The Heusler-type alloy MgZn₂Ce

Single crystals of the MgZn₂Ce alloy (magnesium dizinc cerium) were synthesized by a two-step method which couples the thermal treatment of pressed pellets of the pure elements with subsequent arc melting. The compound crystallizes in a Heusler-type (MnCu₂Al) structure in the space group Fmm, with one Zn site (site symmetry 3m), one Ce and one Mg site (both with mm site symmetry). All inter­atomic distances indicate metallic type bonding.

Key indicators

• Single-crystal X-ray study
• T = 295 K
• Mean [sigma](e-Zn) = 0.000 Å
• R factor = 0.016
• wR factor = 0.045
• Data-to-parameter ratio = 11.0

checkCIF/PLATON results

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Alert level C
PLAT041_ALERT_1_C Calc. and Rep. SumFormula Strings    Differ ....          ?
PLAT045_ALERT_1_C Calculated and Reported Z Differ by ............       0.50 Ratio
PLAT790_ALERT_4_C Centre of Gravity not Within Unit Cell: Resd.  #          1
              Ce2 Zn4

Alert level G
REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is
            correct. If it is not, please give the correct count in the
            _publ_section_exptl_refinement section of the submitted CIF.
           From the CIF: _diffrn_reflns_theta_max           32.96
           From the CIF: _reflns_number_total                 55
           Count of symmetry unique reflns            1
           Completeness (_total/calc)           5500.00%
           TEST3: Check Friedels for noncentro structure
           Estimate of Friedel pairs measured        54
           Fraction of Friedel pairs measured    54.000
           Are heavy atom types Z>Si present        yes

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   3 ALERT level C = Check and explain
   1 ALERT level G = General alerts; check

   2 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   0 ALERT type 2 Indicator that the structure model may be wrong or deficient
   0 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Comment

Rare-earth transition metal intermetallics are intensively studied due to their useful physical properties. Mg-containing alloys tend to be particularly effective because of their high hydrogen uptake capacity at low density and cost. Among RE–T–Mg alloys (RE = rare earth, T = 3d transition metal), the most studied are Ni-containing alloys. Ternary phases with a 1:9:2 stoichiometry were studied by Kadir et al. (1997) and Solokha et al. (2006). Hydrogen uptake capacities of RENi₄Mg (RE = Y, La, Nd) compounds with the MgCu₄Sn structure type were determined by Aono et al. (2000) and Guénée et al. (2003). The crystal structure, chemical bonding and physical properties of RE₂{Cu,Ni,Pd}₂Mg ternaries with the Mo₂FeB₂ structure type were investigated by Lukachuk & Pöttgen (2003). It was shown that the Heusler-type alloy LaNi₂Mg adsorbs H₂ under formation of the quaternary metal hydride LaMg₂NiH₇ (Renaudin et al., 2003). Detailed investigations of fundamental properties of La–Ni–Mg (De Negri et al., 2005) and La–Cu–Mg (De Negri et al., 2006) phases at 673 K have indicated a great number of novel ternary phases in these systems. Recently, Solokha et al. (2007) studied the crystal structure of the novel phase Tb₂Ni₂Mg₃ which does not form with the ligher rare earth metals.

The accurate determination of the crystal structure of new intermetallics is the basic requirement for a more detailed understanding of their properties. Here, we report the crystal structure of the ternary compound MgZn₂Ce (I) based on single-crystal X-ray diffraction data.

The title compound (I) belongs to the Heusler-type alloys (MnCu₂Al structure), with the Al site [Wyckoff position 4a] now occupied by Ce atoms, and the Mn [4b] and Cu [8c] atoms replaced by Mg and Zn atoms, respectively. The unit cell projection of (I) is shown in Fig. 1. The first coordination spheres of all atoms are the same as for the cubic close packed (c.c.p.) structures, viz. a rhombododecahedron with a coordination number of 14. The interatomic distances are in good correlation with the sums of the atomic radii (Emsley, 1991) and indicate a typical metallic type bonding, with the shortest Ce–Zn distance of 3.04659 (17) Å which is 96.2% of the sum of the atomic radii.

Related literature

Other ternary alloys in the system RE–T–Mg (RE = rare earth metal and T = transition metal) and their properties have been studied by: Kadir et al. (1997); Solokha et al. (2006, 2007); Aono et al. (2000); Guénée et al. (2003); Lukachuk & Pöttgen (2003); Renaudin et al. (2003); De Negri et al., (2005, 2007). Tests for centrosymmetric or non-centrosymmetric space groups were performed using the WinGX program by Farrugia (1999), following the advice of Marsh (1995). Atomic radii for the elements were taken from Emsley (1991) and the atomic coordinates were standardized using the STRUCTURE-TIDY program by Gelato & Parthé (1987).

S2. Experimental

With the purpose to avoid losses of magnesium and zinc due to their low vapour pressure at higher temperatures, the alloys were prepared in two steps. In a first step the powders of the pure elements (stoichiometry Mg:Zn:Ce = 1:2:1) were pressed into a pellet and enclosed in evacuated silica ampoules (pressure inside 10^-5 to 10 ^-6 Pa), which were then placed in a resistance furnace with a thermocouple controller. The heating process was carried out in consecutive order. Firstly, the ampoule was heated at 673 K for four d, then the temperature was increased to 873 K and held for two d. The ampoules were then heated up to 1073 K (held for four h and slowly cooled down to room temperature). In a second step, the pellets were remelted in an arc furnace. The samples were stable against air and moisture, irrespective whether they were in the form of compact pellets or as fine-grained powders. Wavelength dispersive spectrometry and electron probe microanalysis (CAMECA SX100 analyser) were used to control the number of phases and their content in the samples. An average result of the microprobe analysis for the title compound is 25.6%_at Ce, 25.8%_at Zn and 48.6%_at Mg which compares well with the composition obtained from the structure refinement. Irregularly shaped single crystals, exhibiting metallic luster, were isolated by mechanical fragmentation from the alloys.

S3. Refinement

Systematic absences indicated possible space groups F23 (#196), Fm3 (#202), F432 (#209), F43m (#216), and centrosymmetric Fm3m (#225). The statistical test of the distribution of the E values (Farrugia, 1999) suggested that the structure is centrosymmetric with a probability of 55.1%. However, following the advice of Marsh (1995) the structure solution and refinement were also performed in non-centrosymmetric space groups. The results clearly indicated that compound (I) crystallizes in the centrosymmetric space group Fm3m. The s.o.f. parameters were refined for this structure in order to check the deviations from the ideal stoichiometry, but no significant deviations were found. Therefore in the final refinement cycles the full occupancy parameters were retained and all atoms were refined with anisotropic displacement parameters. Finally, the atomic coordinates were standardized using the STRUCTURE-TIDY program (Gelato & Parthé, 1987). The final difference Fourier syntheses revealed no significant residual peaks - the highest maximum electron density is 0.95 Å from Ce and the deepest hole is 1.32 Å from Zn.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2004); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2005); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. Perspective view of the unit-cell projection of (I) on the (001) plane. Coordination polyhedra of Ce (a), Zn (b) and Mg (c) are shown.

Magnesium dizinc cerium

Crystal data

MgZn2Ce	Dx = 5.629 Mg m^−3
Mr = 295.17	Mo Kα radiation, λ = 0.71073 Å
Cubic, Fm3m	Cell parameters from 764 reflections
Hall symbol: -F 4 2 3	θ = 5.0–33.0°
a = 7.0358 (4) Å	µ = 26.40 mm^−1
V = 348.29 (3) Å^3	T = 295 K
Z = 4	Irregular, metallic dark-grey
F(000) = 520	0.10 × 0.08 × 0.06 mm

Data collection

Oxford Diffraction Xcalibur CCD diffractometer	55 independent reflections
Radiation source: fine-focus sealed tube	55 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.053
ω scans	θmax = 33.0°, θmin = 5.0°
Absorption correction: analytical (CrysAlis RED; Oxford Diffraction, 2005)	h = −10→7
Tmin = 0.11, Tmax = 0.198	k = −10→10
764 measured reflections	l = −9→10

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.016	w = 1/[σ^2(Fo^2) + (0.0117P)^2 + 19.6603P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.045	(Δ/σ)max < 0.001
S = 1.01	Δρmax = 0.95 e Å^−3
55 reflections	Δρmin = −1.32 e Å^−3
5 parameters	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0038 (5)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Ce	0.0000	0.0000	0.0000	0.0098 (4)
Zn	0.2500	0.2500	0.2500	0.0153 (4)
Mg	0.5000	0.5000	0.5000	0.0112 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ce	0.0098 (4)	0.0098 (4)	0.0098 (4)	0.000	0.000	0.000
Zn	0.0153 (4)	0.0153 (4)	0.0153 (4)	0.000	0.000	0.000
Mg	0.0112 (11)	0.0112 (11)	0.0112 (11)	0.000	0.000	0.000

Geometric parameters (Å, º)

Ce—Zn^i	3.0466 (2)	Zn—Ce^xi	3.0466 (2)
Ce—Zn	3.0466 (2)	Zn—Ce^xii	3.0466 (2)
Ce—Zn^ii	3.0466 (2)	Zn—Mg^v	3.0466 (2)
Ce—Zn^iii	3.0466 (2)	Zn—Mg^iii	3.0466 (2)
Ce—Zn^iv	3.0466 (2)	Mg—Zn^xiii	3.0466 (2)
Ce—Zn^v	3.0466 (2)	Mg—Zn^xiv	3.0466 (2)
Ce—Zn^vi	3.0466 (2)	Mg—Zn^xv	3.0466 (2)
Ce—Zn^vii	3.0466 (2)	Mg—Zn^xvi	3.0466 (2)
Ce—Mg^iv	3.5179 (2)	Mg—Zn^xii	3.0466 (2)
Ce—Mg^viii	3.5179 (2)	Mg—Zn^xi	3.0466 (2)
Ce—Mg^v	3.5179 (2)	Mg—Zn^x	3.0466 (2)
Ce—Mg^ix	3.5179 (2)	Mg—Ce^xvii	3.5179 (2)
Zn—Mg	3.0466 (2)	Mg—Ce^xviii	3.5179 (2)
Zn—Mg^iv	3.0466 (2)	Mg—Ce^xii	3.5179 (2)
Zn—Ce^x	3.0466 (2)	Mg—Ce^x	3.5179 (2)
Zn^i—Ce—Zn	180.0	Ce^xi—Zn—Ce^xii	109.5
Zn^i—Ce—Zn^ii	109.5	Mg—Zn—Mg^v	109.5
Zn—Ce—Zn^ii	70.5	Ce—Zn—Mg^v	70.5
Zn^i—Ce—Zn^iii	70.5	Mg^iv—Zn—Mg^v	109.5
Zn—Ce—Zn^iii	109.5	Ce^x—Zn—Mg^v	180.0
Zn^ii—Ce—Zn^iii	70.5	Ce^xi—Zn—Mg^v	70.5
Zn^i—Ce—Zn^iv	70.5	Ce^xii—Zn—Mg^v	70.5
Zn—Ce—Zn^iv	109.5	Mg—Zn—Mg^iii	109.5
Zn^ii—Ce—Zn^iv	180.0	Ce—Zn—Mg^iii	70.5
Zn^iii—Ce—Zn^iv	109.5	Mg^iv—Zn—Mg^iii	109.5
Zn^i—Ce—Zn^v	70.5	Ce^x—Zn—Mg^iii	70.5
Zn—Ce—Zn^v	109.5	Ce^xi—Zn—Mg^iii	180.0
Zn^ii—Ce—Zn^v	70.5	Ce^xii—Zn—Mg^iii	70.5
Zn^iii—Ce—Zn^v	109.5	Mg^v—Zn—Mg^iii	109.5
Zn^iv—Ce—Zn^v	109.5	Zn^xiii—Mg—Zn	180.0
Zn^i—Ce—Zn^vi	109.5	Zn^xiii—Mg—Zn^xiv	109.5
Zn—Ce—Zn^vi	70.5	Zn—Mg—Zn^xiv	70.5
Zn^ii—Ce—Zn^vi	109.5	Zn^xiii—Mg—Zn^xv	109.5
Zn^iii—Ce—Zn^vi	70.5	Zn—Mg—Zn^xv	70.5
Zn^iv—Ce—Zn^vi	70.5	Zn^xiv—Mg—Zn^xv	109.5
Zn^v—Ce—Zn^vi	180.0	Zn^xiii—Mg—Zn^xvi	109.5
Zn^i—Ce—Zn^vii	109.5	Zn—Mg—Zn^xvi	70.5
Zn—Ce—Zn^vii	70.5	Zn^xiv—Mg—Zn^xvi	109.5
Zn^ii—Ce—Zn^vii	109.5	Zn^xv—Mg—Zn^xvi	109.5
Zn^iii—Ce—Zn^vii	180.0	Zn^xiii—Mg—Zn^xii	70.5
Zn^iv—Ce—Zn^vii	70.5	Zn—Mg—Zn^xii	109.5
Zn^v—Ce—Zn^vii	70.5	Zn^xiv—Mg—Zn^xii	180.0
Zn^vi—Ce—Zn^vii	109.5	Zn^xv—Mg—Zn^xii	70.5
Zn^i—Ce—Mg^iv	125.3	Zn^xvi—Mg—Zn^xii	70.5
Zn—Ce—Mg^iv	54.7	Zn^xiii—Mg—Zn^xi	70.5
Zn^ii—Ce—Mg^iv	125.3	Zn—Mg—Zn^xi	109.5
Zn^iii—Ce—Mg^iv	125.3	Zn^xiv—Mg—Zn^xi	70.5
Zn^iv—Ce—Mg^iv	54.7	Zn^xv—Mg—Zn^xi	180.0
Zn^v—Ce—Mg^iv	125.3	Zn^xvi—Mg—Zn^xi	70.5
Zn^vi—Ce—Mg^iv	54.7	Zn^xii—Mg—Zn^xi	109.5
Zn^vii—Ce—Mg^iv	54.7	Zn^xiii—Mg—Zn^x	70.5
Zn^i—Ce—Mg^viii	54.7	Zn—Mg—Zn^x	109.5
Zn—Ce—Mg^viii	125.3	Zn^xiv—Mg—Zn^x	70.5
Zn^ii—Ce—Mg^viii	125.3	Zn^xv—Mg—Zn^x	70.5
Zn^iii—Ce—Mg^viii	54.7	Zn^xvi—Mg—Zn^x	180.0
Zn^iv—Ce—Mg^viii	54.7	Zn^xii—Mg—Zn^x	109.5
Zn^v—Ce—Mg^viii	125.3	Zn^xi—Mg—Zn^x	109.5
Zn^vi—Ce—Mg^viii	54.7	Zn^xiii—Mg—Ce^xvii	54.7
Zn^vii—Ce—Mg^viii	125.3	Zn—Mg—Ce^xvii	125.3
Mg^iv—Ce—Mg^viii	90.0	Zn^xiv—Mg—Ce^xvii	125.3
Zn^i—Ce—Mg^v	125.3	Zn^xv—Mg—Ce^xvii	125.3
Zn—Ce—Mg^v	54.7	Zn^xvi—Mg—Ce^xvii	54.7
Zn^ii—Ce—Mg^v	54.7	Zn^xii—Mg—Ce^xvii	54.7
Zn^iii—Ce—Mg^v	125.3	Zn^xi—Mg—Ce^xvii	54.7
Zn^iv—Ce—Mg^v	125.3	Zn^x—Mg—Ce^xvii	125.3
