Artikel

Preparation and Crystal Structure of the Ternary Lanthanoid PlatinumAntimonides Ln3Pt7Sb4 (Ln � Ce, Pr, Nd, and Sm) with Er3Pd7P4 TypeStructure

Tobias Schmidt and Wolfgang Jeitschko*

Münster, Anorganisch-Chemisches Institut der Westfälischen Wilhelms-Universität

Received October 31st, 2001.

Dedicated to Professor Joachim Strähle on the Occasion of his 65th Birthday

Abstract. The four compounds Ln3Pt7Sb4 (Ln � Ce, Pr, Nd, andSm) were prepared from the elements by arc-melting and subse-quent heat treatment in resistance and high-frequency furnaces. Thecrystal structure of these isotypic compounds was determined fromfour-circle X-ray diffractometer data of Nd3Pt7Sb4 [C2/m, a �

1644.0(2) pm, b � 429.3(1) pm, c � 1030.6(1) pm, β � 128.58(1)°,Z � 2, R � 0.032 for 698 structure factors and 46 variable parame-ters] and Sm3Pt7Sb4 [a � 1639.5(2) pm, b � 427.1(1) pm, c �

1031.8(1) pm, β � 128.76(1)°, Z � 2, R � 0.025 for 816 F-valuesand 46 variables]. The structure is isotypic with that of the homolo-

Darstellung und Kristallstruktur der ternären Lanthanoid-Platin-AntimonideLn3Pt7Sb4 (Ln � Ce�Nd und Sm) mit Er3Pd7P4-Struktur

Inhaltsübersicht. Die vier Verbindungen Ln3Pt7Sb4 (Ln � Ce�Nd,Sm) wurden durch Schmelzen der Elemente in einem Lichtbogeno-fen und anschließendes Tempern in Widerstands- und Hochfre-quenzöfen dargestellt. Die Kristallstruktur dieser isotypen Verbin-dungen wurde bestimmt anhand von Röntgen-Vierkreis-Diffrakto-meterdaten von Nd3Pt7Sb4 [C2/m, a � 1644.0(2) pm, b �

429.3(1) pm, c � 1030.6(1) pm, β � 128.58(1)°, Z � 2, R � 0.032für 698 Strukturfaktoren und 46 variable Parameter] undSm3Pt7Sb4 [a � 1639.5(2) pm, b � 427.1(1) pm, c � 1031.8(1) pm,β � 128.76(1)°, Z � 2, R � 0.025 für 816 F-Werte and 46 Variable].

IntroductionTernary rare earth (Ln) transition metal (T) antimonidesare investigated since almost thirty years. The cubic com-pounds LnTSb (T � Ni, Pt) with MgAgAs type structure[1] and Ln3Au3Sb4 with Y3Au3Sb4 type structure [2] seemto have been the first ones reported. In a brief review Fergu-son, Hushagen, and Mar list some 40 publications about thepreparation and crystal structures of such ternaries pub-lished up to 1996 [3]. More recently, the outstanding thermo-electric properties of serveral ternary antimonides,especially those with LnFe4P12 type structure havestimulated the research on such antimonides [4�8].

* Prof. Dr. W. JeitschkoAnorganisch-Chemisches Institut der UniversitätWilhelm-Klemm-Straße 8D-48149 Münster, Germany

Z. Anorg. Allg. Chem. 2002, 628, 927�932 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 0044�2313/02/628/927�932 $ 20.00�.50/0 927

gous phosphide Er3Pd7P4. In contrast to the structure of this phos-phide, where the phosphorus atoms have the coordination numbernine, the larger antimony atoms of Nd3Pt7Sb4 obtain the coordina-tion number ten. The structural relationships between the struc-tures of EuNi2�xSb2, EuPd2Sb2, CeNi2�xSb2�x, Ce3Pd6Sb5, andNd3Pt7Sb4, all closely related to the tetragonal BaAl4 (ThCr2Si2)type structure, are briefly discussed emphasizing their spacegroup relationships.

Keywords: Lanthanoids; Platinum; Antimonides

Die Struktur ist isotyp mit der des homologen PhosphidsEr3Pd7P4. Zum Unterschied von der Struktur dieses Phosphids,bei dem die Phosphoratome die Koordinationszahl neun haben,erreichen die größere Antimonatome in Nd3Pt7Sb4 die Koordina-tionszahl zehn. Die Strukturen von EuNi2�xSb2, EuPd2Sb2,CeNi2�xSb2�x, Ce3Pd6Sb5 und Nd3Pt7Sb4 können alle als Ab-kömmlinge der tetragonalen BaAl4- (ThCr2Si2-) Struktur betrach-tet werden. Dies wird mit Hilfe ihrer Raumgruppenbeziehungenverdeutlicht.

