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Er2Au2Sn and other Ternary Rare Earth Metal Gold Stannides with Ordered Zr3Al2-Type StructureRainer PöttgenM ax-Planck-Institut für Festkörperforschung, H eisenbergstraße 1, D-70569 Stuttgart, G erm any

Z. Naturforsch. 49b, 1309-1313 (1994); received May 20, 1994

R are E arth Gold Tin, Interm etallic Com pounds, O rdered Z r3A l2-Structure

The ternary stannides R E 2A u2Sn (R E = Y, Dy, Ho, Er, Tm, Lu) were prepared by arc- melting of the elem ental com ponents and subsequent annealing at 800 °C. The structure of E r2A u2Sn (single crystal. X-ray, P 42/m nm, Z = 4, a = 778.2(2) pm, c = 739.6(3) pm, V = 0.4479 nm 3 and R = 0.026) is described as the ternary ordered version of the Z r3A l2-type structure, a superstructure of the U 3Si2-type. It consists of two-dimensionally infinite layers (A u2Sn)„ which are separated by the erbium atoms. The structure is built up from slightly distorted [SnEr8] square prisms and [A uEr6] trigonal prisms which are condensed in all three directions. These fragm ents are derived from the well known A1B2 and CsCl-type structures.

Introduction

Uranium forms a silicide of composition U3Si2 [1, 2]. Its tetragonal crystal structure (space group P4/mbm) contains two different uranium positions and one silicon site. Several years ago it was ob­served, that these three different crystallographic sites may also be occupied by three different atoms, thus forming a ternary ordered version of the U3Si2-type with the composition R2T2X. Nu­merous investigations of such compounds resulted in the syntheses of several borides [3], aluminides [4-6], indides [7], silicides [8], and phosphides [9], Only very recently the first ternary uranium tran­sition metal stannides U2T2Sn (T = Fe, Co, Ni, Ru, Rh, Pd) and indides U2T2In (T = Co, Ni, Rh, Pd, Ir, Pt) have been reported [10-12].

Interestingly, the binary aluminide Z r3Al2 [13] forms a crystal structure very similar to U3Si2. However, the difference in size between the zir­conium and aluminium atoms results in small dis­tortions and in a doubling of the c lattice constant as compared to U 3Si2. Z r3Al2 may therefore be considered as a superstructure of U3Si2, crystalliz­ing in the klassengleiche supergroup P42/mnm. In the present paper we report on the first rare-earth stannides RE2Au2Sn (RE = Y, Dy, Ho, Er, Tm, Lu) with the ternary ordered Z r3Al2-type. Very re­

* R eprint requests to R. Pöttgen.

cently the same ordered structure has been re­ported for U2Pt2Sn [14] and U2Ir2Sn [15] from in­dependent investigations.

Sample Preparation and Lattice ConstantsStarting materials for the preparation of the ter­

nary stannides were ingots of the rare-earth el­ements (Johnson Matthey, >99.9%), gold wire (Degussa, 99.9%) and tin granules (Merck, 99.9%). The samples were prepared by arc-melt­ing of the elemental components of the ideal com­positions in an argon (99.996%) atmosphere. The argon was further purified by molecular sieves and an oxisorb catalyst [16]. The melted buttons were turned over and remelted several times to ensure good homogeneity. The weight loss after several meltings was always smaller than 0.5%. The pel­lets were subsequently enclosed in evacuated silica tubes and annealed at 800 °C for ten days. All melted and annealed buttons had a light grey color, but the materials are dark grey in powdered form. Single crystals of E r2Au2Sn have metallic lustre. They are stable in air over long periods of time.

The tetragonal lattice constants (see Table I) were obtained by least-squares fits of the Guinier powder data. CuKö! radiation was used with 5N silicon (a = 543.07 pm) as an internal standard. The identification of the diffraction lines was fa­cilitated by intensity calculations [17] using the positional parameters of the refined structure.

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1310 R. Pöttgen • Er2 Au2Sn

Table I. Lattice constants (pm ) of the tetragonal stan- nides with E r2A u2Sn-type structure.

