DOI: 10.1002/zaac.200600266

Discrete Pr6Pd Octahedra in Pr6Pd13Cd4 � An Intermetallic Analogon to theSubnitride Na16Ba6N

Ahmet Dogan, Rolf-Dieter Hoffmann, and Rainer Pöttgen*

Münster, Institut für Anorganische und Analytische Chemie der Universität

Received September 26th, 2006.

Abstract. The new intermetallic compound Pr6Pd13Cd4 was synthe-sized from the elements in a sealed tantalum ampoule in an induc-tion furnace. Pr6Pd13Cd4 was investigated by X-ray powder andsingle crystal diffraction: Na16Ba6N type, Im3m, a � 975.6(1) pm,wR2 � 0.0192, 162 F2 values and 12 variables. The striking motifof the Pr6Pd13Cd4 structure are discrete palladium centred Pr6 octa-

Introduction

The crystal chemistry of rare earth (RE)-transition metal(T)-cadmium compounds strongly depends on the compo-sition of the respective compound. In the structures ofLaNiCd2, PrNiCd2, LaPdCd2 [1, 2] and ErCuCd2 [3], witha relatively high cadmium content, one observes formationof three-dimensional cadmium networks with Cd�Cd dis-tances similar to hcp cadmium. In the equiatomic com-pounds REPdCd [4, 5] and REAuCd [6], the T and Cdatoms build up three-dimensional [PdCd] and [AuCd] poly-anions which leave distorted hexagonal channels for therare earth atoms.

With a higher transition metal content compounds likeRE2T2Cd (T � Ni, Pd, Pt, Au) [7, 8, and ref. therein],RECu5�xCdx (RE � Ce, Gd, Tb, Yb) [9] or the orderedLaves phases CeNi4Cd and RECu4Cd (RE � Ho, Er, Tm,Yb) [10] form. The transition metal atoms build T2 dumb-bells or zig-zag chains in the RE2T2Cd phases buth three-dimensional transition metal networks in the RET4Cd andRECu5�xCdx compounds. With a high rare earth metalcontent we recently discovered new compounds RE4CoCdand RE4RhCd (RE � Tb, Dy, Ho) [11] with Gd4RhIn [12,13] type structure. The striking structural motif of theseintermetallics are three-dimensional networks of cobalt(rhodium) centered trigonal RE prisms. In the course of oursystematic phase analytical studies of RExTyCdz inter-metallic compounds, we have now discovered Pr6Pd13Cd4

which contains discrete palladium centred Pr6 octahedra.

* Prof. Dr. Rainer PöttgenInstitut für Anorganische und Analytische Chemie, UniversitätMünsterCorrensstrasse 30D-48149 Münster/Germanye-mail: pottgen@uni-muenster.de

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hedra (296 pm Pr�Pd1) in bcc packing. The octahedra are embed-ded by a three-dimensional [Pd3Cd] network with short Pd�Pd(282 pm) and Pd�Cd (274 pm) distances. The structural similari-ties with the subnitrides Na16Ba6N and Ag16Ca6N are discussed.

Keywords: Rare Earth Compounds; Crystal Chemistry

This peculiar crystal structure and the crystal chemical re-lations to the subnitrides Na16Ba6N and Ag16Ca6N [14] arediscussed herein.

Experimental Section

Synthesis

Starting materials for the synthesis of Pr6Pd13Cd4 were a praseo-dymium ingot (Johnson Matthey), palladium powder (Degussa-Hüls, ca. 200 mesh) and a cadmium rod (Johnson-Matthey), allwith a stated purity better than 99.9 %. Pieces of the praseodymiumingot were first arc-melted into small buttons under purified argon.The argon was purified before over molecular sieves, silica gel andtitanium sponge (900 K). The elements were then weighed in theideal 6:13:4 atomic ratio and sealed in a small tantalum tube underpurified argon [15]. The tantalum ampoule was then placed in awater-cooled quartz sample chamber of a high frequency furnace(Hüttinger Elektronik, Freiburg, type TIG 1.5/300) under flowingargon [16] and first heated at 1600 K for about one minute followedby annealing at around 900 K for four hours. The temperature wascontrolled through a Sensor Therm Methis MS09 pyrometer withan accuracy of ±30 K. The sample could easily be separated fromthe crucible material. No reaction with the container was observed.Pr6Pd13Cd4 is stable in moist air over months in powdered as wellas in polycrystalline form. Single crystals exhibit metallic lustrewhile ground powder is dark grey.

