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Single crystals of penta­samarium trigallium, Sm5Ga3, display tetra­gonal symmetry. The crystal structure was solved and refined in space group P4/ncc. The present work does not confirm the space group I4/mcm previously deduced from powder data measurements. One Sm atom is in a general position, and the other Sm atom and one of the Ga atoms are at sites with fourfold symmetry. The remaining Ga atom is at a site with twofold symmetry.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270105017324/iz1058sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270105017324/iz1058Isup2.hkl
Contains datablock I

Comment top

Light rare earth–gallium (R–Ga) systems have been investigated in the past and the structures of some binary SmxGay compounds have been established, generally from X-ray powder diffraction data. The title compound, Sm5Ga3, has previously been identified (Palenzona & Franceschi, 1968; Dzyana & Krypyakevich, 1969). In those studies, the unit-cell parameters and the space group (tetragonal, I4/mcm, a = 7.79 Å and c = 14.28 Å) were obtained from powder diffraction data and they were reported without any further structural details.

We have prepared the title compound in the exact 5:3 stoichiometry and obtained a well crystallized and homogeneous product. A small single-crystal was selected for collection of diffracted intensities on an Oxford Xcalibur CCD diffractometer. The unit-cell parameters, a = 7.7802 (5) and c = 14.1880 (9) Å, were determined with good accuracy. The statistical tests from SHELXS97 (Sheldrick, 1997) gave strong evidence for centrosymmetry. The structure was then solved and refined to R(F) = 2.60% in the tetragonal centrosymmetric space group P4/ncc. Careful examination of the reflection intensities did not reveal any systematic absences corresponding to Bravais centring. Nevertheless, refinement was tested in the I4/mcm space group, but about 25% of reflections with mean I/σ(I) of 5.8 were rejected (for absence violations) and structural refinement was unsatisfactory.

The structure of Sm5Ga3 is in agreement with those determined (from single-crystal data) for other related R5Ga3 compounds with R = La, Gd and Y. These structures are also described in the P4/ncc Ba5Si3 structural type (Zhao & Corbett, 1994). Symmetry lowering from I4/mcm (Cr5B3-type) towards the maximal IIa non-isomorphic subgroup P4/ncc (Ba5Si3-type) results from the displacement of atoms Sm1 and Ga1 from the I4/mcm 16(l) and 4(a) special positions.

The P4/ncc unit cell of Sm5Ga3 contains four isolated Ga atoms and four Ga2 dumbbells. The isolated atom Ga1 is in a regular square antiprismatic environment of Sm (eight Sm1 and two Sm2). The Ga2 dimers lie inside fused (rectangular face-sharing) distorted trigonal prisms of Sm1 atoms, capped on two rectangular faces by Sm2. Each Ga2 atom of the dimer is nearly located at the centre of one prism. The Ga—Ga distance of 2.653 (3) Å in the dimer compares well with those in the isostructural compounds La5Ga3 [2.665 (3) Å], Gd5Ga3 [2.637 (5) Å] and Y5Ga3 [2.663 (8) Å]. The 6.5% elongation from Pauling's Ga—Ga single bond (2.49 Å; Reference?) is due to π-antibonding repulsions. Formally, Sm5Ga3 can be formulated as (5Sm)13+Ga5−(Ga2)8−, with a non-integer charge per Sm atom slightly below its usual (+3) oxidation state.

Experimental top

Gallium metal (Rhone Poulenc, 99.9999% pure) and samarium (ingot, AlfaAesar 99.9% pure) were used to prepare the title compound. They were inserted inside a tantalum tube [Quantities?] which was then weld-sealed in an argon atmosphere. The tantalum tube was then enclosed in a sealed silica jacket filled with argon to protect it from oxidation at high temperatures. The mixture of elements was heated to the appropriate temperature (about 1323 K) and kept at this temperature for 10 h, during which it was shaken several times for good homogenization of the melt. It was then allowed to cool slowly for crystal growth, at a rate of 10 K h−1 down to 1103 K and then at 20 K h−1 down to room temperature. Although the product appeared relatively stable in air, crystals were selected under a microscope inside a glove box filled with purified argon. These crystals were protected from air in sealed thin-walled glass capillaries to be checked for singularity and diffracting quality. The best diffracting single-crystal was chosen for data collection at room temperature and mounted on the four-circle diffractometer.

Refinement top

The structure was refined in the P4/ncc space group using the final data set corrected from absorption effects (µ = 42.5 cm−1). Absorption corrections were performed using the Oxford Xcalibur CrysAlis RED absorption-correction module based on optimization of the crystal shape (Clark & Reid, 1995).

Computing details top

Data collection: CrysAlis CCD (Oxford Diffraction, 2001); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Please provide missing details; software used to prepare material for publication: Please provide missing details.

Figures top
[Figure 1] Fig. 1. Representations of the tetragonal Sm5Ga3 unit cell, emphasizing the Ga2 dumbbells and isolated Ga atoms in their respective environments, (a) trigonal prismatic and (b) square antiprismatic.
pentasamarium trigallium top
Crystal data top
Sm5Ga3Dx = 7.389 Mg m3
Mr = 960.91Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4/nccCell parameters from 10930 reflections
Hall symbol: -P 4a 2acθ = 3.7–26.1°
a = 7.8026 (5) ŵ = 42.51 mm1
c = 14.1880 (9) ÅT = 293 K
V = 863.77 (10) Å3Triangular shape, black
Z = 40.18 × 0.11 × 0.09 mm
F(000) = 1612
Data collection top
Oxford Xcalibur CCD area-detector
diffractometer
440 independent reflections
Radiation source: fine-focus sealed tube440 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.073
ω scansθmax = 26.1°, θmin = 3.7°
Absorption correction: numerical
[CrysAlis RED (Clark & Reid, 1995; Oxford Diffraction, 2001)]
h = 99
Tmin = 0.048, Tmax = 0.247k = 99
10930 measured reflectionsl = 1717
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.026 w = 1/[σ2(Fo2) + (0.0092P)2 + 14.6963P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.055(Δ/σ)max < 0.001
S = 1.52Δρmax = 2.51 e Å3
440 reflectionsΔρmin = 2.65 e Å3
22 parametersExtinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.00135 (9)
Crystal data top
Sm5Ga3Z = 4
Mr = 960.91Mo Kα radiation
Tetragonal, P4/nccµ = 42.51 mm1
a = 7.8026 (5) ÅT = 293 K
c = 14.1880 (9) Å0.18 × 0.11 × 0.09 mm
V = 863.77 (10) Å3
Data collection top
Oxford Xcalibur CCD area-detector
diffractometer
440 independent reflections
Absorption correction: numerical
[CrysAlis RED (Clark & Reid, 1995; Oxford Diffraction, 2001)]
440 reflections with I > 2σ(I)
Tmin = 0.048, Tmax = 0.247Rint = 0.073
10930 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0260 restraints
wR(F2) = 0.055 w = 1/[σ2(Fo2) + (0.0092P)2 + 14.6963P]
where P = (Fo2 + 2Fc2)/3
S = 1.52Δρmax = 2.51 e Å3
440 reflectionsΔρmin = 2.65 e Å3
22 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Sm10.08982 (6)0.92781 (6)0.39884 (3)0.0119 (2)
Sm20.25000.25000.23095 (8)0.0193 (3)
Ga10.25000.75000.49634 (13)0.0112 (4)
Ga20.12980 (14)0.62980 (14)0.25000.0101 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Sm10.0123 (3)0.0122 (3)0.0111 (3)0.00040 (17)0.00101 (17)0.00088 (17)
Sm20.0125 (3)0.0125 (3)0.0328 (6)0.0000.0000.000
Ga10.0100 (6)0.0100 (6)0.0138 (9)0.0000.0000.000
Ga20.0094 (4)0.0094 (4)0.0115 (6)0.0005 (6)0.0000 (3)0.0000 (3)
Geometric parameters (Å, º) top
Sm1—Ga2i3.0737 (8)Sm1—Sm2v3.6818 (9)
Sm1—Ga2ii3.1432 (12)Sm1—Sm1i3.7346 (10)
Sm1—Ga23.1565 (8)Sm1—Sm1vi3.9005 (9)
Sm1—Ga1iii3.1770 (10)Sm1—Sm1vii3.9703 (7)
Sm1—Ga13.2968 (9)Sm2—Ga23.1200 (7)
Sm1—Sm1iii3.3873 (10)Sm2—Ga1viii3.225 (2)
Sm1—Sm2iv3.5137 (8)Ga2—Ga2i2.653 (3)
Ga2viii—Sm2—Ga289.570 (4)Ga2—Sm2—Ga1viii94.97 (2)
Ga2ix—Sm2—Ga2170.06 (4)
Symmetry codes: (i) x+1/2, y+3/2, z; (ii) x, y+1/2, z+1/2; (iii) x, y+2, z+1; (iv) x1/2, y+1, z+1/2; (v) x, y+1, z; (vi) y1/2, x+1, z+1; (vii) y+3/2, x+1, z; (viii) x+1/2, y+1, z+1/2; (ix) x+1/2, y+1/2, z.

Experimental details

Crystal data
Chemical formulaSm5Ga3
Mr960.91
Crystal system, space groupTetragonal, P4/ncc
Temperature (K)293
a, c (Å)7.8026 (5), 14.1880 (9)
V3)863.77 (10)
Z4
Radiation typeMo Kα
µ (mm1)42.51
Crystal size (mm)0.18 × 0.11 × 0.09
Data collection
DiffractometerOxford Xcalibur CCD area-detector
diffractometer
Absorption correctionNumerical
[CrysAlis RED (Clark & Reid, 1995; Oxford Diffraction, 2001)]
Tmin, Tmax0.048, 0.247
No. of measured, independent and
observed [I > 2σ(I)] reflections
10930, 440, 440
Rint0.073
(sin θ/λ)max1)0.620
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.026, 0.055, 1.52
No. of reflections440
No. of parameters22
w = 1/[σ2(Fo2) + (0.0092P)2 + 14.6963P]
where P = (Fo2 + 2Fc2)/3
Δρmax, Δρmin (e Å3)2.51, 2.65

Computer programs: CrysAlis CCD (Oxford Diffraction, 2001), CrysAlis CCD, CrysAlis RED (Oxford Diffraction, 2001), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), Please provide missing details.

Selected bond lengths (Å) top
Sm1—Ga2i3.0737 (8)Sm1—Ga13.2968 (9)
Sm1—Ga2ii3.1432 (12)Sm2—Ga23.1200 (7)
Sm1—Ga23.1565 (8)Sm2—Ga1iv3.225 (2)
Sm1—Ga1iii3.1770 (10)Ga2—Ga2i2.653 (3)
Symmetry codes: (i) x+1/2, y+3/2, z; (ii) x, y+1/2, z+1/2; (iii) x, y+2, z+1; (iv) x+1/2, y+1, z+1/2.
 

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