Zn^v—Ce—Mg^v	54.7	Zn^xiii—Mg—Ce^xviii	54.7
Zn^vi—Ce—Mg^v	125.3	Zn—Mg—Ce^xviii	125.3
Zn^vii—Ce—Mg^v	54.7	Zn^xiv—Mg—Ce^xviii	54.7
Mg^iv—Ce—Mg^v	90.0	Zn^xv—Mg—Ce^xviii	125.3
Mg^viii—Ce—Mg^v	180.0	Zn^xvi—Mg—Ce^xviii	125.3
Zn^i—Ce—Mg^ix	54.7	Zn^xii—Mg—Ce^xviii	125.3
Zn—Ce—Mg^ix	125.3	Zn^xi—Mg—Ce^xviii	54.7
Zn^ii—Ce—Mg^ix	125.3	Zn^x—Mg—Ce^xviii	54.7
Zn^iii—Ce—Mg^ix	125.3	Ce^xvii—Mg—Ce^xviii	90.0
Zn^iv—Ce—Mg^ix	54.7	Zn^xiii—Mg—Ce^xii	125.3
Zn^v—Ce—Mg^ix	54.7	Zn—Mg—Ce^xii	54.7
Zn^vi—Ce—Mg^ix	125.3	Zn^xiv—Mg—Ce^xii	125.3
Zn^vii—Ce—Mg^ix	54.7	Zn^xv—Mg—Ce^xii	54.7
Mg^iv—Ce—Mg^ix	90.0	Zn^xvi—Mg—Ce^xii	54.7
Mg^viii—Ce—Mg^ix	90.0	Zn^xii—Mg—Ce^xii	54.7
Mg^v—Ce—Mg^ix	90.0	Zn^xi—Mg—Ce^xii	125.3
Mg—Zn—Ce	180.0	Zn^x—Mg—Ce^xii	125.3
Mg—Zn—Mg^iv	109.5	Ce^xvii—Mg—Ce^xii	90.0
Ce—Zn—Mg^iv	70.5	Ce^xviii—Mg—Ce^xii	180.0
Mg—Zn—Ce^x	70.5	Zn^xiii—Mg—Ce^x	125.3
Ce—Zn—Ce^x	109.5	Zn—Mg—Ce^x	54.7
Mg^iv—Zn—Ce^x	70.5	Zn^xiv—Mg—Ce^x	54.7
Mg—Zn—Ce^xi	70.5	Zn^xv—Mg—Ce^x	54.7
Ce—Zn—Ce^xi	109.5	Zn^xvi—Mg—Ce^x	125.3
Mg^iv—Zn—Ce^xi	70.5	Zn^xii—Mg—Ce^x	125.3
Ce^x—Zn—Ce^xi	109.5	Zn^xi—Mg—Ce^x	125.3
Mg—Zn—Ce^xii	70.5	Zn^x—Mg—Ce^x	54.7
Ce—Zn—Ce^xii	109.5	Ce^xvii—Mg—Ce^x	180.0
Mg^iv—Zn—Ce^xii	180.0	Ce^xviii—Mg—Ce^x	90.0
Ce^x—Zn—Ce^xii	109.5	Ce^xii—Mg—Ce^x	90.0

Symmetry codes: (i) −x, −y, −z; (ii) −x+1/2, −y+1/2, −z; (iii) x−1/2, y, z−1/2; (iv) x−1/2, y−1/2, z; (v) x, y−1/2, z−1/2; (vi) −x, −y+1/2, −z+1/2; (vii) −x+1/2, −y, −z+1/2; (viii) x−1, y−1/2, z−1/2; (ix) x−1/2, y−1, z−1/2; (x) x, y+1/2, z+1/2; (xi) x+1/2, y, z+1/2; (xii) x+1/2, y+1/2, z; (xiii) −x+1, −y+1, −z+1; (xiv) −x+1/2, −y+1/2, −z+1; (xv) −x+1/2, −y+1, −z+1/2; (xvi) −x+1, −y+1/2, −z+1/2; (xvii) x+1, y+1/2, z+1/2; (xviii) x+1/2, y+1/2, z+1.