Ternary Rare Earth Transition Metal Zinc Compounds RT2Zn2o with T = Fe, Ru, Co, Rh, and NiTono Nasch, Wolfgang Jeitschko*, Ute Ch. RodewaldAnorganisch-C hem isches Institut, Universität Münster,W ilhelm -K lem m -Straße 8, D -48149 Münster, Germany

Z. N aturforsch. 52 b, 1023-1030 (1997); received May 6, 1997

Crystal Structure, Transition M etal Atoms

Forty eight new com pounds RT2Z n2o were prepared by annealing cold-pressed pellets o f the elem ental com ponents in an argon atm osphere. They crystallize with the cubic CeCr^Abo type structure (Fd3m , Z = 8), which was refined from single-crystal diffractom eter data o f TbFeiZn^o (a = 1411.1(1) pm), Y Ru2Z n2o (a = 1422.6(1) pm), D yRu2Z n2o (a = 1422.1(1) pm), G dC o2Z n2o (a = 1406.0(1) pm), D yR h2Zn2o (a = 1418.2(1) pm), andT m N i2Z n2o (a= 1401.6(1) pm) to conventional residuals varying between R = 0.011 and R - 0.024. The com pounds have a tendency for tw inning, thus mim icking hexagonal symmetry, with the cubic [111] axis as the axis with the pseudohexagonal symmetry. M inor inconsistencies in the cell volum es of these com pounds indicate slight deviations from the ideal com position. N evertheless, the five atom ic sites o f this structure w ere found to be fully occupied within the error limits with the exception o f one zinc site o f T m N i2Zn2o- The coordination for the site o f the rare earth atom s is a Frank-K asper polyhedron with coordination num ber (CN) 16. The transition metal atoms occupy a site with icosahedral zinc coordination (CN 12). Two o f the three zinc sites are in pentagonal prism atic coordination o f zinc atom s, capped by rare earth and/or transition metal atom s (CN 12), while the third zinc site has 12 zinc neighbors form ing a hexagonal prism, which is capped by two rare earth atom s (CN 14).

Introduction

Very little is known about ternary compounds of the rare earth and transition elements with zinc as the third component. Apparently, only two such ternary compounds have been reported: rhombo- hedral Ce2NisZn2 with a new ordered substitution variant of the Er2Co7 type structure and the hexag­onal compound CeNi2Zn with YRl^Si type struc­ture [1]. We have recently characterized a large number of ternary rare earth (R) transition metal (T) aluminides with a high content of aluminum: R6T4AI43 [2, 3], RT2A120 [4], R7+, Rei2Al61+, [5], and RFe2Alio [6 ]. In this context we were inter­ested to know whether similar compounds could be prepared, where zinc is taking the place of alu­minum. The compounds reported here are isotypic with CeCr2Al2o [V]. This structure was reported for almost 50 aluminides RT2AI20 [4, 7-10]. However, while the aluminides are formed with the early tran­sition elements T = Ti, V, Nb, Ta, Cr, Mo, and W,

* Reprint requests to W. Jeitschko.

the corresponding zinc compounds reported here contain late transition elements.

Sample Preparation

Starting materials for the preparation of the com­pounds RT2Zn2o were ingots of the light rare earth metals and coarse powders of the heavy lanthanoids (Rhone-Poulenc; Kelpin, all > 99.9 %), powders of cobalt (Ventron, 325 mesh, 99.5 %), iron (Ventron, 325 mesh, 99.5 %), nickel (Merck, <10 /im, >99.5 %), rhodium (Merck, <60 /im, 99.9 %), ruthenium (Heraeus, <60 ^m, 99.9 %), and zinc (Merck, 14-50 mesh, 99.9 %). Filings of the light rare earth ele­ments were prepared under dried paraffin oil, which was washed away by dried (sodium) n-hexane. They were stored under vacuum, and were only briefly exposed to air prior to the reactions.

Cold-pressed pellets of the elements in the molar ratio R:T:Zn = 1:1:18 were sealed in silica tubes under argon. While annealing the liquid samples at 850 °C for 120 h they were turned over and shaken several times to enhance their homogene­ity. Then the ampoules were slowly cooled (5 °C/h)

0939-5075/97 /0900-1023 $ 06.00 © 1997 Verlag der Zeitschrift für Naturforschung. All rights reserved. K

This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution-NoDerivs 3.0 Germany License.