Com pound fl(pm) c(pm ) c/a V (nm 3)

Y 2A u2Sn 781.4(1) 753.5(1) 0.964 0.4601D y2A u2Sn 782.2(1) 750.3(1) 0.959 0.4591H o2A u2Sn 779.7(1) 745.5(1) 0.956 0.4532E r2A u2Sn 778.2(2) 739.6(3) 0.950 0.4479Tm^AuoSn 776.0(3) 738.0(4) 0.951 0.4444Lu2A u2Sn 772.8(6) 734.2(6) 0.950 0.4385

Structure Determination

The structural similarity of the stannides with the U 3Si2-type structure was already recognized on the Guinier powder patterns, but several weak reflections remained, and the whole pattern could only be indexed, when doubling the c lattice con­stant, indicating a superstructure. Further investi­gations were then carried out on single-crystals in order to determine the correct structure. Single­crystals of E r2Au2Sn were isolated from an an­nealed button by mechanical fragmentation. They were examined with a Buerger precession camera to establish their symmetry and suitability for intensity data collection. The crystals had the high Laue symmetry 4/mmm, and the systematic extinc­tions (OkI observed only with k + l - 2n, hOO only with h - 2n) led to the space groups P42/mnm, P4n2, and P42nm. The structure refinements eventually showed that the space group with the highest symmetry compatible with these extinc­tions P42/mnm - D 44h was the correct one.

Intensity data were collected on an automated four-circle diffractometer (CAD 4) with graphite monochromated AgKa radiation and a scintil­lation counter with pulse-height discrimination. Further experimental details are summarized in Table II.

The starting atomic parameters were deduced from a Patterson interpretation [18] and the struc­ture was then successfully refined using SHELXL- 93 [19], which minimizes a weighted square re­sidual “wR2” from all data using structure ampli­tudes IF21 rather than structure factors F. The standard residual “R l ” is calculated purely for comparision. Measured intensities more than two sigma below zero (one independent reflection) were suppressed for refinement purposes. The final residuals are listed in Table II. The final difference Fourier synthesis revealed as highest peak an elec-

Table II. Crystal data and structure refinem ent for E r2A u2Sn.

Empirical formula Er2Au2SnFormula weight 847.14Temperature 293(2) KWavelengths 56.087 pmCrystal system 4/mmmSpace group P42/mnmUnit cell dimensions see Table IFormula units per cell Z = 4Calculated density 12.563 Mg/m3Crystal size 4 0 x 5 0 x 6 0 ^m 3Absorption coefficient 578.1 cm “1F(000) 13760 range for data collection 3.70° to 34.99°Range in h k l + 12, ± 12, ±11Total no. reflections 3729Independent reflections 569 (R mt = 0.0552)Refinement method Full-matrix least-squares on

F2Data/restraints/parameters 568/0/18Goodness-of-fit on F2 1.211Final R indices [I>2cr(I)] R 1=0.0257, wR 2 = 0.0558R indices (all data) R 1 = 0.0330, wR 2 = 0.0625Extinction coefficient 0.00020(7)Largest diff. peak and hole 5186 and -2 7 0 4 e/nm3

Table III. A tom ic coordinates and anisotropic displace­m ent param eters (pm 2x l0 _1) for E r2A u2Sn. U eq is de­fined as one third of the trace of the orthogonalized U,y tensor. The anisotropic displacem ent factor exponent takes the form: - 2 j i2[(ha*)2\J n -\-----\-2hka*b* \J i2]-

Atom P42/mnm X y z u eq

E rl 4 f 0.1836(1) X 0 10(1)Er 2 4g 0.3427(1) - X 0 10(1)Au 8j 0.3724(1) X 0.2784(1) 9(1)Sn 4d 0 1/2 1/4 13(1)

Atom u„ = U 22 u 33 u 23 = u13 u12E rl 12(1) 7(1) 0 -4 (1 )Er 2 12( 1) 7(1) 0 5(1)Au 7(1) 12(1) 0( 1) - 2(1)Sn 8(1) 23(1) 0 0

tron density of 5186 e/nm3, too close to the Au pos­ition to be suitable for an additional atomic site. It most likely resulted from an incomplete absorption correction of the data. Atomic coordinates and an­isotropic thermal parameters are given in Table III, interatomic distances in Table IV*.

* Further details may be obtained from the Fachinfor- m ationszentrum K arlsruhe, Gesellschaft für wissen­schaftlich-technische Inform ation m bH . D-76344 Eg- genstein-Leopoldshafen (G erm any) on quoting the depository num ber CSD 58358. the nam e of the author and the journal citation.

R. Pöttgen • Er2 Au2Sn 1311

Table IV. Interatom ic distances (pm) in the structure of E r2Au2Sn. All distances shorter than 530 pm (E r-A u , E r-S n ) , 410 pm (A u -A u , A u -S n ) and 365 pm (E r - E r , S n -S n ) are listed. S tandard deviations are all equal or less than 0.1 pm.

