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Single crystals of erbium dicobalt disilicide were synthesized from the corresponding elements by arc melting. The ternary intermetallic compound crystallizes in the body-centred tetragonal space group I4/mmm and adopts the CeGa2Al2 structure type, with all atoms in special positions.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S1600536805030722/wm6091sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S1600536805030722/wm6091Isup2.hkl
Contains datablock I

Key indicators

  • Single-crystal X-ray study
  • T = 295 K
  • Mean [sigma](i-Si)= 0.004 Å
  • R factor = 0.022
  • wR factor = 0.050
  • Data-to-parameter ratio = 12.1

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Comment top

Ternary intermetallics of rare earth metals with the general formula RET2X2 (where RE is a rare earth metal, T is a transition metal or, in a few cases, a p-block element, and X is a p-block element) crystallize mostly in ten structure types: tetragonal CeGa2Al2 (also called ThCr2Si2), CaBe2Ge2 and YB2C2, trigonal La2O2S, orthorhombic CaRh2B2, LaRe2Si2, HfFe2Si2 and ScB2C2, and monoclinic LaPt2Ge2 and HoNi2B2. In the majority of cases, RET2X2 phases belong to the CeGa2Al2 structure type (body-centred tetragonal lattice) or to the very similar CaBe2Ge2 structure type (primitive tetragonal lattice), which are both ordered derivatives of the BaAl4 structure (Parthé et al., 1983).

The RET2X2 phases have received special attention due to their interesting physical properties. The compound CeCu2Si2 (CeGa2Al2 type) was the first representative of a heavy-fermion system. The accurate determination of the crystal structure for phases of this composition is necessary for a better understanding of their physical properties. The existence of the ErCo2Si2 phase was first reported by Rossi et al. (1978), and the crystal structure was determined by means of X-ray powder diffraction measurements. Subsequently, neutron powder diffraction measurements and structure refinements have been performed by Yakinthos et al. (1983) and Leciejewicz et al. (1983). In view of the close relation of the structure types CeGa2Al2 (space group I4/mmm) and CaBe2Ge2 (space group P4/nmm), it was necessary to determine precisely the structure type for ErCo2Si2 on the basis of single-crystal diffraction data, and we present these results here.

ErCo2Si2 adopts the CeGa2Al2 structure type. A clinographic projection of the unit cell is shown in Fig. 1. The coordination sphere around Er (site symmetry 4/mmm) consists of 22 atoms, resulting in a polyhedron with 22 vertices [ErSi8Co8Si2Er4] (Fig. 2a), and 12 quadrangular and 24 triangular faces. The coordination polyhedron of the Co atom (site symmetry 4m2) is a distorted cuboctahedron [CoSi4Co4Er4] (Fig. 2 b). The coordination polyhedron for Si (site symmetry 4 mm) is a monocapped tetragonal antiprism [SiCo4Si1Er4] with one additional Si as the capping atom (Fig. 2c). The interatomic distances are in good agreement with the sums of the atomic radii (Pauling, 1967). The shortest distance (Table 1) is observed between Co and Si atoms (94% of the sum of the atomic radii of the corresponding atoms).

Experimental top

The single-crystal used in this work was extracted from an alloy with nominal composition Er20Co40Si40, which was prepared by arc melting of the initial components (purity better than 99.9%) in an electric arc furnace with a water-cooled copper bottom (Ti-getter) under an argon atmosphere and annealed at 870 K. A preliminary crystal investigation was performed using Laue and rotation methods (RKV-86 and RGNS-2 chambers, Mo Kα radiation).

Refinement top

The structure refinement of ErCo2Si2 clearly indicated that this phase crystallizes in the tetragonal crystal system in space group I4/mmm, adopting the CeGa2Al2 structure type. Refinements in space group P4/nmm (CaBe2Ge2 structure type) were less satisfactory and resulted in higher values of the R factors and atomic displacement factors. The highest maximum residual electron density is located at a distance of 0.73 Å from the Er atom, and the deepest hole is 1.80 Å from the same atom.

Computing details top

Data collection: CrysAlis CCD (Oxford Diffraction, 2003); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2004); 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 top
[Figure 1] Fig. 1. A clinographic projection of the ErCo2Si2 unit cell, with displacement ellipsoids drawn at the 95% probability level.
[Figure 2] Fig. 2. Coordination polyhedra around (a) the Er atom, (b) the Co atom and (c) the Si atom. Er atoms are blue, Co atoms are red and Si atoms are green.
Dicobalt erbium disilicon top
Crystal data top
Co2ErSi2Dx = 7.783 Mg m3
Mr = 341.30Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/mmmCell parameters from 561 reflections
Hall symbol: -I 4 2θ = 2.4–29.3°
a = 3.874 (2) ŵ = 40.29 mm1
c = 9.704 (4) ÅT = 295 K
V = 145.64 (12) Å3Plate, metallic light grey
Z = 20.14 × 0.13 × 0.04 mm
F(000) = 300
Data collection top
Oxford Xcalibur3 CCD area–detector
diffractometer
109 independent reflections
Radiation source: fine-focus sealed tube106 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.040
ω scansθmax = 33.8°, θmin = 4.2°
Absorption correction: analytical
(CrysAlis RED; Oxford Diffraction, 2004)
h = 46
Tmin = 0.008, Tmax = 0.261k = 56
694 measured reflectionsl = 1414
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)] = 0.022 w = 1/[σ2(Fo2) + (0.0245P)2 + 2.3748P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.050(Δ/σ)max < 0.001
S = 1.17Δρmax = 2.09 e Å3
109 reflectionsΔρmin = 2.41 e Å3
9 parametersExtinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.018 (3)
Crystal data top
Co2ErSi2Z = 2
Mr = 341.30Mo Kα radiation
Tetragonal, I4/mmmµ = 40.29 mm1
a = 3.874 (2) ÅT = 295 K
c = 9.704 (4) Å0.14 × 0.13 × 0.04 mm
V = 145.64 (12) Å3
Data collection top
Oxford Xcalibur3 CCD area–detector
diffractometer
109 independent reflections
Absorption correction: analytical
(CrysAlis RED; Oxford Diffraction, 2004)
106 reflections with I > 2σ(I)
Tmin = 0.008, Tmax = 0.261Rint = 0.040
694 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0229 parameters
wR(F2) = 0.0500 restraints
S = 1.17Δρmax = 2.09 e Å3
109 reflectionsΔρmin = 2.41 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
Er0.00000.00000.00000.0074 (3)
Co0.00000.50000.25000.0065 (3)
Si0.00000.00000.3745 (3)0.0071 (6)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Er0.0070 (3)0.0070 (3)0.0082 (4)0.0000.0000.000
Co0.0061 (4)0.0061 (4)0.0072 (6)0.0000.0000.000
Si0.0068 (8)0.0068 (8)0.0079 (12)0.0000.0000.000
Geometric parameters (Å, º) top
Er—Sii2.9977 (17)Co—Coxi2.7393 (14)
Er—Siii2.9977 (17)Co—Covii2.7393 (14)
Er—Siiii2.9977 (17)Co—Coxii2.7393 (14)
Er—Siiv2.9977 (17)Co—Coii2.7393 (14)