On 01.01.2015 it is planned to change the License Conditions (the removal of the Creative Commons License condition “no derivative works”). This is to allow reuse in the area of future scientific usage.

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschungin Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung derWissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht:Creative Commons Namensnennung-Keine Bearbeitung 3.0 DeutschlandLizenz.

Zum 01.01.2015 ist eine Anpassung der Lizenzbedingungen (Entfall der Creative Commons Lizenzbedingung „Keine Bearbeitung“) beabsichtigt, um eine Nachnutzung auch im Rahmen zukünftiger wissenschaftlicher Nutzungsformen zu ermöglichen.

1024 T. Nasch et al. ■ Ternary Rare Earth Transition Metal Zinc Compounds

C om pound a [pm] V [nm 3] Com pound a[pm ] V [nm3]

GdFezZnao 1411.3(1) 2.8110 GdCo2Zn2o 1406.0(1) 2.7794TbFe2Zn2o 1411.1(1) 2.8098 TbCo2Zn2o 1404.6(1) 2.7711D yFeiZm o 1410.7(1) 2.8074 DyCo2Zn2o 1404.3(1) 2.7694HoFe2Zn2o 1410.2(1) 2.8044 HoCo2Zn2o 1403.9(1) 2.7670ErFe2Zn2o 1409.0(1) 2.7973 ErCo2Zn2o 1402.0(1) 2.7558TmFe2Zn2o 1408.6(1) 2.7949 TmCo2Zn2o 1401.7(1) 2.7540LuFe2Zn2o 1407.9(1) 2.7907 LuCo2Zn2o 1400.2(1) 2.7452ScRu2Zn2o 1411.7(1) 2.8134 CeRhbZmo 1430.5(1) 2.9273YRu2Zn2o 1422.6(1) 2.8790 PrRhbZmo 1428.7(1) 2.9162CeRu^Znao 1436.9(1) 2.9667 NdRh2Zn2o 1425.1(1) 2.8942PrRu2Zn2o 1433.6(1) 2.9463 SmRh2Zn2o 1421.8(1) 2.8742NdRu2Zn2o 1433.0(1) 2.9426 GdRh2Zn2o 1419.8(1) 2.8621SmRu2Zn2o 1428.3(1) 2.9138 T bR hbZ ^o 1418.7(1) 2.8554GdRu2Zn2o 1424.6(1) 2.8912 D yR hbZ^o 1418.2(1) 2.8524TbRu2Zn2o 1423.6(1) 2.8851 HoRhbZmo 1416.3(1) 2.8410DyRu2Zn2o 1422.1(1) 2.8760 ErRhbZmo 1414.3(1) 2.8289HoRu2Zn2o 1421.0(1) 2.8693 TmRh2Zn2o 1414.2(1) 2.8283ErRu2Zn2o 1419.8(1) 2.8621 YbRh2Zn2o 1414.1(1) 2.8277TmRu2Zn2o 1419.5(1) 2.8603 LuRh2Zn2o 1414.0(1) 2.8271YbRu2Zn2o 1418.8(1) 2.8560 DyNi2Zn2o 1405.5(1) 2.7765LuRu2Zn2o 1417.1(1) 2.8458 HoNi2Zn2o 1402.8(1) 2.7605ScCo2Zn2o 1393.9(1) 2.7083 ErNi2Zn2o 1402.2(1) 2.7570YCo2Zn2o 1403.4(1) 2.7640 TmNi2Zn2o 1401.6(1) 2.7534SmCo2Zn2o 1408.2(1) 2.7925 LuNi2Zn2o 1400.2(1) 2.7452

Table I. Lattice constants o f the com pounds R T:Zn2o. Standard deviations in the positions of the least significant digits are given in parentheses throughout this paper.

to 500 °C and annealed further for 120 h, followed by quenching in air. The Guinier powder patterns of the resulting products showed elemental zinc as the major impurity. This may be dissolved by treating the sample with dilute hydrochloric acid, which at­tacks the ternary compounds at a slower rate. The single crystals used for the structure determinations were also obtained this way.

