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Scandium magnesium gallide, Sc2MgGa2, and yttrium magnesium gallide, Y2MgGa2, were synthesized from the corresponding elements by heating under an argon atmosphere in an induction furnace. These inter­metallic compounds crystallize in the tetra­gonal Mo2FeB2-type structure. All three crystallographically unique atoms occupy special positions and the site symmetries of (Sc/Y, Ga) and Mg are m2m and 4/m, respectively. The coordinations around Sc/Y, Mg and Ga are penta­gonal (Sc/Y), tetra­gonal (Mg) and triangular (Ga) prisms, with four (Mg) or three (Ga) additional capping atoms leading to the coordination numbers [10], [8+4] and [6+3], respectively. The crystal structure of Sc2MgGa2 was determined from single-crystal diffraction intensities and the isostructural Y2MgGa2 was identified from powder diffraction data.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109000766/bd3001sup1.cif
Contains datablocks I, global

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270109000766/bd3001Isup2.hkl
Contains datablock I

Comment top

The potential use of magnesium alloys as storage materials for hydrogen has led to a large number of investigations on different magnesium alloys (Selvam et al., 1986; Sahlberg & Andersson, 2007; Sahlberg et al., 2007; Zlotea et al., 2008). Different compositions in the ternary Sc–Mg–Ga and Y–Mg–Ga systems have been synthesized and several intermediate phases have been found, in order to investigate the hydrogen absorption properties of these compounds. We have synthesized single crystals of Sc2MgGa2, and determined the crystal structure. There are numerous RE2T2X compounds (where RE is a rare earth metal, T is a transition metal and X is a p-block metal) which often crystallize in the W2CoB2- (Rieger et al., 1964) or Mo2FeB2-type structures (Rieger et al., 1964). These types of compounds were reviewed by Lukachuk & Pöttgen (2003). The Mo2FeB2-type structure is a ternary ordered version of the U3Si2-type structure (Zachariasen, 1949), where the two uranium sites are occupied by different atoms.

Prior to this investigation, there were no ternary gallide phases reported with the Mo2FeB2-type structure, but only the binary Ce3Ga2 and Gd3Ga2. It should also be noted that in the Sc–Mg–Ga ternary system this is, to our knowledge, the first reported ternary phase.

The Mo2FeB2-type structure, space group P4/mbm, can be described as two intergrown slabs of CsCl and AlB2 types, as shown in Fig. 1. The composition in the CsCl and AlB2 slabs is ScMg and ScGa2, respectively. The structure is layered in the z direction, and in Sc2MgGa2 the Mg and Ga atoms form a Ga2Mg layer at z = 0 and the Sc atoms form a layer at z = 0.5.

The coordination around Mg is approximately a tetragonal prism of eight Sc atoms with four additional Ga atoms capping the rectangular faces (Fig. 2a). Ga is coordinated by nine atoms in a capped triangular prismatic arangement, six Sc atoms in the prism plus two Mg and one Ga atom outside the rectangular faces (Fig. 2b). Sc is surrounded by two [Ga3Mg2] rings, forming a distorted pentagonal prism (Fig. 2c).

The interatomic distances are in agreement with the corresponding binary compounds. The shortest interatomic distance is the Ga—Ga distance, 2.535 (2) Å, which is slightly longer than the interatomic distance, 2.48 Å, in orthorhombic α-Ga. The bonding in the compound is believed to be metallic.

The unit cell for the isostructural Y2MgGa2 was determined to be a = 7.428 (2) Å, c = 4.2537 (2) Å.

Related literature top

For related literature, see: Rieger et al. (1964); Rietveld (1969); Rodriguez-Carvajal (1993, 2001); Sahlberg & Andersson (2007); Sahlberg, Gustafsson & Andersson (2007); Selvam (1986); Zachariasen (1949); Zlotea et al. (2008).

Experimental top

Appropriate amounts of the elements (Mg 99.99%, Sc 99.95%, Ga 99%) were melted inside a tantalum tube sealed in an argon atmosphere, using a high-frequency induction furnace. The tubes were heated to ~1373 K for 10 min, and then cooled to room temperature. Large single crystals of Sc2MgGa2 were obtained on the surface for [of?] magnesium-rich samples. The mm-sized single crystals were cut into smaller pieces. The bulk samples were characterized by X-ray powder diffraction.