Recent publications on new ternary rare earth platinummetal antimonides and their crystal structures report on theseries LnRhSb and LnIrSb with the orthorhombic TiNiSi(PbCl2) type structure [9]. Other compounds containingrhodium as transition metal are the cubic antimonidesLn3Rh3Sb4 (Ln � La, Ce) with Y3Au3Sb4 type structure[10] and the series of the antimonides LnRh2Sb2 (Ln �La�Nd), which are isotypic with the closely related tetrago-nal structures of ThCr2Si2 or CaBe2Ge2 [11].

All of the other ternary rare earth platinum metal anti-monides reported recently contain palladium. Of these, theseries of compounds LnPd2Sb (Ln � Gd, Dy�Er, Yb) cry-stallize with the cubic Heusler (MnCu2Al) type structure[12]. Relatively complicated structures have been reportedfor the cerium palladium antimonides Ce3Pd6Sb5 [13],Ce2Pd9Sb3 [14], and Ce8Pd24Sb [15], while the antimonidesLn5Pd2Sb with Ln � Gd�Tm and Lu crystallize with orde-

T. Schmidt, W Jeitschko

red Cr5B3 (Mo5B2Si) type structure [16, 17]. Aside from thecompounds LnPtSb [1] and Ln3Pt3Sb4 [18], the presentlyreported series Ln3Pt7Sb4 seem to be the only known rareearth platinum antimonides. A brief account of the presentwork has already been given at a conference [19].

Sample Preparation and Lattice Constants

Starting materials were ingots of the rare earth elements (Heraeus,Kelpin), platinum powder (Degussa), and shots of antimony (John-son Matthey), all with stated purities equal or greater than 99.9 %.Pieces of the rare earth elements were arc-melted to buttons in or-der to minimize a shattering of these elements during the reaction.In a second step the platinum powder (pressed to pellets) and theantimony shots were added in the ideal 3 : 7 : 4 atomic ratio andreacted by arc-melting under 0.6 bar argon (99.996 %) which was(further) purified in a titanium sponge furnace. The arc-melted but-tons were turned over and remelted at least twice to enhance theirhomogeneity. The weight losses after several melting procedureswere smaller than 1 % in all cases. The pellets were subsequentlysealed into evacuated silica tubes, annealed at 1073 K for twoweeks, and quenched in air.

Well crystallized samples of Nd3Pt7Sb4 and Sm3Pt7Sb4 were ob-tained by annealing the arc-melted buttons in a water-cooled silicatube in a high-frequency furnace slightly below the melting pointfor 3 hours.

Compact ingots of the ternary antimonides have silveryshiny luster. The powders are dark grey and not sensitiveto air. Energy-dispersive X-ray fluorescence analyses in ascanning electron microscope did not reveal any impurityelements heavier than sodium.

The samples were characterized by Guinier powder X-raydiagrams recorded with Cu K�1 radiation using α-quartz(a � 491.30 pm, c � 540.46 pm) as an internal standard.The observed diffraction diagrams were identified by com-parision with calculated diagrams [20] eventually using thepositional parameters of the refined structures. The latticeconstants listed in Table 1 were obtained by least-squaresfits.

Structure Refinements

The single-crystals of Nd3Pt7Sb4 and Sm3Pt7Sb4 were isolated fromthe crushed samples annealed in a high-frequency furnace. Theywere examined by Laue photographs on a Weissenberg camera toestablish their suitability for the intensity data collection. Thesedata were collected on a four-circle diffractometer (Enraf-NoniusCAD4) with graphite-monochromated Mo K� radiation and a

Table 1 Lattice constants of the monoclinic lanthanoid platinumantimonides as obtained from Gunier powder dataa)

Compound a (pm) b (pm) c (pm) β (°) V (nm3)

Ce3Pt7Sb4 1658.2(3) 431.4(1) 1030.5(2) 128.50(1) 0.5769(1)Pr3Pt7Sb4 1646.2(2) 430.6(1) 1031.1(1) 128.48(1) 0.5722(2)Nd3Pt7Sb4 1646.6(2) 429.9(1) 1030.9(1) 128.58(1) 0.5705(2)Sm3Pt7Sb4 1637.9(2) 426.7(1) 1031.1(1) 128.74(1) 0.5621(2)

a) Standard deviations in the place values of the least significant digits aregiven in parentheses throughout the paper.

