 0.2 mm

Data collection

2-axis goniometer diffractometer	Data collection mode: transmission
Radiation source: synchrotron, Swiss-Norwegian Beam Line BM1B	Scan method: step
Channel-cut Si 111 monochromator	2θmin = 2.221°, 2θmax = 41.896°, 2θstep = 0.003°
Specimen mounting: glass capillary

Refinement

Refinement on Inet	Profile function: pseudo-Voigt
Least-squares matrix: full with fixed elements per cycle	70 parameters
Rp = 2.903	0 restraints
Rwp = 3.651	Weighting scheme based on measured s.u.'s
Rexp = 2.100
RBragg = 5.602	Background function: linear interpolation between 52 estimated points
χ^2 = NOT FOUND	Preferred orientation correction: No
15870 data points

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

	x	y	z	Uiso*/Ueq
Ir1	0.06679 (11)	0.11580 (12)	0.41605 (12)	0.01646 (7)*
Ir2	0.06885 (11)	0.39256 (13)	0.41144 (13)	0.01646 (7)*
Ir3	0.07161 (9)	0.19842 (10)	0.16277 (14)	0.01646 (7)*
Ir4	0.13722 (11)	0.15639 (13)	0.04111 (11)	0.01646 (7)*
Ir5	0.13842 (11)	0.15641 (13)	0.28575 (11)	0.01646 (7)*
Ir6	0.36484 (9)	0.25075 (17)	0.08809 (11)	0.01646 (7)*
Ir7	0.00000	0.33637 (16)	0.17102 (18)	0.01646 (7)*
Ir8	0.00000	0.43289 (17)	0.28126 (16)	0.01646 (7)*
Ir9	0.25000	0.0786 (2)	0.25000	0.01646 (7)*
Ir10	0.25000	0.42625 (18)	0.25000	0.01646 (7)*
Ir11	0.13616 (15)	0.00000	0.00000	0.01646 (7)*
Ir12	0.37442 (16)	0.00000	0.00000	0.01646 (7)*
Ir13	0.25000	0.25000	0.00000	0.01646 (7)*
Mg1	0.1228 (8)	0.3301 (9)	0.0621 (8)	0.0075 (9)*
Mg2	0.1375 (8)	0.0278 (7)	0.1609 (10)	0.0075 (9)*
Mg3	0.1352 (7)	0.3375 (9)	0.2721 (8)	0.0075 (9)*
Mg4	0.1235 (7)	0.4779 (8)	0.1736 (9)	0.0075 (9)*
Mg5	0.2230 (6)	0.2243 (7)	0.1678 (8)	0.0075 (9)*
Mg6	0.2750 (8)	0.0874 (10)	0.0738 (8)	0.0075 (9)*
Mg7	0.2658 (7)	0.3949 (9)	0.0980 (8)	0.0075 (9)*
Mg8	0.00000	0.0650 (13)	0.0704 (13)	0.0075 (9)*
Mg9	0.00000	0.0863 (13)	0.2650 (13)	0.0075 (9)*
Mg10	0.00000	0.2415 (13)	1.0037 (12)	0.0075 (9)*
Mg11	0.00000	0.2526 (14)	0.3277 (12)	0.0075 (9)*
Mg12	0.00000	0.4505 (13)	0.0612 (13)	0.0075 (9)*