EDX Analyses

The single crystal investigated on the diffractometer and the bulksample of Pr6Pd13Cd4 were studied by energy dispersive analysesof X-rays (EDX) using a Leica 420i scanning electron microscopewith PrF3, palladium, and cadmium as standards. The experimen-tally observed composition (25±2 at.-% Pr : 58±2 at.-% Pd : 17±2at.-% Cd) was close to the ideal one (26.1 : 56.5 : 17.4) and noimpurity elements were observed. The standard uncertainty ac-counts for the measurements at different points of the irregularlyshaped sample.

A. Dogan, R.-D. Hoffmann, R. Pöttgen

X-ray Image Plate Data and Data Collection

The annealed sample was studied by X-ray powder diffraction(Guinier technique) using CuK�1 radiation and �-quartz (a �

491.30, c � 540.46 pm) as an internal standard. The Guiniercamera was equipped with a Fujifilm / BAS�1800 image platesystem. The cubic lattice parameter was refined from the Guinierdata. To ensure correct indexing, the observed pattern was com-pared with a calculated one [17] taking the atomic positionsobtained from the structure refinement. The single crystal (a �

975.64(6) pm) and powder lattice parameter were in good agree-ment.

High-quality single crystals were selected from the annealed sampleby mechanical fragmentation. They were first investigated by Lauephotographs on a Buerger camera (white molybdenum radiation;imaging plate technique, Fujifilm, BAS-1800) in order to check thequality for intensity data collection. Intensity data of a suitablecrystal were collected at room temperature by use of a four-circlediffractometer (CAD4) with graphite monochromatized Mo Kα

(71.073 pm) radiation and a scintillation counter with pulse heightdiscrimination. The scans were taken in the ω/2θ mode and anempirical absorption correction was applied on the basis of psi-scan data, followed by a spherical absorption correction. All rel-evant crystallographic data and details of the data collections arelisted in Table 1.

Results and Discussion

Structure Determination and Refinement

The diffractometer data set revealed a body-centered cubiccell and no other systematic extinctions, leading to the pos-sible space groups Im3m, I43m, Im3, and I432. The spacegroup with the highest symmetry, Im3m was found to becorrect during the structure refinement. The starting atomicparameters were deduced from an automatic interpretationof direct methods with S-97 [18] and the structure wassucessfully refined using S-97 [19] (full-matrix least-squares on F2) with anisotropic atomic displacement par-ameters for all atoms. A literature search readily revealedisotypism with the subnitride Na16Ba6N [14]. In the sub-sequent cycles, the structure was refined with the standardsetting of Na16Ba6N. Since palladium and cadmium differonly by two electrons, the site occupancy parameters wererefined in a separate series of least-squares cycles, in orderto check the correct assignment of the Wyckoff sites. Thehigh quality of the data set (Table 1) revealed precise occu-pancy parameters (Table 2). All sites were fully occupiedwithin less than two standard uncertainties and in the finalcycles the ideal occupancy parameters were assumed again.Final difference Fourier syntheses revealed no significantresidual peaks. The positional parameters and interatomicdistances of the refinement are listed in Tables 2 and 3.Further details on the structure refinement are available.1)

1) Details may be obtained from: Fachinformationszentrum Karls-ruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quotingthe Registry No. CSD�417049.

www.zaac.wiley-vch.de 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2007, 219�222220

Table 1 Crystal data and structure refinement for Pr6Pd13Cd4.

Empirical formula Pr6Pd13Cd4

Space group, Z Im3m, 2Molar mass 2678.26 g/molUnit cell dimensions a � 975.6(1) pm(powder data) V � 0.9286 nm3

Calculated density 9.58 g/cm3

Crystal size 30 x 30 x 70 µm3

Transm. ratio (max/min) 1.99Absorption coefficient 32.0 mm�1

F(000) 2288θ range 2° to 30°Range in hkl ±13, ±13, ±13Total no. reflections 5424Independent reflections 162 (Rint � 0.0609)Reflections with I > 2σ(I) 149 (Rsigma � 0.0125)Data/parameters 162 / 12Goodness-of-fit on F2 1.437Final R indices [I > 2σ(I)] R1 � 0.0082; wR2 � 0.0190R indices (all data) R1 � 0.0098; wR2 � 0.0192Extinction coefficient 0.00216(5)Largest diff. peak and hole 0.55 / �0.57 e/A3

Table 2 Atomic coordinates and isotropic displacement parame-ters (pm2) for Pr6Pd13Cd4. Ueq is defined as one third of the traceof the orthogonalized Uij tensor. The occupancy parameters havebeen obtained in a separate series of least-squares cycles.