E r l : 2 Au 292.5 Au: 1 Au 280.84 Au 295.7 1 E r 2 288.04 Sn 339.4 1 E r l 292.5

2 E r l 295.7E r 2: 2 Au 288.0 2 E r 2 303.4

4 Au 303.4 2 Sn 307.11 E r 2 346.1 1 Au 327.94 Sn 346.9

Sn: 4 Au 307.14 E r l 339.44 E r 2 346.9

0 1/2

u ORh •

Sn #

U2Rh2Sn

Discussion

Six ternary stannides RE2Au2Sn (RE = Y, Dy, Ho, Er, Tm, Lu) were synthesized and their crystal structure was determined from single crystal dif­fractometer data for the erbium compound. The structure of E r2Au2Sn represents a new type. It is derived from the structure of binary Z r3Al2 [13] by an ordered arrangement of the E r l , E r2, Au and Sn atoms on the Z r l , Zr2, Al and Zr3 po­sitions of Z r3Al2, respectively. Ternary stannides with the ordered U3Si2 type structure have been reported recently: U2T2Sn (T = Fe, Co, Ni, Ru, Rh, Pd) [10-12]. Interestingly, Z r3Al2 (space group P42/mnm) is a superstructure of the binary uranium silicide U3Si2 (space group P4/mbm) [1, 2]. Small distortions, due to the difference in size between zirconium and aluminium [13], result in a doubling of the lattice constant c, when com­pared to U 3Si2. Thus, Z r3Al2 crystallizes in the klassengleiche supergroup P42/mnm of U3Si2. The crystallographic relationship between the struc­tures of U 3Si2 [1, 2] and Z r3Al2 [13] and their ter-

P4/m2i/b2/m P4/m2i/b27m

|U 2 R h 2 S n | M----- | U3Si21k2

31, 32, 2c 1/2 , 0 , 0

P42/m2i/n2/m P42/m2i/n2/m

|Zf3Al2| ---- ► |Er2Au2Sn]

Sn 2a Rh 4g

U2 2a Si 4g

r —► Zr1 4f '— ► Zr2 4g

Zr3 4d Al 8j

Er1 4fEr2 4gSn 4dAu 8j

Fig. 1. Crystal chemical relationship betw een the struc­tures of U 3Si2, Z r3A l2, U 2R h2Sn and E r2A u2Sn. The space group, the group-subgroup relationship and occu­pancy of the different Wyckoff sites is indicated.

Fig. 2. Projections of the crystal structures of U 2R h2Sn (ordered U 3Si2-type) and E r2Au2Sn (ordered Z r3A l2- type) on the xy plane. The z param eters of the atom s are indicated. The A1B2- and CsCl-like fragm ents are outlined. The two different erbium positions in the struc­ture of E r2A u2Sn are indicated.

nary ordered variants U 2Rh2Sn [10-12] and E r2Au2Sn is shown in Fig. 1 in the manner for­malized by Bärnighausen [20],

The crystal structures of Er2Au2Sn and the un­distorted variant U2Rh2Sn are shown in Fig. 2 as projections on the xy planes. From this Figure it can clearly be seen that both structures are built

1312 R. Pöttgen • Er2 Au2Sn

up from [SnU8] and [SnEr8] tetragonal prisms and [RhU6] and [AuEr6] trigonal prisms, respectively. However, both types of prisms are distorted in the structure of Er2Au2Sn, while they are more or less regular in the uranium compound. Always two of the [AuEr6] prisms are face-shared forming an AlB2-like fragment. The [SnEr8] fragments are de­rived from the well known CsCl-type structure. However, no binary compound of the composition ErSn is known [3] and ErAu2 crystallizes in the tetragonal MoSi2-type structure [21], not in the AlB2-type. Such an AlB2-type arrangement was up to now only observed for the binary gold inter- metallics BaAu2 [22], NbAu2 [23,24], ThAu2 [25,26], and UAu2 [27-29],

In Er2Au2Sn the A1B2- and CsCl-like fragments are condensed within the xy plane by common square faces in such a way, that every CsCl-frag- ment is connected to four AlB2-fragments and vice versa. These layers are stacked one upon the other in c direction.

Alternatively, the structure of Er2Au2Sn may also be described as consisting of slightly waved two-dimensionally infinite layers (Au2Sn)„ within the xy plane which are separated by the rare- earth atoms. The main difference between the undistorted structure of U2Rh2Sn and the dis­torted one of Er2Au2Sn is the slight puckering of the Au2Sn-nets. A similar behavior was observed recently for the silicides ThAuSi [30] and LuAuSi [31]. While ThAuSi crystallizes with an ordered AlB2-type (LiBaSi-type) structure, LuAuSi shows a slight puckering of the BN-like hexagonal AuSi-nets, resulting in the doubling of the c lat­tice constant.

In the structure of Er2Au2Sn there are two different crystallographic erbium positions, while there is only one uranium position in U2Rh2Sn. The Er 1 atoms have a coordination number (C.N.) of 10 (6 Au and 4 Sn). The average E r l - Au and E r l -S n distances amount ato 294.6 and339.4 pm, respectively. A similar near neighbor environment is observed for the Er2 atoms with average E r2 -A u and E r2 -S n distances of 298.3 and 346.9 pm, respectively. However, the E r2 atoms have a further Er 2 atom at 346.1 pm in their coordination shell, while the nearest E r l - E r l contact is 370.9 pm with a negligible bond­ing character. This difference in the coordination shell of the erbium atoms is certainly due to

the distortions in the superstructure. The larger coordination number of the Er 2 atoms is also reflected by the somewhat larger average Er 2 - Au and E r2 -S n distances.

The gold atoms in E r2Au2Sn are all within the distorted trigonal [Er6] prisms forming the A1B2- like fragment. Each gold atom has two other gold neighbors, one Au atom within the Au2Sn net­work at 280.8 pm and one other Au atom in the next Au2Sn plane at 327.9 pm. The coordination shell of the gold atoms is completed by six Er atoms at an average A u -E r distance of 296.5 pm and two Sn atoms, each at 307.1 pm. The differ­ence between the two A u-A u distances is quite large. The short A u-A u contact of 280.8 pm within the A1B2 fragments may certainly be con­sidered as strongly bonding. This distance is even somewhat smaller than the interatomic distance of288.4 pm in elemental gold [32]. Similar short A u- Au distances have also be determined in KAu5 [33] (277.4 and 283.1 pm), NaAu2 [34] (276.2 pm), and UAu2 [29] (274.6 pm). The A u-A u distances of 327.9 pm between the Au2Sn layers are about 40 pm longer than the corresponding distances in elemental gold and may therefore only be con­sidered as due to very weak interactions. However, in molecular compounds like [Au(/-C3H70 ) 2PS2]2 [35], i-C3H7NH2A u O C C 6H5 [36] or Au(III)(DM G )2Au(I)C12 [37] such secondary bonds (291.4 pm up to 327 pm) are sufficiently strong to cause dimerization in solution and poly­merization in the solid state. Similar weak A u- Au interactions were also observed in Au2P3 and Au7P 10l [38],

The tin atoms are located in the distorted square prisms of the erbium atoms. They have four Er neighbours at 339.4 pm and four Er neighbours at346.9 pm. The average S n -E r distance of 343.2 pm is only somewhat longer than the S n -E r bond length of 328.7 pm in binary ErSn3 [39] with Cu3Au-type structure. The coordination shell of the tin atoms is completed by four gold atoms at a distance of 307.1 pm. This is essentially the same value as the E r-A u distances of 306.1 pm in ErAu [40] with CsCl-type structure.

AcknowledgmentsI am grateful to Prof. Dr. Arndt Simon for his

interest and steady support of this work. I thank Dr. H. Borrmann for the collection of the four-

R. Pöttgen • Er2 Au2Sn

circle diffractometer data, W. Röthenbach for the Guinier powder patterns, and Dr. W. Gerhartz (Degussa AG) for a generous gift of gold metal. I

1313

am also indebted to the Stiftung Stipendienfonds des Verbandes der Chemischen Industrie for a Liebig fellowship.

[1] W. H. Zachariasen, A cta Crystallogr. 1, 265 (1948). [19[2] K. Remschnig, T. Le Bihan, H. Noel, P. Rogl, J.

Solid State Chem. 97, 391 (1992).[3] P. Villars, L. D. Calvert, Pearson 's H andbook of [20

Crystallographic D ata for Interm etallic Phases, Se­cond Edition, A m erican Society for Metals, M ate- [21 rials Park, O H 44073 (1991).

[4] A. O. Sampaio, E. Santa M arta, H. L. Lukas, G. Pet- [22 zow, J. Less-Common Met. 14, 472 (1968). [23

[5] A. P. Prevarskiy, Yu. B. Kuz’ma, M. S. Z rada, R us­sian Metallurgy 177 (1979).