Properties and Lattice Constants

Well crystallized samples of the ternary com­pounds show metallic luster, the powders are black. They are stable in air for long periods of time, but are slowly attacked by dilute hydrochloric acid. Energy-dispersive X-ray analyses of some samples in a scanning electron microscope were in agree­ment with the ideal composition and did not reveal any impurity elements heavier than sodium.

All samples were characterized by Guinier pow­der patterns recorded with CuKa radiation and with a-quartz (a = 491.30, c = 540.46 pm) as an internal standard. Indices could be assigned on the basis of the cubic cell found by the single crystal investiga­tions. The lattice constants (Table I) were obtained by least-squares fits of the powder data.

Structure Refinements

Single-crystals of the ternary compounds were examined on an automated four-circle diffractome­ter (Enraf-Nonius CAD4). They showed a pro­nounced tendency for twinning as was observed earlier also for the aluminides RT2A12o [4]. The twins mimic hexagonal symmetry with the hexag­onal (h) axis corresponding to one trigonal axis of each cubic (c) twin domain. In agreement with this interpretation the hexagonal lattice constants are: ah = ac/>/2 and Ch = ac\/3. The intensity data, however, were collected from single domain crystals. They were recorded for the six compounds TbFe^Z^o, YRu2Zn2o, DyRu2Zn2o, GdCo2Zn2o, DyRhbZ^o, and TmNi2Zn2o with graphite-monochromated Mo Ka radiation, a scintillation counter with pulse- height discrimination, and background counts at both ends of each 6/29 scan. The crystallographic data are summarized in Table II.

The X-ray powder patterns of the compounds RT2Zn2o showed good agreement with the patterns calculated [ 1 1 ] assuming the compounds to be iso­typic with CeCnALo [7], Therefore, the starting values for the refinement of the positional param­eters were taken from that earlier investigation.

T. Nasch et al. • Ternary Rare Earth Transition Metal Zinc Compounds_______________________________________1025

Table II. Crystal data for T bF e^Z ^o, YRu2Zn2o, DyRu^Zmo, G dCo2Z n2o, D yR hbZ^o, and Tm N i2Z n2o-

TbFe:Zn2o YRu2Zn2o DyRu2Zn2o GdCo2Zn2o DyRh2Zn2o TmNi2Zn2o

Space group Fd3m Fd3m Fd3m Fd3m Fd3m Fd3mForm ula units/cell Z = 8 Z = 8 Z = 8 Z = 8 Z = 8 Z = 8Form ula w eight 1578.2 1598.6 1672.2 1582.7 1675.9 1593.9C alculated density [g/cm3] 7.46 7.38 7.72 7.56 7.81 7.69Crystal dim ensions [pm3] 25 x 50 x 60 45 x 45 x 35 35 x 35 x 45 50 x 60 x 60 50 x 70 x 70 60 x 70 x 709/29 Scans up to 29 = 80° 29 = 65°

oOooII<N 29 = 80° 29 = 11° 29 = 66°Range in h ± 2 5 ±21 ± 2 5 ± 2 5 ± 2 5 ±21

k ± 2 5 ±21 ± 2 5 ± 2 5 ± 2 5 ± 21I 0-25 0-21 0-25 -25-1 0-24 0-21

Total num ber o f reflections 8871 5386 9052 9237 8004 5463A bsorption corrections from psi scans H ighest/low est transm ission 1.21 1.08 1.73 2.22 1.65 2.21Unique reflections 493 314 506 489 462 333Reflections w ith I > 3a (I) 189 149 261 231 292 163N um ber o f variables 17 17 17 17 17 15H ighest residual electron density [e/A3] 0.39 0.17 0.22 0.27 0.38 0.81Conventional residual R = 0.014 R = 0.011 R = 0.013 R = 0.013 R = 0.019 R = 0 .024W eighted residual /?w = 0.017 /?w =0.011 /fw = 0 .015 /?w = 0.014 Rw = 0.023 Rw = 0.030

This structure type was confirmed during the full- matrix least-squares refinements with atomic scat­tering factors [1 2 ], corrected for anomalous disper­sion [13]. The weighting scheme accounted for the counting statistics and a parameter correcting for isotropic secondary extinction was varied as a least- squares parameter.