Atom Wyckoff occup. / % x y z Ueq

position

Pr 12e 99.8(2) 0.30303(3) 0 0 71(1)Pd1 2a 99.9(6) 0 0 0 92(2)Pd2 24h 100.3(2) 0 0.34071(2) y 70(1)Cd 8c 100.1(3) 1/4 1/4 1/4 87(1)

Table 3 Interatomic distances (pm), calculated with the powderlattice parameters in Pr6Pd13Cd4. Standard deviations are all equalor smaller than 0.1 pm. All distances within the first coordinationspheres are listed.

Pr: 4 Pd2 291.9 Pd2: 2 Cd 274.11 Pd1 295.6 4 Pd2 282.24 Pd2 334.4 2 Pr 291.94 Cd 348.8 2 Pd2 310.81 Pr 384.3 2 Pr 334.44 Pr 418.1 Cd: 6 Pd2 274.1

Pd1: 6 Pr 295.6 6 Pr 348.8

Crystal Chemistry

The new intermetallic compound Pr6Pd13Cd4 crystallizes with a siteoccupancy variant of the subnitride Na16Ba6N [14]. The praseo-dymium atoms occupy the barium site and the Pd1 atoms take thenitrogen position. The two crystallographically independent so-dium sites 24h and 8c are occupied in an ordered manner by thePd2 and Cd atoms.

A view of the Pr6Pd13Cd4 unit cell is presented in Figure 1. Themain structural motif is defined by Pd1 centered Pr6 octahedrawhich show a bcc packing, similar to the nitrogen centered Ba6

octahedra in the subnitride. Considering the course of the electro-negativities (palladium as the most electronegative component inPr6Pd13Cd4 takes the role of the nitride anions), this crystal chemi-cal analogy is reasonable. The subnitride shows ionic bonding

The Structure of Pr6Pd13Cd4

Fig. 1 Crystal structure of Pr6Pd13Cd4. Praseodymium, palla-dium, and cadmium atoms are drawn as medium grey, black, andopen circles, respectively. The palladium centred Pr6 octahedraare emphasized.

within the Ba6N unit with short Ba�N distances of 283 pm [14].In the Pr6Pd1 octahedra of Pr6Pd13Cd4 we can assume a high de-gree of covalent Pr�Pd1 bonding since the Pr�Pd1 distances of296 pm are close to the sum of the covalent radii of 293 pm [20].Similar distances (291 pm) have also been observed in the pal-ladium centered trigonal prisms in Pr2Pd2Cd [11].

In Na16Ba6N the Ba6N octahedra are embedded in a metallic ma-trix of sodium atoms. Similar coordination is observed forPr6Pd13Cd4. As emphasized in Figure 2, each Pr6Pd1 octahedronis surrounded in an ordered manner by a complex [Pd3Cd] networkthat completely separates the Pr6Pd1 octahedra. Within the[Pd3Cd] network we observe short Pd�Pd (282 pm) and Pd�Cd(274 pm) distances. The Pd�Pd distances compare well with thosein fcc palladium (275 pm) [21] and the Pd�Pd distance of 281 pmin the Pd2 dumb-bell of Pr2Pd2Cd [11]. The Pd�Cd distances areclose to the sum of the covalent radii (269 pm) and shorter than inPr2Pd2Cd (305 pm) [11].

The motif of transition metal centred trigonal prisms or antiprismsseems to govern the crystal chemistry of rare earth metal richRExTyCdz cadmium and RExTyInz indium compounds. Whilethree-dimensional networks of corner-sharing trigonal prisms areobserved in the series of RE4TX [11�13, 22] and RE14T3In3 [23,and ref. therein] compounds, Pr6Pd13Cd4 is the first example withdiscrete RE6 octahedra.

Finally we need to point out that the Pr6Pd13Cd4 structure belongsto a larger family of compounds with transition metal centeredoctahedra. Such structural units, filled with manganese, iron, orosmium atoms have been observed for many rare earth metal clus-ter compounds like LiLa6I12Os, Cs4Pr6I13Os, KPr6I10Os,CsPr6I10Fe, CsLa6I10Fe, or CsLa6I10Mn [24�26]. In that contextone should also consider the close structural relationship betweenBa2Pt and Ba2N [27] and the isotypy of [PtIn6](GeO4)2O with themineral [Na6F](SO4)2Cl [28].

Z. Anorg. Allg. Chem. 2007, 219�222 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 221

Fig. 2 View of the Pr6Pd13Cd4 structure along the threefold axis.Praseodymium, palladium, and cadmium atoms are drawn asmedium grey, black, and open circles, respectively. The Pr6Pd1octahedron at the top corner of the unit cell is not drawn. Thethree-dimensional [Pd3Cd] network surrounding the octahedra isemphasized.

We thank Dipl.-Ing. U. Ch. Rodewald for the intensity datacollection. This work was financially supported by the DeutscheForschungsgemeinschaft.

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