Z. Anorg. Allg. Chem. 2002, 628, 927�932928

Table 2 Crystallographic data of Nd3Pt7Sb4 and Sm3Pt7Sb4

Compound Nd3Pt7Sb4 Sm3Pt7Sb4

Formula mass 2285.3 2303.7Space group C2/m (No. 12) C2/m (No. 12)Lattice constantsa/pm 1644.0(2) 1639.5(2)b/pm 429.3(1) 427.1(1)c/pm 1030.6(1) 1031.8(1)β/° 128.58(1) 128.76(1)V/nm3 0.5686(1) 0.5634(1)Formula units/cell, Z 2 2Pearson symbol mS28 mS28Calculated density (g/cm3) 13.35 13.58Crystal size (µm3) 10 � 10 � 20 10 � 20 � 20Scan range up to 2θ/° 60 60Range in hkl ±22, ±6, ±14 ±22, ±6, ±14Total no. of reflections 3288 3228Independent F values 927 911Internal residual, Ri (F2) 0.084 0.067Reflections with Io �2σ (Io) 698 816Number of variables 46 46Conv. residual, R (Fo �2σ) 0.032 0.025Weighted residual, Rw (all F2) 0.076 0.055Highest residual peak (e/A3) 4.84 3.33

scintillation counter with pulse-height discrimination. The scanswere along θ with background counts at both ends of each scan.Empirical absorption corrections were applied from ψ scan data.Further details of the data collections are summarized in Table 2.The lattice constants, refined from the four-circle diffractometerdata, are in good agreement with those determined from the Gunierpowder data (Table 1).The structures were determined and refined with the program pack-age SHELX-97 [21]. The C centered centrosymmetric monoclinicspace group C2/m (No. 12) suggested by the program turned outto be correct during the structure refinements. The positions ofmost heavy atoms were determined by direct methods, and the re-maining atoms were located by difference Fourier syntheses. Thestructures were refined with a full-matrix least-squares programusing atomic scattering factors, corrected for anomalous disper-sion, as provided by the program [21]. The weighting schemes re-flected the counting statistics, and a parameter correcting for iso-

Table 3 Atomic parameters of Nd3Pt7Sb4 and Sm3Pt7Sb4a)

Atom C2/m x y z U11 U22 U33 U13 U/pm2

Nd3Pt7Sb4

Nd1 4i 0.1740(1) 0 0.6853(2) 73(6) 57(5) 77(6) 48(5) 68(3)Nd2 2b 0 1/2 0 81(9) 105(8) 96(9) 53(8) 96(4)Pt1 4i 0.01363(8) 0 0.3176(1) 63(5) 73(4) 104(5) 42(4) 87(2)Pt2 4i 0.17030(8) 0 0.2750(1) 124(5) 99(4) 117(5) 93(4) 101(2)Pt3 4i 0.37658(8) 0 0.1530(1) 70(5) 56(4) 66(4) 39(4) 67(2)Pt4 2d 0 1/2 1/2 59(6) 70(5) 52(6) 32(5) 62(3)Sb1 4i 0.1714(1) 0 0.0257(2) 86(8) 44(6) 71(7) 48(6) 68(3)Sb2 4i 0.3629(1) 0 0.5916(2) 87(8) 56(6) 61(7) 47(7) 68(3)

Sm3Pt7Sb4

Sm1 4i 0.17458(5) 0 0.68670(8) 97(3) 75(3) 91(3) 64(2) 84(2)Sm2 2b 0 1/2 0 88(4) 128(5) 106(4) 59(4) 109(2)Pt1 4i 0.01438(4) 0 0.32157(6) 88(2) 84(3) 115(2) 62(2) 97(1)Pt2 4i 0.16957(4) 0 0.27277(6) 141(3) 124(3) 130(2) 107(2) 116(1)Pt3 4i 0.37646(4) 0 0.15376(6) 96(2) 74(2) 81(2) 54(2) 85(1)Pt4 2d 0 1/2 1/2 102(3) 84(3) 74(3) 60(3) 83(2)Sb1 4i 0.17082(7) 0 0.0250(1) 103(4) 75(4) 75(3) 58(3) 83(2)Sb2 4i 0.36204(7) 0 0.5908(1) 108(4) 59(4) 86(4) 62(3) 83(2)

a) The positional parameters were standardized with the program STRUCTURE TIDY[22]. The last column contains the equivalent isotropic displacement parameters. Theanisotropic parameters U12 and U23 equal zero for all atoms of these structures.