Occupancy parameters were refined together with the thermal parameters in separate series of least- squares cycles to check for deviations from the ideal compositions. These parameters varied be­tween 98.7(3) % for the Zn3 position of GdCo2Zn2o and 101.6(3) % for the Co position of the same compound. The only exception was the Zn3 site of the compound TmNi2Zn2o. For this position an oc­cupancy of 108.0(9) % was obtained. Since only the thulium atoms have a higher scattering power than the zinc atoms, we refined this position with a mixed Zn/Tm occupancy. The thermal parame­ters of the Zn and Tm atoms were fixed at rea­sonable values (as judged from the corresponding thermal parameters of the other compounds) during this refinement series. A Zn/Tm ratio of 93( 1 )/7( 1) was obtained. Even though the standard deviation is relatively large, a mixed occupancy for this site

is highly probable for the following reasons. The Zn3 site has a higher coordination number (CN 14) than the Znl and Zn2 sites with CN 12. It is there­fore better suited to accommodate the large thulium atoms than the other two zinc sites. Also, all com­pounds were prepared with a small excess of the rare earth metal component (R:T:Zn = 1:1:18). Of the six crystals whose structures we have refined, the com­pound TmNi2Zn2o has the smallest rare earth atom. Hence, if any of these compounds should have a significant homogeneity range extending towards the rare earth components, the thulium compound is best suited for it. Nevertheless, we expect the ho­mogeneity range of this compound to include the ideal composition TmNi2Zn2o and we therefore use the ideal formula for most purposes.

In the final structure refinements of the five com­pounds all atomic sites were refined with anisotropic thermal parameters and the ideal occupancy values with the exception of the Zn3/Tm site, as discussed above. Final difference Fourier analyses showed no significant electron densities at sites suitable for ad­ditional atomic positions. The atomic parameters and the interatomic distances are given in the Tables III and IV. Listings of the anisotropic displacement

1026 T. Nasch et al. ■ Ternary Rare Earth Transition Metal Zinc Compounds

Table III. A tomic param eters o f TbFe^Zmo, Y R u:Z n2o. D yRu2Zn2o, G dC o2Z n2o, DyRh2Z n2o, and Tm N i2Z n2o- The last colum n contains the equivalent isotropic B values (in units of 104 p m ') . The occupancy param eters listed in the third colum n were obtained in previous least-squares cycles. During the last cycles full occupancy was assum ed for all atom ic sites with the exception o f the Zn3/Tm site o f Tm N i2Zn2o, which was refined w ith mixed occupancy and with fixed isotropic thermal param eters. Hence, the exact composition o f this crystal corresponds to the form ula T m i . i 4 ( 2 ) N i 2 Z n i 9 . 8 6 ( 2 ) -

Atoms Fd3m (No. 227) Occup. X y B

TbFeTZmoTb 8a 0.994(3) 1/8 1/8 1/8 0.174(3)Fe 16d 1.003(5) 1/2 1/2 1/2 0.161(7)Z nl 96g 0.999(2) 0.05882(4) 0.05882 0.32621(4) 0.777(7)Zn2 48f 1.012(2) 0.48931(6) 1/8 1/8 0.451(9)Zn3 16c 1.014(5) 0 0 0 1.177(9)YRu^ZmoY 8a 0.991(4) 1/8 1/8 1/8 0.336(6)Ru 16d 0.997(2) Ml 1/2 1/2 0.266(3)Z nl 96g 0.994(1) 0.05906(3) 0.05906 0.32555(3) 0.921(6)Zn2 48f 1.010(2) 0.48873(5) 1/8 1/8 0.626(8)Zn3 16c 1.005(2) 0 0 0 1.474(7)

DyRu-Zn^oDy 8a 0.994(2) 1/8 1/8 1/8 0.347(2)Ru 16d 1.005(2) 1/2 1/2 1/2 0.287(2)Z nl 96g 1.000(1) 0.05916(2) 0.05916 0.32541(3) 0.937(5)Zn2 48f 1.003(2) 0.48875(4) 1/8 1/8 0.655(7)Zn3 16c 1.013(4) 0 0 0 1.363(6)

