YbAgxSi2−x [x = 0 28 (1)] with the tetragonal α ThSi2 structure type

inorganic papers

i96

1.72 doi:10.1107/S1600536805013103 Acta Cryst.(2005). E61, i96–i98 Acta Crystallographica Section E

Structure Reports Online

ISSN 1600-5368

YbAg

Si

[

x

= 0.28 (1)] with the tetragonal

a

-ThSi

structure type

Svilen Bobeva* and Eric D. Bauerb

a

Department of Chemistry and Biochemistry, 304A Drake Hall, University of Delaware, Newark, DE 19716, USA, andb_{MST-10 Mail} Stop K764, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Correspondence e-mail: sbobev@chem.udel.edu

Key indicators

Single-crystal X-ray study T= 293 K

Mean(SiAg–Si/Ag) = 0.001 A˚ Disorder in main residue Rfactor = 0.009 wRfactor = 0.020

Data-to-parameter ratio = 13.1

For details of how these key indicators were automatically derived from the article, see http://journals.iucr.org/e.

#2005 International Union of Crystallography Printed in Great Britain – all rights reserved

Single crystals of the title compound, ytterbium silver silicide, were synthesized from the corresponding elements using a eutectic Ag/Si mixture as a solvent. Structure determination suggested the composition of the product to be YbAgxSi2x

[x = 0.28 (1)], i.e. a new ternary derivative of the -ThSi2

structure type, which crystallizes in the body-centered tetragonal space group I41/amd. The two atoms in the

asymmetric unit lie on special positions with Wyckoff symbols 4a(Yb), and 8e(disordered Ag and Si).

Comment

Binary rare-earth silicides and germanides are important materials, which have been extensively studied in the last two or three decades (Gschneider & Eyring, 1979). Of specific interest to us was the divalent oxidation state of Eu and the mixed valency of Yb in some silicides and germanides and their derivatives. The pronounced stability of the divalent Eu2+and Yb2+oxidation states can be explained by their half-filled and completely half-filled 4f-shells, respectively.

The purpose of the present work was to study the variations of the polyanionic network of such europium and ytterbium compounds as a function of electron count, electronegativity, and constituent size. Some results from the systematic inves-tigation of the physical properties and chemical bonding in REAlxSi2x compounds (RE is a rare earth) adopting the -ThSi2structure type (Villars & Calvert, 1991) have already

been published (Bobevet al., 2005). Those studies confirmed wide homogeneity regions in both systems, which present a significant challenge for obtaining the phases as pure products and with defined composition. We report here the synthesis and structural characterization of a new member of the family, YbAgxSi2x[x= 0.28 (1)], which also crystallizes in the

body-centered tetragonal-ThSi2structure type. Detailed physical

property studies of this new material will be reported in a forthcoming publication.

The-ThSi2type is a very common structure among such

intermetallics. As described already, many of these are indeed non-stoichiometric phases with large stoichiometry ranges. This fact, together with the rather anisotropic physical prop-erties one might expect from the crystal structure type, presents challenges for researchers in this field.

To circumvent these difficulties, we employed the flux-growth technique (Canfield & Fisk, 1992) to obtain high-quality single crystals of YbAgxSi2x [x = 0.28 (1)]. The

availability of sizeable single crystals in this and many other cases proved very important for unequivocal structure deter-mination and precise property measurements. The title compound was successfully prepared in good yield from an Ag–Si low-melting eutectic mixture.

The structure of YbAgxSi2x[x= 0.28 (1)] is a new ternary

derivative of the -ThSi2 structure type (Villars & Calvert,

1991). Notably, the binary phase YbSi2is known, although it is

also not fully stoichiometric and adopts the AlB2 structure

type (Villars & Calvert, 1991). YbAgxSi2x [x = 0.28 (1)],

therefore, is a rare example of a ternary phase, based on a structure type very common among the binary rare earth metals, but yet not realised in the Yb–Si system.

The lattice parameters a = 4.0757 (7) A˚ and c = 14.1965 (11) A˚ compare well with those for other REAgxSi2x

phases, such as CeAgxSi2x (Cordruwisch et al., 2001).

Although YbSi2(or rather YbSi2x) with the-ThSi2structure

type does not exist, an elongation of the crystal axes, especially the c axis, is clearly seen for YbAgxSi2x [x = 0.28 (1)] in

comparison with those for other unsubstituted RESi2x

compounds. Such expansion of the unit cell is typical in other solid solutions REMxSi2x (M = main-group or transition

metal), and is due to the larger atomic size of Ag compared with that of Si. Thus, all interatomic Si—Si distances are slightly longer than those found in pure binary phases (Villars & Calvert, 1991).

YbAgxSi2x[x= 0.28 (1)] and its parent structure,-ThSi2,

can be viewed as polar intermetallics,i.e.compounds formed by electropositive and electronegative metals and semi-metals. The structure can be considered as made up of an Si-based polyanionic subnetwork, with the rare-earth cations occupying the voids and channels within it, as shown in Fig. 1. The Yb atom is situated on a site with 4m2 symmetry, whereas the Si site has 2mm symmetry. The Si—Si contacts fall within the narrow range 2.3463 (16)–2.3664 (8) A˚ , which agrees with the

description above. The shortest Yb—Si contact is 3.1116 (3) A˚ , a distance normal for such a high coordination number (Fig. 2).

Experimental

All starting materials were used as received: Yb (Ames Laboratory, ingot, 99.99% metal basis), Ag (Alfa, foil, 99.999%) and Si (Alfa, pieces, 99.999%). A mixture of the elements in a ratio Yb:Ag:Si = 1:0.11:0.89 was loaded in an alumina crucible, which was subsequently enclosed in an evacuated fused silica jacket by flame-sealing. The reaction was carried out at a temperature of 1423 K for 2 h, followed by slow cooling (4 K h1) to 1148 K. At this point, the molten flux was removed by centrifugation. The product of the reaction were small crystals with silver metallic luster. These were later identified as YbAgxSi2x[x= 0.28 (1)]. The crystals are stable in air and moisture

over extended periods of time.

Crystal data

YbAg28Si1.72

Mr= 251.76

Tetragonal,I41=amd

a= 4.0757 (2) A˚

c= 14.1965 (11) A˚

V= 235.82 (2) A˚3

Z= 4

Dx= 7.091 Mg m

3

MoKradiation Cell parameters from 796

reflections

= 5.2–30.9 = 42.37 mm1

T= 293 (2) K Bar, grey

0.050.040.03 mm

Data collection

Bruker SMART APEX diffractometer

!scans

Absorption correction: multi-scan (SADABS; Sheldrick, 2003)

Tmin= 0.155,Tmax= 0.280

796 measured reflections

118 independent reflections 107 reflections withI> 2(I)

Rint= 0.018

max= 30.9

h=5!5

k=5!4

l=20!19

Refinement

Refinement onF2

R[F2_{> 2(F}2_{)] = 0.009}

wR(F2) = 0.020

S= 1.23 118 reflections 9 parameters

w= 1/[2_(F

o2) + (0.0097P)2

+ 0.0681P]

whereP= (Fo2+ 2Fc2)/3

(/)max< 0.001 max= 0.45 e A˚

3 min=0.66 e A˚

3

Extinction correction:SHELXL97

Extinction coefficient: 0.0086 (6)

inorganic papers

Acta Cryst.(2005). E61, i96–i98 Bobev and Bauer _YbAg28Si

1.72

i97

A view of the Yb coordination polyhedron in YbAgxSi2x[x= 0.28 (1)]. Displacement ellipsoids are drawn at the 90% probability level.

Figure 1

Table 1

Selected interatomic distances (A˚ ).

Yb—Si/Agi

3.1116 (3) Si/Ag—Si/Agii 2.3463 (16) Si/Ag—Si/Agi

2.3664 (8)

Si/Ag—Ybi

3.1116 (3)

Si/Ag—Ybiii 3.1302 (6)

Symmetry codes: (i)1 2x;

1 2y;

1

2z; (ii)y 1 4;

1 4x;

3

4z; (iii)x;y1;z.

Structure solution and refinement were performed with origin choice 2 of the space groupI41/amd. The structure refinement based

on a composition ‘YbSi2’ converged at poor residuals and two

crys-tallographically unique sites (Yb and Si) exhibited unusually aniso-tropic displacement parameters. By freeing the site-occupation factor for an individual atom, while other remaining parameters were kept fixed, it became evident that the Si site is statistically occupied by Si and Ag atoms. The Si/Ag site was found to be a nearly 85:15 statistical mixture of Si and Ag, whereas the Yb site is fully occupied with deviations from full occupancy within less than 3.

Data collection:SMART(Bruker, 2002); cell refinement:SAINT

(Bruker, 2002); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine

structure:SHELXTL; molecular graphics:XPin SHELXTL; soft-ware used to prepare material for publication:SHELXTL.

This work was funded in part by a University of Delaware start-up grant. Work at LANL is performed under the auspices of the DOE.

References

Bobev, S., Tobash, P. H., Fritsch, V., Hundley, M. F., Thompson, J. D., Sarrao, J. L. & Fisk, Z. (2005).J. Solid State Chem.In the press.

Bruker (2002).SMARTandSAINT.Bruker AXS Inc., Madison, Wisconsin, USA.

Canfield, P. C. & Fisk, Z. (1992).Philos. Mag. B,65, 1117–1123.

Cordruwisch, E., Kaczorowski, D., Rogl, P., Saccone, A. & Ferro, R. (2001).J. Alloys Compds,320, 308–319.

Gschneider, K. A. Jr. & Eyring, L. (1979).Handbook on the Physics and Chemistry of Rare Earths. Amsterdam: North Holland.

Sheldrick, G. M. (2001).SHELXTL.Bruker AXS Inc., Madison, Wisconsin, USA.

Sheldrick, G. M. (2003).SADABS.University of Go¨ttingen, Germany. Villars, P. & Calvert, L. D. (1991).Pearson’s Handbook of Crystallographic

Data for Intermetallic Compounds, 2nd ed. Materials Park, Ohio, USA: American Society for Metals.

inorganic papers

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Acta Cryst. (2005). E61, i96–i98

supporting information

Acta Cryst. (2005). E61, i96–i98 [https://doi.org/10.1107/S1600536805013103]

YbAg

Si

[

x

= 0.28

(1)] with the tetragonal

α

-ThSi

structure type

Svilen Bobev and Eric D. Bauer

ytterbium silver silicide

Crystal data YbAg.28Si1.72

Mr = 251.76

Tetragonal, I41/amd

Hall symbol: -I 4bd 2 a = 4.0757 (2) Å c = 14.1965 (11) Å V = 235.82 (2) Å3

Z = 4 F(000) = 429

Dx = 7.091 Mg m−3

Mo Kα radiation, λ = 0.71073 Å Cell parameters from 796 reflections θ = 5.2–30.9°

µ = 42.37 mm−1

T = 293 K Bar, grey

0.05 × 0.04 × 0.03 mm

Data collection Bruker APEX SMART

diffractometer

Radiation source: fine-focus sealed tube Graphite monochromator

Detector resolution: 8.3 pixels mm-1

ω scans

Absorption correction: multi-scan (SADABS; Sheldrick, 2003) Tmin = 0.155, Tmax = 0.280

796 measured reflections 118 independent reflections 107 reflections with I > 2σ(I) Rint = 0.018

θmax = 30.9°, θmin = 5.2°

h = −5→5 k = −5→4 l = −20→19

Refinement Refinement on F2

Least-squares matrix: full R[F2 _{> 2σ(F}2_{)] = 0.009}

wR(F2_{) = 0.020}

S = 1.23 118 reflections 9 parameters 0 restraints

Primary atom site location: structure-invariant direct methods

Secondary atom site location: difference Fourier map

w = 1/[σ2_(F

o2) + (0.0097P)2 + 0.0681P]

where P = (Fo2 + 2Fc2)/3

(Δ/σ)max < 0.001

Δρmax = 0.45 e Å−3

Δρmin = −0.66 e Å−3

Extinction correction: SHELXL97, Fc*_{=kFc[1+0.001xFc}2_λ3_/sin(2θ)]-1/4

Extinction coefficient: 0.0086 (6)

Special details

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Acta Cryst. (2005). E61, i96–i98

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 F}2_,

conventional R-factors R are based on F, with F set to zero for negative F2_{. The threshold expression of F}2 _{> σ(F}2_{) 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₎

x y z Uiso*/Ueq Occ. (<1)

Yb 0.0000 0.7500 0.1250 0.00700 (14)

Si 0.0000 0.2500 0.29237 (6) 0.0077 (4) 0.858 (4) Ag 0.0000 0.2500 0.29237 (6) 0.0077 (4) 0.142 (4)

Atomic displacement parameters (Å2₎

U11 _U22 _U33 _U12 _U13 _U23

Yb 0.00575 (15) 0.00575 (15) 0.00951 (17) 0.000 0.000 0.000 Si 0.0044 (5) 0.0145 (6) 0.0041 (4) 0.000 0.000 0.000 Ag 0.0044 (5) 0.0145 (6) 0.0041 (4) 0.000 0.000 0.000

Geometric parameters (Å, º)

Yb—Agi _{3.1116 (3) Si—Ag}ix _{2.3463 (16)}

Yb—Agii _{3.1116 (3) Si—Si}ix _{2.3463 (16)}

Yb—Sii _{3.1116 (3) Si—Ag}iv _{2.3664 (8)}

Yb—Siii _{3.1116 (3) Si—Si}iv _{2.3664 (8)}

Yb—Agiii _{3.1116 (3) Si—Ag}viii _{2.3664 (8)}

Yb—Agiv _{3.1116 (3) Si—Si}viii _{2.3664 (8)}

Yb—Siiii _{3.1116 (3) Si—Yb}ii _{3.1116 (3)}

Yb—Siiv _{3.1116 (3) Si—Yb}iv _{3.1116 (3)}

Yb—Agv _{3.1116 (3) Si—Yb}vii _{3.1116 (3)}

Yb—Agvi _{3.1116 (3) Si—Yb}viii _{3.1116 (3)}

Yb—Agvii _{3.1116 (3) Si—Yb}x _{3.1302 (6)}

Yb—Agviii _{3.1116 (3)}

Agi_—Yb—Agii _{180.00 (3) Ag}vi_—Yb—Agviii _{44.30 (3)}

Agi_—Yb—Sii _{0.00 (3) Ag}vii_—Yb—Agviii _{135.70 (3)}

Agii_—Yb—Sii _{180.0 Ag}ix_—Si—Agiv _{120.55 (3)}

Agi_—Yb—Siii _{180.00 (3) Si}ix_—Si—Agiv _{120.55 (3)}

Agii_—Yb—Siii _{0.00 (3) Si}ix_—Si—Siiv _{120.55 (3)}

Sii_—Yb—Siii _{180.00 (3) Ag}ix_—Si—Agviii _{120.55 (3)}

Agi_—Yb—Agiii _{135.70 (3) Si}ix_—Si—Agviii _{120.55 (3)}

Agii_—Yb—Agiii _{44.30 (3) Ag}iv_—Si—Agviii _{118.90 (7)}

Sii_—Yb—Agiii _{135.70 (3) Si}iv_—Si—Agviii _{118.90 (7)}

Siii_—Yb—Agiii _{44.30 (3) Ag}ix_—Si—Siviii _{120.55 (3)}

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Acta Cryst. (2005). E61, i96–i98

Agii_—Yb—Agiv _{135.70 (3) Ag}iv_—Si—Siviii _{118.90 (7)}

Sii_—Yb—Agiv _{44.30 (3) Si}iv_—Si—Siviii _{118.90 (7)}

Siii_—Yb—Agiv _{135.70 (3) Ag}ix_—Si—Ybii _{67.851 (14)}

Agiii_—Yb—Agiv _{180.0 Si}ix_—Si—Ybii _{67.851 (14)}

Agi_—Yb—Siiii _{135.70 (3) Ag}iv_—Si—Ybii _{139.082 (6)}

Agii_—Yb—Siiii _{44.30 (3) Si}iv_—Si—Ybii _{139.082 (6)}

Sii_—Yb—Siiii _{135.70 (3) Ag}viii_—Si—Ybii _{68.139 (14)}

Siii_—Yb—Siiii _{44.30 (3) Si}viii_—Si—Ybii _{68.139 (14)}

Agiii_—Yb—Siiii _{0.00 (3) Ag}ix_—Si—Ybiv _{67.851 (14)}

Agiv_—Yb—Siiii _{180.0 Si}ix_—Si—Ybiv _{67.851 (14)}

Agi_—Yb—Siiv _{44.30 (3) Ag}iv_—Si—Ybiv _{68.139 (14)}

Agii_—Yb—Siiv _{135.70 (3) Si}iv_—Si—Ybiv _{68.139 (14)}

Sii_—Yb—Siiv _{44.30 (3) Ag}viii_—Si—Ybiv _{139.082 (6)}

Siii_—Yb—Siiv _{135.70 (3) Si}viii_—Si—Ybiv _{139.082 (6)}

Agiii_—Yb—Siiv _{180.0 Yb}ii_—Si—Ybiv _{135.70 (3)}

Agiv_—Yb—Siiv _{0.0 Ag}ix_—Si—Ybvii _{67.851 (14)}

Siiii_—Yb—Siiv _{180.0 Si}ix_—Si—Ybvii _{67.851 (14)}

Agi_—Yb—Agv _{81.828 (10) Ag}iv_—Si—Ybvii _{68.139 (14)}

Agii_—Yb—Agv _{98.172 (10) Si}iv_—Si—Ybvii _{68.139 (14)}

Sii_—Yb—Agv _{81.828 (10) Ag}viii_—Si—Ybvii _{139.082 (6)}

Siii_—Yb—Agv _{98.172 (10) Si}viii_—Si—Ybvii _{139.082 (6)}

Agiii_—Yb—Agv _{81.828 (10) Yb}ii_—Si—Ybvii _{81.828 (10)}

Agiv_—Yb—Agv _{98.172 (10) Yb}iv_—Si—Ybvii _{81.828 (10)}

Siiii_—Yb—Agv _{81.828 (10) Ag}ix_—Si—Ybviii _{67.851 (14)}

Siiv_—Yb—Agv _{98.172 (10) Si}ix_—Si—Ybviii _{67.851 (14)}

Agi_—Yb—Agvi _{81.828 (10) Ag}iv_—Si—Ybviii _{139.082 (6)}

Agii_—Yb—Agvi _{98.172 (10) Si}iv_—Si—Ybviii _{139.082 (6)}

Sii_—Yb—Agvi _{81.828 (10) Ag}viii_—Si—Ybviii _{68.139 (14)}

Siii_—Yb—Agvi _{98.172 (10) Si}viii_—Si—Ybviii _{68.139 (14)}

Agiii_—Yb—Agvi _{81.828 (10) Yb}ii_—Si—Ybviii _{81.828 (10)}

Agiv_—Yb—Agvi _{98.172 (10) Yb}iv_—Si—Ybviii _{81.828 (10)}

Siiii_—Yb—Agvi _{81.828 (10) Yb}vii_—Si—Ybviii _{135.70 (3)}

Siiv_—Yb—Agvi _{98.172 (10) Ag}ix_—Si—Ybx _{139.381 (10)}

Agv_—Yb—Agvi _{135.70 (3) Si}ix_—Si—Ybx _{139.381 (10)}

Agi_—Yb—Agvii _{98.172 (10) Ag}iv_—Si—Ybx _{67.30 (3)}

Agii_—Yb—Agvii _{81.828 (10) Si}iv_—Si—Ybx _{67.30 (3)}

Sii_—Yb—Agvii _{98.172 (10) Ag}viii_—Si—Ybx _{67.30 (3)}

Siii_—Yb—Agvii _{81.828 (10) Si}viii_—Si—Ybx _{67.30 (3)}

Agiii_—Yb—Agvii _{98.172 (10) Yb}ii_—Si—Ybx _{135.443 (14)}

Agiv_—Yb—Agvii _{81.828 (10) Yb}iv_—Si—Ybx _{81.941 (6)}

Siiii_—Yb—Agvii _{98.172 (10) Yb}vii_—Si—Ybx _{135.443 (14)}

Siiv_—Yb—Agvii _{81.828 (10) Yb}viii_—Si—Ybx _{81.941 (6)}

Agv_—Yb—Agvii _{44.30 (3) Ag}ix_{—Si—Yb 139.381 (10)}

Agvi_—Yb—Agvii _{180.0 Si}ix_{—Si—Yb 139.381 (10)}

Agi_—Yb—Agviii _{98.172 (10) Ag}iv_{—Si—Yb 67.30 (3)}

Agii_—Yb—Agviii _{81.828 (10) Si}iv_{—Si—Yb 67.30 (3)}

Sii_—Yb—Agviii _{98.172 (10) Ag}viii_{—Si—Yb 67.30 (3)}

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Agiii_—Yb—Agviii _{98.172 (10) Yb}ii_{—Si—Yb 81.941 (6)}

Agiv_—Yb—Agviii _{81.828 (10) Yb}iv_{—Si—Yb 135.443 (14)}

Siiii_—Yb—Agviii _{98.172 (10) Yb}vii_{—Si—Yb 81.941 (6)}

Siiv_—Yb—Agviii _{81.828 (10) Yb}viii_{—Si—Yb 135.443 (14)}

Agv_—Yb—Agviii _{180.00 (3) Yb}x_{—Si—Yb 81.24 (2)}