The Ternary Lanthanoid Platinum Antimonides Ln3Pt7Sb4 (Ln � Ce, Pr, Nd, and Sm)

Table 4 Interatomic distances in the structures of the antimonidesLn3Pt7Sb4 (Ln � Nd and Sm)a)

Nd/Sm Nd/Sm

Ln1: 1Pt1 297.6/294.9 Pt3: 2Sb1 261.2/260.11Pt1 307.2/304.2 1Sb1 275.8/274.12Pt4 310.7/308.9 2Pt1 279.6/278.52Pt3 313.5/311.1 1Pt4 280.4/279.32Pt2 317.0/314.8 2Ln1 313.5/311.12Sb1 324.3/322.2 1Ln2 325.8/325.92Sb2 331.2/330.9 1Sb2 335.4/334.11Sb1 353.6/352.7 [1Ln1 378.3/376.8][1Pt3 378.3/376.8] Pt4: 2Pt3 280.4/279.3[1Sb2 378.9/376.9] 2Sb2 294.0/293.7

Ln2: 4Pt2 325.7/322.5 4Pt1 295.5/292.12Pt3 325.8/325.9 4Ln1 310.7/308.92Sb2 331.2/331.3 Sb1: 1Pt2 258.2/256.74Sb1 342.4/339.8 2Pt3 261.2/260.1[4Pt1 380.5/382.4] 1Pt3 275.8/274.1

Pt1: 2Sb2 269.4/267.9 1Pt1 289.1/289.92Pt3 279.6/278.5 2Ln1 324.3/322.21Pt2 287.4/288.4 2Ln2 342.4/339.81Sb1 289.1/289.9 1Ln1 353.6/352.72Pt4 295.5/292.1 [2Sb1 364.9/363.6]1Ln1 297.6/294.9 Sb2: 2Pt1 269.4/267.91Ln1 307.2/304.2 2Pt2 278.4/277.7[2Ln2 380.5/382.4] 1Pt2 279.3/278.4

Pt2: 1Sb1 258.2/256.7 1Pt4 294.0/293.72Sb2 278.4/277.7 1Ln2 331.2/331.31Sb2 279.3/278.4 2Ln1 331.2/330.91Pt1 287.4/288.4 1Pt3 335.4/334.12Ln1 317.0/314.8 [2Sb2 364.1/359.6]2Ln2 325.7/322.5 [1Ln1 378.9/376.9]

a) The distances were calculated with the lattice constants as obtained fromthe Guinier powder data (Table 1). All distances shorter than 400 pm(Ln�Ln, Ln�Pt, Ln�Sb) and 370 pm (all other distances), respectively, arelisted. Standard deviations are all equal or less than 0.3 pm. Atoms withdistances listed in brackets are not shown as neighbors in Fig. 3.

tropic secondary extinction was optimized as a least-squares varia-ble. To check for deviations from the ideal compositions, the occu-pancy parameters were varied in one series of least-squares cyclestogether with variable thermal parameters. The occupancy parame-ters of all atomic positions were very close to the ideal values. ForNd3Pt7Sb4 they varied between 98.9(8) % for the Pt3 and 101(1) %for the Sb2 positions; for Sm3Pt7Sb4 the corresponding values were99.3(7) for Sb1 and 100.2(7) % for the Sb2 position. Consequently,in the final least-squares cycles all atomic positions were refinedwith the ideal occupancies. The resulting atomic parameters andthe interatomic distances are listed in the Tables 3 and 4.

Discussion

We report the crystal structure of the four new lanthanoidplatinum antimonides Ln3Pt7Sb4 (Ln � Ce�Nd, Sm) whichwe have determined and refined for the two compoundsNd3Pt7Sb4 and Sm3Pt7Sb4. The structure is shown in Fig.1 with the neodymium compound as representative exam-ple. All atoms are situated at two equivalent mirror planes,which extend perpendicular to the short two-fold axis. Wehave connected the atoms at the two different heights of theprojection direction by thick and thin lines. These linesshould help in visualizing the structure. They should notmislead the reader to believe that the structure has a layeredcharacter, since chemical bonding within and between these

Z. Anorg. Allg. Chem. 2002, 628, 927�932 929

Fig. 1 Projection of the crystal structure of Nd3Pt7Sb4 along they axis. All atoms are situated on mirror planes, which are separatedfrom each other by half a translation period of the projection direc-tion. These atoms are connected by heavy (y � 1/2) and light lines(y � 0), which do not necessarily correspond to chemical bonds.The crystal structures of the present publication were all drawnusing the program DIAMOND [24].

Fig. 2 Near-neighbor environments of the atoms in the structure ofthe compound Nd3Pt7Sb4.

topological layers is of equal strength, as can be judged bylooking at the near-neighbor coordinations (Fig. 2).

The neodymium atoms occupy two different crystallo-graphic sites. Both with high coordination numbers whichare not readily defined. This is especially true for the Nd1atom, where the Sb1 neighbor at 353.6 pm is shown in Fig.2 as belonging to its coordination, while the next antimony

T. Schmidt, W Jeitschko

atom (Sb2) at 378.9 pm may already be considered as notbelonging to the near-neighbor environment of Nd1. Consi-dering the Nd�Pt interactions, we may draw a line between325.8 pm (Nd2�Pt3) as the longest weakly bonding inte-raction and 378.3 pm (Nd1�Pt3) as the shortest non-bon-ding Nd�Pt distance. Thus, including the weakly bondingNd1�Sb2 interaction of 378.9 pm, the Nd1 and Nd2 atomsobtain the coordination numbers (CN) 14 (6Sb � 8Pt) and12 (6Sb � 6 Pt), respectively. Certainly, the bonding interac-tions between the most electropositive (neodymium) andthe most electronegative (antimony) atoms of the com-pound will be more important than the Nd�Pt interactions.However, considering the high platinum content of thecompound, the latter must not be discounted. The Nd�Ptdistance of 287.0 pm, computed from the lattice constantof Cu3Au type NdPt3 [25], is not that much shorter thanthe average Nd1�Pt and Nd2�Pt distances of 310.9 pmand 325.7 pm, respectively. The shortest Nd�Nd distanceamounts to 419.7 (Nd1�Nd2) pm. It is considerably longerthan the average Nd�Nd distance of 362.8 pm in the hexa-gonal close packed structure of the element, calculated byus from the lattice constants [26]. Thus, in view of the factthat the neodymium atoms have � since they are the mostelectropositive component of the compound � largelytransferred their valence electrons to the platinum and anti-mony atoms, there is very little Nd�Nd bonding.

The four different platinum sites have rather differingcoordinations. One extreme case is the Pt2 atom with fourantimony atoms in distorted tetrahedral coordination andonly one platinum neighbor. The other extreme is the Pt4atom with only two antimony neighbors in linear arrange-ment and six platinum neighbors. The Pt�Sb distances cov-er the range from 258.2 pm (Pt2�Sb1) to 335.4 pm(Pt3�Sb2). The Pt�Pt distances are all of rather similarlength ranging between 279.6 pm (Pt1�Pt3) and 295.5 pm(Pt1�Pt4). The average Pt�Pt distance of 287.2 pm is onlysomewhat greater than the Pt�Pt distance of 277.4 pm inthe cubic close packed element [26]. Thus, there is conside-rable Pt�Pt bonding, in agreement with the fact that thecompound contains 50 at-% platinum. In addition to thebetween five and eight antimony and platinum neighborseach platinum atom has four neodymium neighbors in-creasing their CN to between nine and twelve.

There are only two kinds of antimony atoms in the struc-ture. They are isolated from each other and located in trigo-nal prisms formed by the metal atoms. The rectangular fa-ces of these prisms are capped, two of them by one metalatom each, and the third one is capped by two metal atoms,thus increasing their CN to ten.

Using oxidation numbers, where the more or less cova-lently bonding electrons are counted at the bonding partnerwith the higher electronegativity, the compound may be re-presented by the formula (Nd3�)3(Pt7Sb4)9�. Since there areno Sb�Sb bonds, and since the 5s and 5p orbitals of theantimony atoms can be expected to fully participate in themore or less covalent bonding with the metal atoms (octettrule), we can assign the oxidation number (formal charge)

Z. Anorg. Allg. Chem. 2002, 628, 927�932930

3� to the antimony atoms, resulting in the formula(Nd3�)3(Pt7)3�(Sb3�)4. Since a neutral platinum atom has10 valence electrons, the average platinum atom obtains 9.6electrons which are available for the many Pt�Pt bondinginteractions. However, since the electron count of the plati-num atoms should not exceed 18, and since the platinumatoms already use a great portion of their valence orbitalsfor bonding with the other atoms, most of the remaining 9.6electrons of an average platinum metal will be nonbonding.

The structure of Nd3Pt7Sb4 is isotypic with a structurefirst reported by Johrendt and Mewis for the isoelectronicphosphide Er3Pd7P4 [23]. These two structures and theirunit cells are compared in Fig. 3. The standardized [22] Ccentered monoclinic cells are drawn with continuous linesfor both structures. For these two cells the positional para-meters of the two structures are completely different, eventhough the two structures are isotypic. The main differenceof the two structures can be seen in the coordination of thephosphorus and antimony atoms. The phosphorus atomswith CN 9 are situated in the centers of the trigonal prismsoutlined in Fig. 3, whereas the antimony atoms are locatedoff these centers and have moved towards a neodymiumatom, which becomes their tenth neighbor. As alreadynoted by Johrendt and Mewis the Er3Pd7P4 type structure

Fig. 3 Projections of the crystal structures of Er3Pd7P4 andNd3Pt7Sb4 along the short axes. All atoms lie on mirror planes attwo different heights of the projection direction. The trigonalprisms around the pnictogen atoms are emphasized. In the upperpart of the figure the monoclinic cell is shown in the setting of theoriginal publication on Er3Pd7P4 [23]. A monoclinic cell, with β �

128.58° is outlined for Nd3Pt7Sb4 in the lower part of the figure.This is the cell as obtained by standardization from the programSTRUCTURE TIDY [22]. A pseudo-orthorhombic body-centeredmonoclinic cell I2/m11 with a � 90.17° is outlined with dottedlines. This is the cell as shown for this structure in Fig. 4.

The Ternary Lanthanoid Platinum Antimonides Ln3Pt7Sb4 (Ln � Ce, Pr, Nd, and Sm)

Fig. 4 The crystal structure of Nd3Pt7Sb4 and some other closely related antimonides as derived from the tetragonal BaAl4 (ThCr2Si2) typestructure. The group-subgroup relationships are shown, giving the space groups symbols, the unit cell transformations, and the indices ofthe translationsgleiche (t), klassengleiche (k), and isomorphic (i; the same space group with multiple translational symmetry) subgroups.The symbol � indicates nonoccupied atomic sites.

may be considered as belonging to a large family of structureswith a metal : nonmetal ratio of 2 : 1 or close to that ratio[27�30]. These structures are usually classified by the waythe trigonal prisms, emphasized in Fig. 3, are linked to eachother. The Er3Pd7P4 type structure deviates from the ideal2 : 1 ratio. This has in part to do with the Pd3 atom ofEr3Pd7P4 which corresponds to the Pt4 atom in the standar-dized setting of Nd3Pt7Sb4. This atom does not participate

Z. Anorg. Allg. Chem. 2002, 628, 927�932 931

in the formation of the trigonal prisms surrounding thephosphorous and antimony atoms.

In Fig. 3 we show a pseudo-orthorhombic cell forNd3Pt7Sb4, as indicated by dotted lines. This cell corre-sponds to the tripled pseudo-tetragonal cell of Nd3Pt7Sb4,as outlined in the lower right-hand corner of Fig. 4. In thisfigure we show that this structure can be derived from thewell-known ThCr2Si2 type structure using mostly examples

T. Schmidt, W Jeitschko

of ternary antimonides, i.e., the ThCr2Si2 type structure ofEuNi2�xSb2 [31] and the CaBe2Ge2 type structure ofEuPd2Sb2 [32]. The structure of CeNi2�xSb2�x [33] is re-markable, since the atomic positions have nearly tetragonalsymmetry and only the lattice dimensions are orthorhom-bic. This description of the structure probably only repre-sents a subcell. The tripled pseudo-tetragonal cells ofCe3Pd6Sb5 [13], La3Al11 [34], and Dy3Co6Sn5 [35] have alre-ady been shown to be derivable from the ThCr2Si2 typestructure [36].

Acknowledgments. We thank Mrs. Dipl.-Ing. U. Ch. Rodewald forcollecting the single-crystal diffractometer data and Mr. H.-J.Göcke for the work at the scanning electron microscope. We alsoappreciate the generous gifts of silica tubes and platinum powderfrom Dr. C. Höfer (Heraeus Quarzschmelze) and Dr. W. Gerhartz(Degussa). Furthermore, this work was supported by the DeutscheForschungsgemeinschaft, the Fonds der Chemischen Industrie, andthe International Centre for Diffraction Data.

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