GdCo^Zn-ioGd 8a 1.009(2) 1/8 1/8 1/8 0.367(2)Co 16d 1.016(3) 1/2 1/2 1/2 0.375(4)Z nl 96g 0.998(1) 0.05893(2) 0.05893 0.32606(3) 0.953(4)Zn2 48f 0.997(2) 0.48886(4) 1/8 1/8 0.662(6)Zn3 16c 0.987(3) 0 0 0 1.450(5)D yRh-ZmoDy 8a 1.009(2) 1/8 1/8 1/8 0.170(3)Rh 16d 1.006(3) 1/2 1/2 1/2 0.154(3)Znl 96g 0.996(2) 0.05971(3) 0.05971 0.32415(4) 0.765(6)Zn2 48f 0.988(2) 0.48741(6) 1/8 1/8 0.543(9)Zn3 16c 0.993(5) 0 0 0 1.218(7)

TmNhZn2oTm 8a 1.002(5) 1/8 1/8 1/8 0.237(6)Ni 16d 1.015(9) 1/2 1/2 1/2 0.31(1)Z nl 96g 0.995(3) 0.05955(7) 0.05955 0.32471(9) 0.83(1)Zn2 48f 0.993(4) 0.4884(1) 1/8 1/8 0.47(2)Zn3/Tm 16c 0 .93(l)/0 .07 0 0 0 1.3/0.23

parameters and the structure factors are available from the authors [14]. They are also deposited*. A stereoplot and the near-neighbor coordinations are shown in Figs 1 and 2.

’They may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft für wissenschaftlich-technische Infor­mation mbH, D-76344 Eggenstein-Leopoldshafen, by quoting the registration No. CSD-59443.

Discussion

While there are some 50 rare earth transition metal aluminides with CeC^Aho type structure [4, 7-10], the presently reported compounds are the first ones containing zinc. The cell volumes of these cubic compounds (Fig. 3) reflect the well known lanthanoid contraction. The volumes of the cerium and ytterbium compounds CeRu2Zn2o, CeRh2Zn2o, YbRu2Zn20, and YbRh2Zn2o do not greatly deviate

T. Nasch et al. ■ Ternary Rare Earth Transition Metal Zinc Compounds 1027

Table IV. Interatom ic distances in the structures o f the com pounds RT2Z n2o- All distances shorter than 380 pm are listed. The standard deviations com puted from those of the lattice constants and the positional param eters are all equal to or sm aller than 0.2 pm.

T bFe2Z n 2o YRu2Zn2o DyRu2Zn2o GdCo2Zn2o DyRhbZmo TmNi22

R: 4Zn3 305.5 308.0 307.9 304.4 307.1 303.512Znl 313.1 314.6 314.3 311.8 311.3 308.5

T: 6Zn2 249.9 252.0 251.9 249.1 251.3 248.36Z nl 271.9 275.2 275.3 271.2 276.7 272.6

Z n l : lZ n l 264.1 265.3 264.8 262.8 261.9 259.5lZ n2 265.4 267.4 267.4 263.9 266.0 263.52Z nl 269.5 270.8 270.6 268.4 268.5 266.1IT 271.9 275.2 275.3 271.2 276.7 272.62Zn2 276.8 279.5 279.7 275.9 279.8 276.42Znl 283.2 286.6 286.7 282.4 288.0 283.72Zn3 302.1 304.0 303.7 300.8 301.8 298.6IR 313.1 314.6 314.3 311.8 311.3 308.5

Zn2: 2T 249.9 252.0 251.9 249.1 251.3 248.32Z nl 265.4 267.4 267.4 263.9 266.0 263.54Zn2 270.8 274.2 274.0 270.7 276.0 270.84Z nl 276.8 279.5 279.7 275.9 279.8 276.4

Zn3: 12Znl 302.1 304.0 303.7 300.8 301.8 298.62R 305.5 308.0 307.9 304.4 307.1 303.5

Fig. 1. Stereoplot o f one unit cell o f the cubic com pounds RT2Zn2o- The rare earth and the transition metal atom s are represented by large and m edium large spheres. Only the R-Zn and T-Zn bonds are shown.