Er—Siv2.9977 (17)Co—Erxiii3.1044 (10)
Er—Sivi2.9977 (17)Co—Erx3.1044 (10)
Er—Sivii2.9977 (17)Co—Erxiv3.1044 (10)
Er—Siviii2.9977 (17)Si—Covii2.2831 (17)
Er—Coi3.1044 (10)Si—Coxv2.2831 (17)
Er—Coix3.1044 (10)Si—Coii2.2831 (17)
Er—Co3.1044 (10)Si—Sixvi2.435 (6)
Er—Coii3.1044 (10)Si—Erxiii2.9977 (17)
Co—Sivii2.2831 (17)Si—Erxvii2.9977 (17)
Co—Six2.2831 (17)Si—Erxiv2.9977 (17)
Co—Siii2.2831 (17)Si—Erxviii2.9977 (17)
Co—Si2.2831 (17)
Sii—Er—Siii180.0Si—Co—Coxii126.86 (3)
Sii—Er—Siiii132.07 (10)Coxi—Co—Coxii90.0
Siii—Er—Siiii47.93 (10)Covii—Co—Coxii90.0
Sii—Er—Siiv47.93 (10)Sivii—Co—Coii126.86 (3)
Siii—Er—Siiv132.07 (10)Six—Co—Coii126.86 (3)
Siiii—Er—Siiv180.0Siii—Co—Coii53.14 (3)
Sii—Er—Siv80.51 (4)Si—Co—Coii53.14 (3)
Siii—Er—Siv99.49 (4)Coxi—Co—Coii90.0
Siiii—Er—Siv80.51 (4)Covii—Co—Coii90.0
Siiv—Er—Siv99.49 (4)Coxii—Co—Coii180.0
Sii—Er—Sivi80.51 (4)Sivii—Co—Er65.57 (5)
Siii—Er—Sivi99.49 (4)Six—Co—Er160.56 (6)
Siiii—Er—Sivi80.51 (4)Siii—Co—Er65.57 (5)
Siiv—Er—Sivi99.49 (4)Si—Co—Er83.35 (7)
Siv—Er—Sivi132.07 (10)Coxi—Co—Er116.180 (11)
Sii—Er—Sivii99.49 (4)Covii—Co—Er63.820 (11)
Siii—Er—Sivii80.51 (4)Coxii—Co—Er116.180 (11)
Siiii—Er—Sivii99.49 (4)Coii—Co—Er63.820 (11)
Siiv—Er—Sivii80.51 (4)Sivii—Co—Erxiii160.56 (6)
Siv—Er—Sivii47.93 (10)Six—Co—Erxiii65.57 (5)
Sivi—Er—Sivii180.0Siii—Co—Erxiii83.35 (7)
Sii—Er—Siviii99.49 (4)Si—Co—Erxiii65.57 (5)
Siii—Er—Siviii80.51 (4)Coxi—Co—Erxiii63.820 (11)
Siiii—Er—Siviii99.49 (4)Covii—Co—Erxiii116.180 (11)
Siiv—Er—Siviii80.51 (4)Coxii—Co—Erxiii116.180 (11)
Siv—Er—Siviii180.0Coii—Co—Erxiii63.820 (11)
Sivi—Er—Siviii47.93 (10)Er—Co—Erxiii127.64 (2)
Sivii—Er—Siviii132.07 (10)Sivii—Co—Erx65.57 (5)
Sii—Er—Coi43.90 (4)Six—Co—Erx83.35 (7)
Siii—Er—Coi136.10 (4)Siii—Co—Erx65.57 (5)
Siiii—Er—Coi94.92 (5)Si—Co—Erx160.56 (6)
Siiv—Er—Coi85.08 (5)Coxi—Co—Erx63.820 (11)
Siv—Er—Coi43.90 (4)Covii—Co—Erx116.180 (11)
Sivi—Er—Coi94.92 (5)Coxii—Co—Erx63.820 (11)
Sivii—Er—Coi85.08 (5)Coii—Co—Erx116.180 (11)
Siviii—Er—Coi136.10 (4)Er—Co—Erx77.21 (4)
Sii—Er—Coix43.90 (4)Erxiii—Co—Erx127.64 (2)
Siii—Er—Coix136.10 (4)Sivii—Co—Erxiv83.35 (7)
Siiii—Er—Coix94.92 (5)Six—Co—Erxiv65.57 (5)
Siiv—Er—Coix85.08 (5)Siii—Co—Erxiv160.56 (6)
Siv—Er—Coix94.92 (5)Si—Co—Erxiv65.57 (5)
Sivi—Er—Coix43.90 (4)Coxi—Co—Erxiv116.180 (11)
Sivii—Er—Coix136.10 (4)Covii—Co—Erxiv63.820 (11)
Siviii—Er—Coix85.08 (5)Coxii—Co—Erxiv63.820 (11)
Coi—Er—Coix52.36 (2)Coii—Co—Erxiv116.180 (11)
Sii—Er—Co136.10 (4)Er—Co—Erxiv127.64 (2)
Siii—Er—Co43.90 (4)Erxiii—Co—Erxiv77.21 (4)
Siiii—Er—Co85.08 (5)Erx—Co—Erxiv127.64 (2)
Siiv—Er—Co94.92 (5)Covii—Si—Coxv73.73 (6)
Siv—Er—Co85.08 (5)Covii—Si—Coii116.08 (13)
Sivi—Er—Co136.10 (4)Coxv—Si—Coii73.73 (6)
Sivii—Er—Co43.90 (4)Covii—Si—Co73.73 (6)
Siviii—Er—Co94.92 (5)Coxv—Si—Co116.08 (13)
Coi—Er—Co127.64 (2)Coii—Si—Co73.73 (6)
Coix—Er—Co180.0Covii—Si—Sixvi121.96 (6)
Sii—Er—Coii136.10 (4)Coxv—Si—Sixvi121.96 (6)
Siii—Er—Coii43.90 (4)Coii—Si—Sixvi121.96 (6)
Siiii—Er—Coii85.08 (5)Co—Si—Sixvi121.96 (6)
Siiv—Er—Coii94.92 (5)Covii—Si—Erxiii139.747 (19)
Siv—Er—Coii136.10 (4)Coxv—Si—Erxiii139.747 (19)
Sivi—Er—Coii85.08 (5)Coii—Si—Erxiii70.54 (2)
Sivii—Er—Coii94.92 (5)Co—Si—Erxiii70.54 (2)
Siviii—Er—Coii43.90 (4)Sixvi—Si—Erxiii66.04 (5)
Coi—Er—Coii180.0Covii—Si—Erxvii70.54 (2)
Coix—Er—Coii127.64 (2)Coxv—Si—Erxvii70.54 (2)
Co—Er—Coii52.36 (2)Coii—Si—Erxvii139.747 (19)
Sivii—Co—Six106.27 (6)Co—Si—Erxvii139.747 (19)
Sivii—Co—Siii116.08 (13)Sixvi—Si—Erxvii66.04 (5)
Six—Co—Siii106.27 (6)Erxiii—Si—Erxvii132.07 (10)
Sivii—Co—Si106.27 (6)Covii—Si—Erxiv70.54 (2)
Six—Co—Si116.08 (13)Coxv—Si—Erxiv139.747 (19)
Siii—Co—Si106.27 (6)Coii—Si—Erxiv139.747 (19)
Sivii—Co—Coxi126.86 (3)Co—Si—Erxiv70.54 (2)
Six—Co—Coxi53.14 (3)Sixvi—Si—Erxiv66.04 (5)
Siii—Co—Coxi53.14 (3)Erxiii—Si—Erxiv80.51 (4)
Si—Co—Coxi126.86 (3)Erxvii—Si—Erxiv80.51 (4)
Sivii—Co—Covii53.14 (3)Covii—Si—Erxviii139.747 (19)
Six—Co—Covii126.86 (3)Coxv—Si—Erxviii70.54 (2)
Siii—Co—Covii126.86 (3)Coii—Si—Erxviii70.54 (2)
Si—Co—Covii53.14 (3)Co—Si—Erxviii139.747 (19)
Coxi—Co—Covii180.0Sixvi—Si—Erxviii66.04 (5)
Sivii—Co—Coxii53.14 (3)Erxiii—Si—Erxviii80.51 (4)
Six—Co—Coxii53.14 (3)Erxvii—Si—Erxviii80.51 (4)
Siii—Co—Coxii126.86 (3)Erxiv—Si—Erxviii132.07 (10)
Symmetry codes: (i) x1/2, y1/2, z1/2; (ii) x+1/2, y+1/2, z+1/2; (iii) x+1/2, y+1/2, z1/2; (iv) x1/2, y1/2, z+1/2; (v) x1/2, y+1/2, z1/2; (vi) x+1/2, y1/2, z1/2; (vii) x1/2, y+1/2, z+1/2; (viii) x+1/2, y1/2, z+1/2; (ix) x, y, z; (x) x, y+1, z; (xi) x+1/2, y+3/2, z+1/2; (xii) x1/2, y+3/2, z+1/2; (xiii) x+1/2, y+1/2, z+1/2; (xiv) x1/2, y+1/2, z+1/2; (xv) x, y1, z; (xvi) x, y, z+1; (xvii) x1/2, y1/2, z+1/2; (xviii) x+1/2, y1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaCo2ErSi2
Mr341.30
Crystal system, space groupTetragonal, I4/mmm
Temperature (K)295
a, c (Å)3.874 (2), 9.704 (4)
V3)145.64 (12)
Z2
Radiation typeMo Kα
µ (mm1)40.29
Crystal size (mm)0.14 × 0.13 × 0.04
Data collection
DiffractometerOxford Xcalibur3 CCD area–detector
diffractometer
Absorption correctionAnalytical
(CrysAlis RED; Oxford Diffraction, 2004)
Tmin, Tmax0.008, 0.261
No. of measured, independent and
observed [I > 2σ(I)] reflections
694, 109, 106
Rint0.040
(sin θ/λ)max1)0.783
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.022, 0.050, 1.17
No. of reflections109
No. of parameters9
Δρmax, Δρmin (e Å3)2.09, 2.41

Computer programs: CrysAlis CCD (Oxford Diffraction, 2003), CrysAlis CCD, CrysAlis RED (Oxford Diffraction, 2004), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), DIAMOND (Brandenburg, 1999), SHELXL97.

Selected geometric parameters (Å, º) top
Er—Sii2.9977 (17)Co—Coiii2.7393 (14)
Er—Coi3.1044 (10)Si—Siiv2.435 (6)
Co—Siii2.2831 (17)
Sii—Er—Siv180.0Six—Co—Coiii53.14 (3)
Sii—Er—Sivi132.07 (10)Coiii—Co—Coii180.0
Siv—Er—Sivi47.93 (10)Coiii—Co—Coxi90.0
Sii—Er—Sivii80.51 (4)Siii—Co—Er65.57 (5)
Siv—Er—Sivii99.49 (4)Six—Co—Er160.56 (6)
Sii—Er—Coi43.90 (4)Si—Co—Er83.35 (7)
Siv—Er—Coi136.10 (4)Coii—Si—Coxii73.73 (6)
Sivi—Er—Coi94.92 (5)Coii—Si—Cov116.08 (13)
Siviii—Er—Coi85.08 (5)Coii—Si—Siiv121.96 (6)
Coi—Er—Coix52.36 (2)Coii—Si—Erxiii139.747 (19)
Siii—Co—Six106.27 (6)Cov—Si—Erxiii70.54 (2)
Siii—Co—Siv116.08 (13)Siiv—Si—Erxiii66.04 (5)
Siii—Co—Coiii126.86 (3)Erxiii—Si—Erxiv132.07 (10)
Symmetry codes: (i) x1/2, y1/2, z1/2; (ii) x1/2, y+1/2, z+1/2; (iii) x+1/2, y+3/2, z+1/2; (iv) x, y, z+1; (v) x+1/2, y+1/2, z+1/2; (vi) x+1/2, y+1/2, z1/2; (vii) x1/2, y+1/2, z1/2; (viii) x1/2, y1/2, z+1/2; (ix) x, y, z; (x) x, y+1, z; (xi) x1/2, y+3/2, z+1/2; (xii) x, y1, z; (xiii) x+1/2, y+1/2, z+1/2; (xiv) x1/2, y1/2, z+1/2.
 

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