The chemical composition of the single crystals was also analysed with SEM-EDS (scanning electron microscopy EDS energy-dispersive X-ray spectroscopy?). The image in Fig. 3 is taken from a sample with the nominal composition ScMgGa. The composition from EDS was found to be Sc0.37Mg0.20Ga0.43 after correction by the ZAF method, which corrects for atomic number (Z), absorption (A) and fluorescence (F).

Y2MgGa2 was synthesized by the same method as for Sc2MgGa2. The phase was found in a multiphase sample with the overall composition YMgGa. Attempts to synthesize large single crystals were not successful, probably due to the high stability of YMgGa and YGa2. The crystal structure was refined from powder X-ray diffraction data using the Rietveld method (Rietveld, 1969). The unit cell was determined using the CHECKCELL program (Laguier & Bochu, Year?).

Refinement top

Crystal structure refinements on Y2MgGa2 were performed using the Rietveld method (Rietveld, 1969) implemented in the program FULLPROF (Rodriguez-Carvajal, 1993, 2001). The peak shape was described by the pseudo-Voigt function and the background was modelled by interpolation between fixed points.

Computing details top

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2007); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2009).

Figures top
[Figure 1] Fig. 1. The crystal structure of Sc2MgGa2, viewed down the c axis, showing the Ga2Mg and Sc layers. The two slabs of `CsCl' and `AlB2' are emphasized. Mg - white, Ga - black and Sc - red.
[Figure 2] Fig. 2. Coordination around: (a) the Mg atom, (b) the Ga atom and (c) the Sc atom. Displacement ellipsoids are drawn at the 95% probability level.
[Figure 3] Fig. 3. SEM image of a sample with the overall composition ScMg4Ga. The Sc2MgGa2 crystals are shown in the matrix.
Scandium magnesium gallide top
Crystal data top
Sc2MgGa2Dx = 4.198 Mg m3
Mr = 253.67Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P4/mbmCell parameters from 503 reflections
a = 7.1577 (10) Åθ = 4.0–24.1°
c = 3.9166 (8) ŵ = 16.43 mm1
V = 200.66 (6) Å3T = 293 K
Z = 2Block, grey
F(000) = 2320.14 × 0.09 × 0.04 mm
Data collection top
Bruker APEXI
diffractometer
154 independent reflections
Radiation source: fine-focus sealed tube135 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.069
Detector resolution: 7.31 pixels mm-1θmax = 27.9°, θmin = 4.0°
profile data from ω scanh = 98
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)
k = 98
Tmin = 0.20, Tmax = 0.52l = 55
2538 measured reflections
Refinement top
Refinement on F20 restraints
Least-squares matrix: fullPrimary atom site location: structure-invariant direct methods
R[F2 > 2σ(F2)] = 0.028Secondary atom site location: difference Fourier map
wR(F2) = 0.069 w = 1/[σ2(Fo2) + (0.0361P)2 + 0.5388P]
where P = (Fo2 + 2Fc2)/3
S = 1.16(Δ/σ)max = 0.001
154 reflectionsΔρmax = 0.84 e Å3