from these trends, hence, the cerium and ytterbium atoms are essentially trivalent in these compounds. The cell volume of YRu2Zn2o fits between the vol­umes of the corresponding terbium and dysprosium compounds, as it is also the case for the series R6O 4 AI4 3, R6M0 4 AI43 [3] as well as for many rare earth transition metal carbides [15]. In contrast, the volume of YCo2Zn2o is in between the volumes of HoCo2Zn2o and ErC^Zmo- As can be seen from

the volume plot of the RCo2Zn2o series (Fig. 3), this somewhat unusual result may be ascribed to the fact, that the cell volumes of HoCo2Zn2o and ErCo2Zn2o are slightly too large. Small inconsis­tencies in the cell volumes (however, much larger than the error limits of the lattice constants) also occur for other compounds RT2Zn2o- These indi­cate slight differences in the compositions. Proba­bly, all compounds have small homogeneity ranges.

1028 T. Nasch et al. ■ Ternary Rare Earth Transition Metal Zinc Compounds

R (43m)

Zn1 (m)

T (3m)

Zn2 (mm)

Fig. 2. Coordination polyhedra in the structure of the com pounds R T iZ ^o- The site sym m etries o f the central atom s are indicated in parentheses.

Nevertheless, the refinement of the occupancy pa­rameters resulted in most cases in the ideal values, with the only exception of the Zn3/Tm position of TmNiiZ^o, as already discussed above.

It is remarkable that the aluminum compounds RT2AI20 are formed with the early transition metals, whereas the corresponding zinc compounds contain the late transition metals T = Fe, Ru, Co, Rh, and Ni. This may be rationalized by both atomic volume and valence electron arguments. The metallic radius for CN 12 of aluminum (143.2 pm) is greater than that of zinc (139.4 pm) [16, 17J. Hence, assum­ing equally dense packing of aluminum and zinc atoms, the space available for the transition element atoms is larger in the aluminides than in the zinc compounds, and therefore, the aluminides are bet­ter suited to accommodate the large early transition

Fig.3. Cell volumes o f the com pounds RT2Z n 2o-

elements. The valence electron count also favours the early transition elements for the aluminides, as­suming three valence electrons for aluminum and two for zinc, although the higher valence electron count of the late transition elements cannot com­pletely compensate the lower count of zinc.

The thermal parameters of our six structure re­finements of the zinc compounds RT2Zn2o as well as our earlier structure refinements of the aluminides CeTi2Al2o [4] and CeMo2-.r AI20+* [4] show some

Fig. 4. Crystal structure o f the cubic com pounds RT2Z n2o- In the upper part of the draw ing all rare earth (large cir­cles), transition metal (m edium sized circles), and zinc atoms (filled circles) are projected along one translation period. In the middle and low er part the packing o f the coordination polyhedra R Z ni6 and T Z n i2 is shown.

T. Nasch et al. ■ Ternary Rare Earth Transition Metal Zinc Compounds 1029

regularities. Generally, the smallest displacement parameters are those of the transition metal atoms. Those of the rare earth metal atoms are usually somewhat greater, and the greatest ones are those of the aluminum and zinc atoms. These thermal pa­rameters thus reflect the complex interplay of the differing atomic weights, bond strengths, and coor­dination numbers. The dependency on the coordi­nation numbers is well demonstrated by the Znl, Zn2, and Zn3 as well as by the A ll, A12, and A13 atoms. The Zn3 and A13 atoms have CN 14 and very large displacement parameters, while the dis­placement parameters of the Zn 1, Zn2, AI 1, and A12 atoms with CN 12 are smaller.

The rare earth atoms are coordinated by 16 zinc atoms forming the Frank-Kasper polyhe­dron CN 16, while the transition metal atoms have icosahedral zinc coordination (Figs 2 and 4). One might expect, that the average R-Zn dis­tances decrease with the size of the R atoms. This, however, is not entirely the case; these distances also reflect the size of the unit cell, which is also determined by the size of the tran­sition metal atoms. Thus, the average R-Zn dis­tances (in pm) for GdCooZ^o (310.1), TbFe2Zn2o(311.2), YRu2Zn20 (313.0), DyRu2Zn20 (312.7), DyRh2Zn2o (310.3), and TmNi2Zn2o (307.3) should decrease in that order, which is not the case, be­cause the lattice constants of the ruthenium and rhodium compounds are substantially larger than those of the compounds containing iron, cobalt and nickel. On the other hand, the average T-Zn dis­tances for TbFe2Zn2o (260.9), GdCo2Zn2o (260.2), TmNi2Zn2o (260.5), YRu2Zn2o (263.6), DyR^Zmo (263.6), and DyRfbZmo (264.0) rather well reflect the size of the metallic radii for Fe (127.4), Co(125.2), Ni (124.6), Ru (133.9), and Rh (134.5) [16,17].