[6] A. T. Tyvanchuk, T. I. Yanson, B. Ya. Kotur, O. S. [24 Zarechnyuk, M. L. K harakterova, Russian M etal­lurgy 190 (1988). [25

[7] Ya. M. Kalychak, V. I. Zarem ba, V. M. B aranyak, [26 P. Yu. Zavalii, V. A. Bruskov, L. V. Sysa, O. V. D m ytrakh, Inorg. M ater. 26, 94 (1990). [27

[8] G. Steinberg, H.-U. Schuster, Z. N aturforsch. 34b,1237 (1979). [28

[9] Ya. F. Lomnitskaya, Yu. B. Kuz’ma, Inorg. M ater.19, 1191 (1983). [29

[10] F. M iram bet, P. G ravereau, B. Chevalier, L. Trut, J. E tourneau, J. Alloys Com pounds 191, L I (1993). [30

[11] M. N. Peron, Y. K ergadallan, J. R ebizant, D. Meyer,J. M. W inand, S. Zwirner, L. H avela, H. N akotte,J. C. Spirlet, G. M. Kalvius, E. Colineau, J. L. O d- [31 dou, C. Jeandey, J. P. Sanchez, J. Alloys Com pounds 201, 203 (1993). [32

[12] F. M iram bet, B. Chevalier, L. Fournes, P. G ravereau,J. E tourneau, J. Alloys C om pounds 203, 29 (1994). [33

[13] C. G. Wilson, F. J. Spooner, A cta Crystallogr. 13, [34 358 (1960). [35

[14] P. G ravereau, F. M irambet, B. Chevalier, F. Weill, L. Fournes, D. Laffargue, J. E tourneau , 24lfemes Jour- [36 nees des Actinides, A bstract PB.12, O bergurgl (A u­stria) (1994). [37

[15] L. C. J. Pereira, J. M. W inand, F. Wastin, J. R ebizant, [38 J. C. Spirlet, 24lfemes Journees des Actinides, A b ­stract PB.9, O bergurgl (Austria) (1994). [39

[16] H. L. Krauss, H. Stach, Z. Anorg. Allg. Chem. 366, [40 34 (1969).

[17] K. Yvon, W. Jeitschko, E. Parthe, J. Appl. Crys­tallogr. 10, 73 (1977).

[18] G. M. Sheldrick, SHELX-86, Program for the Solu­tion of Crystal Structures, University of G öttingen, G erm any (1986).

G. M. Sheldrick, SHELXL-93, Program for the Crystal S tructure R efinem ent, University of G öttingen, G erm any (1993).H. B ärnighausen, Commun. M ath. Chem. 9, 139(1980).A. E. Dwight, J. W. Downey, R. A. C onner (Jr.), A cta Crystallogr. 22, 745 (1967).G. Bruzzone, A tti Acad. Naz. Lincei 48, 235 (1970). K. Schubert, T. R. A nantharam an, H. O. K. A ta,H. G. M eissner, M. Pötzschke, W. Rosstdeutscher, E. Stolz, Naturw issenschaften 47, 512 (1960).A. E. Dwight, M etallurgical Society Conferences, Proceedings 10, 383 (1961).A. Brown, A cta Crystallogr. 14, 860 (1961).A. Palenzona, S. Cirafici, J. Less-Common Met. 124, 245 (1986).A. D om m ann, F. Hulliger, J. Less-Common Met. 141, 261 (1988).A. Palenzona, S. Cirafici, J. Less-Common M et. 143, 167 (1988).R. Pöttgen, D. Kaczorowski, R.-D. Hoffm ann, V. H. Tran, to be published.J. H. A lbering, R. Pöttgen, W. Jeitschko, R.-D. H offm ann, B. Chevalier, J. E tourneau, J. Alloys C om pounds 206, 133 (1994).M. L. Fornasini, A. Iandelli, M. Pani, J. Alloys C om ­pounds 187, 243 (1992).J. D onohue, The S tructures of the Elem ents, Wiley, New York (1974).U. Zachw ieja, J. Alloys C om pounds 196,187 (1993). U. Zachw ieja, J. Alloys C om pounds 196, 171 (1993). S. L. Law ton, W. J. R ohrbaugh, G. T. Kokotailo, In ­org. Chem . 11, 2227 (1972).P. W. R. Corfield, H. M. M. Shearer, A cta Crys­tallogr. 23, 156 (1967).R. E. Rundle, J. Am. Chem. Soc. 76, 3101 (1954). W. Jeitschko, M. H. M öller, A cta Crystallogr. B35, 573 (1979).K. Miller, H. T. Hall, Inorg. Chem. 11, 1188 (1972).O. D. Me Masters, K. A. G schneidner (Jr.), G. Bruz­zone, A. Palenzona, J. Less-Com mon Met. 25, 135(1971).