11 parametersΔρmin = 0.73 e Å3
Crystal data top
Sc2MgGa2Z = 2
Mr = 253.67Mo Kα radiation
Tetragonal, P4/mbmµ = 16.43 mm1
a = 7.1577 (10) ÅT = 293 K
c = 3.9166 (8) Å0.14 × 0.09 × 0.04 mm
V = 200.66 (6) Å3
Data collection top
Bruker APEXI
diffractometer
154 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2004)
135 reflections with I > 2σ(I)
Tmin = 0.20, Tmax = 0.52Rint = 0.069
2538 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.02811 parameters
wR(F2) = 0.0690 restraints
S = 1.16Δρmax = 0.84 e Å3
154 reflectionsΔρmin = 0.73 e Å3
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
Sc10.17314 (18)0.67314 (18)0.50000.0120 (4)
Ga10.37478 (10)0.87478 (10)0.00000.0111 (4)
Mg10.00001.00000.00000.0078 (9)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Sc10.0135 (6)0.0135 (6)0.0089 (8)0.0024 (7)0.0000.000
Ga10.0117 (4)0.0117 (4)0.0099 (5)0.0009 (4)0.0000.000
Mg10.0044 (12)0.0044 (12)0.0146 (19)0.0000.0000.000
Geometric parameters (Å, º) top
Sc1—Ga1i2.8286 (15)Ga1—Sc1xi2.9177 (12)
Sc1—Ga12.8286 (15)Ga1—Sc1xii2.9177 (12)
Sc1—Ga1ii2.9177 (12)Ga1—Sc1xiii2.9177 (12)
Sc1—Ga1iii2.9177 (12)Ga1—Sc1xiv2.9177 (12)
Sc1—Ga1iv2.9177 (12)Mg1—Ga1ii2.8283 (6)
Sc1—Ga1v2.9177 (12)Mg1—Ga1xi2.8283 (6)
Sc1—Mg1i3.2931 (6)Mg1—Ga1xv2.8283 (6)
Sc1—Mg1vi3.2931 (6)Mg1—Sc1ii3.2931 (6)
Sc1—Mg13.2931 (6)Mg1—Sc1xi3.2931 (6)
Sc1—Mg1vii3.2931 (6)Mg1—Sc1x3.2931 (6)
Sc1—Sc1viii3.505 (4)Mg1—Sc1xvi3.2931 (6)
Ga1—Ga1ix2.535 (2)Mg1—Sc1iv3.2931 (6)
Ga1—Mg1vii2.8283 (6)Mg1—Sc1xiii3.2931 (6)
Ga1—Mg12.8283 (6)Mg1—Sc1xv3.2931 (6)
Ga1—Sc1x2.8286 (15)
Ga1i—Sc1—Ga187.63 (6)Sc1x—Ga1—Sc1xiii81.30 (2)
Ga1i—Sc1—Ga1ii153.91 (3)Sc1—Ga1—Sc1xiii141.09 (3)
Ga1—Sc1—Ga1ii88.211 (18)Sc1xi—Ga1—Sc1xiii84.32 (4)
Ga1i—Sc1—Ga1iii153.91 (3)Sc1xii—Ga1—Sc1xiii128.50 (4)
Ga1—Sc1—Ga1iii88.211 (18)Ga1ix—Ga1—Sc1xiv64.25 (2)
Ga1ii—Sc1—Ga1iii51.50 (4)Mg1vii—Ga1—Sc1xiv69.91 (3)
Ga1i—Sc1—Ga1iv88.211 (18)Mg1—Ga1—Sc1xiv137.01 (3)
Ga1—Sc1—Ga1iv153.91 (3)Sc1x—Ga1—Sc1xiv81.30 (2)
Ga1ii—Sc1—Ga1iv84.32 (4)Sc1—Ga1—Sc1xiv141.09 (3)
Ga1iii—Sc1—Ga1iv106.16 (6)Sc1xi—Ga1—Sc1xiv128.50 (4)
Ga1i—Sc1—Ga1v88.211 (18)Sc1xii—Ga1—Sc1xiv84.32 (4)
Ga1—Sc1—Ga1v153.91 (3)Sc1xiii—Ga1—Sc1xiv73.84 (6)
Ga1ii—Sc1—Ga1v106.16 (6)Ga1ii—Mg1—Ga1xi180.0
Ga1iii—Sc1—Ga1v84.32 (4)Ga1ii—Mg1—Ga190.0
Ga1iv—Sc1—Ga1v51.50 (4)Ga1xi—Mg1—Ga190.0
Ga1i—Sc1—Mg1i54.396 (17)Ga1ii—Mg1—Ga1xv90.0
Ga1—Sc1—Mg1i103.96 (4)Ga1xi—Mg1—Ga1xv90.0
Ga1ii—Sc1—Mg1i101.96 (2)Ga1—Mg1—Ga1xv180.0
Ga1iii—Sc1—Mg1i151.03 (5)Ga1ii—Mg1—Sc1ii54.40 (3)
Ga1iv—Sc1—Mg1i53.768 (14)Ga1xi—Mg1—Sc1ii125.60 (3)
Ga1v—Sc1—Mg1i94.39 (2)Ga1—Mg1—Sc1ii123.68 (3)
Ga1i—Sc1—Mg1vi54.396 (17)Ga1xv—Mg1—Sc1ii56.32 (3)
Ga1—Sc1—Mg1vi103.96 (4)Ga1ii—Mg1—Sc1xi125.60 (3)
Ga1ii—Sc1—Mg1vi151.03 (5)Ga1xi—Mg1—Sc1xi54.40 (3)
Ga1iii—Sc1—Mg1vi101.96 (2)Ga1—Mg1—Sc1xi56.32 (3)
Ga1iv—Sc1—Mg1vi94.39 (2)Ga1xv—Mg1—Sc1xi123.68 (3)
Ga1v—Sc1—Mg1vi53.768 (14)Sc1ii—Mg1—Sc1xi180.0
Mg1i—Sc1—Mg1vi100.43 (2)Ga1ii—Mg1—Sc1x56.32 (3)