As could be expected, the Zn-Zn distances show the same tendency. They are larger for the com­pounds with the larger lattice constants. They also reflect the size of their coordination polyhedra, i.e. the average Zn-Zn distances of the Znl and Zn2 atoms with CN 12 - varying between 277.3 (TmNi2Zn2o) and 281.5 pm (YRu2Zn2o) for Znl and between 271.4 (GdCxnZmo) and 275.5 pm (DyRhbZ^o) for Zn2 - are all considerably smaller than the Zn-Zn distances for the Zn3 atoms - varying between 298.6 (T m ^Z m o) and 304.0 pm (YRu2Zn2o) _ with CN 14.

Since these compounds have a very high zinc content considerable Zn-Zn bonding must be as­sumed, and this is also supported by a comparison of the Zn-Zn distances. The shortest of these are the Znl-Znl distances, which vary between 259.5 pm for TmNi2Zn2o and 265.3 pm for YR^Zmo- Thus, they are close to the six short Zn-Zn distances of264.4 pm for each zinc atom in the hexagonal "close packed" structure of elemental zinc [18]. The latter structure has an unusually large c/a ratio, which re­sults in six short (264.4 pm) and six large (291.2 pm) Zn-Zn distances. The average Zn-Zn distance of 277.8 pm in the element compares well with the average Zn-Zn distances for the Znl and Zn2 atoms enumerated above, which also have CN 12.

The CeCr2Al2o type structure, found for the ternary compounds RT2Zn2o described here, is an ordered ternary version of the structure of the binary compound ZrZn22 [19]. The latter structure was re­cently refined and analyzed [20]. Relatively large voids were found in this structure. It was argued, that these voids are needed to accommodate non­bonding electrons of the zinc atoms. This can also be assumed for the ternary compounds RT2Zn2o, since the voids in the ternary compounds are formed solely by zinc atoms; the transition metal atoms are not coordinated to the void positions. These voids are large enough to accommodate interstitial atoms like carbon, nitrogen, or oxygen. Our difference Fourier analyses of the structures refined here, how­ever, showed that these voids are not filled. For an interstitial oxygen atom an electron density of about 14 to 20 e/A3 could be expected, whereas the highest residual electron densities of the present structure refinements were all smaller than 1 e/A3 (Table II).

Acknowledgements

We thank Mr. K. W agner for the work at the scan­ning electron microscope. We are also obliged to Dr.G. H öfer (Heraeus Q uarzschm elze) and to the Rhone- Poulenc com pany for generous gifts of silica tubes and rare earth metals. This work was supported by the D eutsche Forschungsgem einschaft and the Fonds der Chem ischen Industrie. Finally we acknowledge the I.S.R M otorenprüfstände GmbH for a stipend to one o f us (T .N .).

1030 T. Nasch et al. ■ Ternary Rare Earth Transition Metal Zinc Compounds

[1] V. V. Pavlyuk, I. M. Opainych. O. I. Bodak, R. Cerny, K. Yvon, Acta Crystallogr. C51, 2464 (1995).

[2] S. N iem ann, W. Jeitschko, Z. M etallkd. 85, 345(1994).

[3] S. N iem ann, W. Jeitschko, J. Solid State Chem. 116, 131 (1995).

[4] S. N iem ann. W. Jeitschko, J. Solid State Chem. 114, 337 (1995).

[5] S. N iem ann, W. Jeitschko, J. Alloys Compd. 221, 235 (1995).

[6] S. N iem ann, W. Jeitschko, Z. Kristallogr. 210, 338(1995).

[7] R 1. Kripyakevich, O. S. Zarechnyuk, Dopov. Akad. N auk Ukr. RSR, Ser. A, 30, 364 (1968).

[8] R. M. Rykhal, O. S. Zarechnyuk, O. P. