Crystal structure of the defect clathrate C s s S i l m ^

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Ζ. Kristallogr. NCS 215 (2000) 205-206 205

© by Oldenbourg Wissenschaftsverlag, München

Crystal structure of the defect clathrate C s s S i l m ^

H. G. von Schnering

¹

, R. Kröner

¹

, M. Baitinger

¹

, K. Peters

¹

, R. Nesper

¹¹

**and Yu. Grin*·**

¹¹¹

I Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, D-70506 Stuttgart, Germany

II ΕΤΗ Zürich, Laboratorium für Anorganische Chemie, Universitätstraße 6, CH-8092 Zürich, Switzerland

III Max-Planck-Institut für Chemische Physik fester Stoffe, Pirnaer Landstraße 176, D-01257 Dresden, Germany Received August 20, 1999, CSD-No. 409448

d3

Sn2

Sn3

Sn2 Sn3

Sn1 Abstract

CsgSn44, cubic, Pm3n (No. 223), a = 12.105(1) Ä,

V = 1773.8 Ä

3

, Ζ =

Γ = 2 9 3 Κ.

1, R

g

i(F) = 0.021, wR

K

f(F

A

) = 0.038,

Discussion

The first refinement of the title structure from the powder data gave the composition CssSrus with relatively large displacement for the Sn 1 position [ 1 ]. The standard refinement (averaged struc- ture, without split positions) results in the composition CsgSn,, with η = 44.0+0.1 (R(F) = 0.059; see Table 2 and also [2,3]). The Cs2 anisotropic displacement is usually observed and is related to the shape of the Sn24 cage (Fig., lower part), but the large £/

eq

= 0.0437(8) for Sn and the strong anisotropy at the Sn3 position (Table 2) is unexpected. Bond lengths for the averaged structure:

dl = 2.805 Ä, d2 = 2.846 Ä, d3 = 2.882 Ä, d4 = 2.735 Ä and d

w

= 2.819 A. The Sn3 displacement of about 0.3 Ä is related to the de- fects at the Snl position (note the short d4 distance). Further re- finements [4] yield in CssSn„ with η = 44.00+0.02 and show unambiguously that the anisotropic Sn3 displacement results from the presence of two split positions, Sn31 and Sn32 (/?

gt

(F) = 0.021 for 249 F

ohs

> 4a(F

0

bs) and R

M

(F) = 0.032 for all 310 re- flections, Table 3). The Sn31 and Sn32 positions are occupied al- ternatively in the ratio 4:2 which agrees perfectly with the distribution of 4 Sn atoms and two defects ( • ) at the 6c position of Sn 1. The Sn32 positions mark the four 3-fold bonded Sn" anions around the defects (J4(Sn32— • ) = 2.32 Ä, J(Sn32—Sn32) = 3.79 Ä. Bond lengths are as follows: dl = 2.806 Ä, d2 = 2.801 A or 3.03 Ä, d3 = 2.77 Ä or 3.05 A, d4 = 2.86 Ä. The displacement el- lipsoids indicate local equilibrations in the d2 and c/3 distances.

Furthermore, the splittings of the Sn3 and Cs2 positions shorten Cs

+

—Sn~ distances. For more details see [5].

Source of material

The compound was synthesized f r o m the elements in the stoichiometric ratio (sealed Ta tube, heating up to 1273 Κ for 3 h, annealing at 1270 Κ for 1 h and at 970 Κ for 2 d). C s g S n + ö forms well shaped grey crystals with metallic luster. The brittle compound is a semiconductor (E

g

= 0.14 eV) and is stable in air and against dilute acids and bases.

Table 1. Data collection and handling.

Crystal:

Wavelength:

μ:

Diffractometer, scan mode:

29max:

N(hkl)mcasured, N(hkl)u„iquC:

Criterion for I

0

b

S

, N(hkl)

g

N(param) refined:

Program:

grey with metallic lustre, size 0.08 χ 0.08 χ 0.08 mm Mo Ka radiation (0.71069 Ä)

1 9 8 . 9 6 c m "1

Syntex P2, Wyckoff

4 9 . 8 4 °

1 6 4 9 , 3 1 0

U s > 2 o ( / o b s j , 2 4 9 2 4

SHELXS-97 [6]

* Correspondence author (e-mail: grin@cpfs.mpg.de)

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206

D e f e c t c l a t h r a t e Cs8Sn44D2

Table 2. Atomic coordinates and displacement parameters (in Ä2). Refinement without split positions.

Atom Site Occ. X y

ζ Uu U22

^t/33

Un Un U23

C s ( l ) 2

a

⁰ ⁰ ⁰ ^0.018(1)

Uu

^u^u ⁰ ⁰ ⁰

Cs(2) 6 d 1/4 1/2 0 0.031(2) 0.048(2)

U22

⁰ ⁰ ⁰

S n ( l ) 6c 0.66(2) 1/4 0 1/2 0.017(3)

U

^π t/11 0 0 0

Sn(2) 16

i

^0.1831(1) ^X ^X ^0.0242(9)

Uu Uu

-0.0036(5) t/12

U\2

Sn(3) 26k 0 0.3159(2) 0.1191(2) 0.021(1) 0.079(2) 0.038(2) 0 0 0.031(1)

Table 3. Atomic coordinates and displacement parameters (in Ä2). Refinement with split positions.

Atom Site Occ. X y

ζ Uu U22

^C/33 ^i/12

Un

C/23

C s ( l ) 2

a

0 0

0

^0.0203(4)

Uu Uu

⁰ ⁰ ⁰

Cs(2)" 6

d

^1/4 ^1/2 ⁰ ^0.0248(7) ^0.0486(6)

U22

⁰ ⁰ ⁰

S n ( l ) 6c 0.666(4) 1/4 0 1/2 0.019(1) 0.0159(7)

U22

⁰ ⁰ ⁰

Sn(2) 16 i 0.18309(3) X X 0.0236(2) U11

Uu

-0.0040(2) I/12

Un

Sn(31) 24k 0.70(4) 0 0.307(1) 0.1143(5) 0.0217(5) 0.022(3) 0.0221(9) 0 0 -0.001(1)

Sn(32) 24 k 0.31(4) 0 0.347(2) 0.135(1) 0.022(1) 0.022(6) 0.022(3) 0 0 0.002(3)

a: Alternatively: Cs(2) at 24/ 1/4, 0.512(1), 1/2+y with occ. 0.25.

Acknowledgments. This work was also supported by the Fonds der Chemi- schen Industrie.

References

1. Grin, Yu. N.; Melekhov, L. Z.;Chuntonov, Κ. Α.; Yatsenko, S. P.: Crystal structure of the compound CsgSn46. Sov. Phys. Crystallogr. 32 (1987) 290-291.

2. Kröner, R.: Zintl-Phasen der A l k a l i m e t a l l e und des B a r i u m s mit Clathratstruktur. Dissertation, Universität Stuttgart, Germany 1989.

3. Zhao, J.-T.; Corbett, J. D.: Zintl Phases in Alkali-Metal-Tin Systems:

K8S n2 5 with Condensed Pentagonal Dodecahedra of Tin. T w o ArSii44 Phases with a defect Clathrate Structure. Inorg. C h e m . 33 (1994) 5721-5726.

4. Baitinger, M.; Grin, Yu.; von Schnering, H. G.: Valence determined defects in the clathrate-I structures of AgSn46-jr with A = K, Rb, Cs. In: Vlth Europ. Conf. Solid State Chemistry, Zürich, 1997. Book of Abstracts, PA116.

5. von Schnering, H. G.; Curda, J.; Carillo-Cabrera, W.; Baitinger, M.; Pe- ters, K.; Grin, Yu.: KgC„Sn44_„n2 and K8C„Ge44-nD2 - Covalent Clathrate-I Networks with Valence Determined Defects. Z. Anorg. Allg.

Chem., in preparation.

6. Sheldrick, G. M.: SHELXL-97. Program for refining crystal structures.

University of Göttingen, Germany 1997.