Ga1i—Sc1—Mg1103.96 (4)Ga1xi—Mg1—Sc1x123.68 (3)
Ga1—Sc1—Mg154.396 (17)Ga1—Mg1—Sc1x54.40 (3)
Ga1ii—Sc1—Mg153.768 (14)Ga1xv—Mg1—Sc1x125.60 (3)
Ga1iii—Sc1—Mg194.39 (2)Sc1ii—Mg1—Sc1x69.290 (9)
Ga1iv—Sc1—Mg1101.96 (2)Sc1xi—Mg1—Sc1x110.710 (9)
Ga1v—Sc1—Mg1151.03 (5)Ga1ii—Mg1—Sc1xvi123.68 (3)
Mg1i—Sc1—Mg172.978 (18)Ga1xi—Mg1—Sc1xvi56.32 (3)
Mg1vi—Sc1—Mg1152.67 (6)Ga1—Mg1—Sc1xvi125.60 (3)
Ga1i—Sc1—Mg1vii103.96 (4)Ga1xv—Mg1—Sc1xvi54.40 (3)
Ga1—Sc1—Mg1vii54.396 (17)Sc1ii—Mg1—Sc1xvi110.710 (9)
Ga1ii—Sc1—Mg1vii94.39 (2)Sc1xi—Mg1—Sc1xvi69.290 (9)
Ga1iii—Sc1—Mg1vii53.768 (14)Sc1x—Mg1—Sc1xvi180.0
Ga1iv—Sc1—Mg1vii151.03 (5)Ga1ii—Mg1—Sc1iv54.40 (3)
Ga1v—Sc1—Mg1vii101.96 (2)Ga1xi—Mg1—Sc1iv125.60 (3)
Mg1i—Sc1—Mg1vii152.67 (6)Ga1—Mg1—Sc1iv123.68 (3)
Mg1vi—Sc1—Mg1vii72.978 (18)Ga1xv—Mg1—Sc1iv56.32 (3)
Mg1—Sc1—Mg1vii100.43 (2)Sc1ii—Mg1—Sc1iv72.978 (18)
Ga1i—Sc1—Sc1viii136.19 (3)Sc1xi—Mg1—Sc1iv107.022 (18)
Ga1—Sc1—Sc1viii136.19 (3)Sc1x—Mg1—Sc1iv110.710 (9)
Ga1ii—Sc1—Sc1viii53.08 (3)Sc1xvi—Mg1—Sc1iv69.290 (9)
Ga1iii—Sc1—Sc1viii53.08 (3)Ga1ii—Mg1—Sc1xiii125.60 (3)
Ga1iv—Sc1—Sc1viii53.08 (3)Ga1xi—Mg1—Sc1xiii54.40 (3)
Ga1v—Sc1—Sc1viii53.08 (3)Ga1—Mg1—Sc1xiii56.32 (3)
Mg1i—Sc1—Sc1viii103.67 (3)Ga1xv—Mg1—Sc1xiii123.68 (3)
Mg1vi—Sc1—Sc1viii103.67 (3)Sc1ii—Mg1—Sc1xiii107.022 (18)
Mg1—Sc1—Sc1viii103.67 (3)Sc1xi—Mg1—Sc1xiii72.978 (17)
Mg1vii—Sc1—Sc1viii103.67 (3)Sc1x—Mg1—Sc1xiii69.290 (9)
Ga1ix—Ga1—Mg1vii116.525 (18)Sc1xvi—Mg1—Sc1xiii110.710 (9)
Ga1ix—Ga1—Mg1116.525 (18)Sc1iv—Mg1—Sc1xiii180.00 (4)
Mg1vii—Ga1—Mg1126.95 (4)Ga1ii—Mg1—Sc156.32 (3)
Ga1ix—Ga1—Sc1x136.19 (3)Ga1xi—Mg1—Sc1123.68 (3)
Mg1vii—Ga1—Sc1x71.201 (18)Ga1—Mg1—Sc154.40 (3)
Mg1—Ga1—Sc1x71.201 (18)Ga1xv—Mg1—Sc1125.60 (3)
Ga1ix—Ga1—Sc1136.19 (3)Sc1ii—Mg1—Sc1110.710 (9)
Mg1vii—Ga1—Sc171.201 (18)Sc1xi—Mg1—Sc169.290 (9)
Mg1—Ga1—Sc171.201 (18)Sc1x—Mg1—Sc172.978 (18)
Sc1x—Ga1—Sc187.63 (6)Sc1xvi—Mg1—Sc1107.022 (18)
Ga1ix—Ga1—Sc1xi64.25 (2)Sc1iv—Mg1—Sc169.290 (9)
Mg1vii—Ga1—Sc1xi137.01 (3)Sc1xiii—Mg1—Sc1110.710 (9)
Mg1—Ga1—Sc1xi69.91 (3)Ga1ii—Mg1—Sc1xv123.68 (3)
Sc1x—Ga1—Sc1xi141.09 (3)Ga1xi—Mg1—Sc1xv56.32 (3)
Sc1—Ga1—Sc1xi81.30 (2)Ga1—Mg1—Sc1xv125.60 (3)
Ga1ix—Ga1—Sc1xii64.25 (2)Ga1xv—Mg1—Sc1xv54.40 (3)
Mg1vii—Ga1—Sc1xii69.91 (3)Sc1ii—Mg1—Sc1xv69.290 (9)
Mg1—Ga1—Sc1xii137.01 (3)Sc1xi—Mg1—Sc1xv110.710 (9)
Sc1x—Ga1—Sc1xii141.09 (3)Sc1x—Mg1—Sc1xv107.022 (18)
Sc1—Ga1—Sc1xii81.30 (2)Sc1xvi—Mg1—Sc1xv72.978 (18)
Sc1xi—Ga1—Sc1xii73.84 (6)Sc1iv—Mg1—Sc1xv110.710 (9)
Ga1ix—Ga1—Sc1xiii64.25 (2)Sc1xiii—Mg1—Sc1xv69.290 (9)
Mg1vii—Ga1—Sc1xiii137.01 (3)Sc1—Mg1—Sc1xv180.0
Mg1—Ga1—Sc1xiii69.91 (3)
Symmetry codes: (i) x, y, z+1; (ii) y1, x+1, z; (iii) y+1, x, z; (iv) y1, x+1, z+1; (v) y+1, x, z+1; (vi) x+1/2, y1/2, z+1; (vii) x+1/2, y1/2, z; (viii) x, y+1, z+1; (ix) x+1, y+2, z; (x) x, y, z1; (xi) y+1, x+1, z; (xii) y, x+1, z+1; (xiii) y+1, x+1, z1; (xiv) y, x+1, z; (xv) x, y+2, z; (xvi) x, y+2, z+1.

Experimental details

Crystal data
Chemical formulaSc2MgGa2
Mr253.67
Crystal system, space groupTetragonal, P4/mbm
Temperature (K)293
a, c (Å)7.1577 (10), 3.9166 (8)
V3)200.66 (6)
Z2
Radiation typeMo Kα
µ (mm1)16.43
Crystal size (mm)0.14 × 0.09 × 0.04
Data collection
DiffractometerBruker APEXI
diffractometer
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2004)
Tmin, Tmax0.20, 0.52
No. of measured, independent and
observed [I > 2σ(I)] reflections
2538, 154, 135
Rint0.069
(sin θ/λ)max1)0.659
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.028, 0.069, 1.16
No. of reflections154
No. of parameters11
Δρmax, Δρmin (e Å3)0.84, 0.73

Computer programs: SMART (Bruker, 2001), SAINT (Bruker, 2001), SHELXS97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2007), SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2009).

Selected bond lengths (Å) top
Sc1—Ga1i2.8286 (15)Ga1—Ga1iii2.535 (2)
Sc1—Mg1i3.2931 (6)Ga1—Mg1iv2.8283 (6)
Sc1—Sc1ii3.505 (4)
Symmetry codes: (i) x, y, z+1; (ii) x, y+1, z+1; (iii) x+1, y+2, z; (iv) x+1/2, y1/2, z.
 

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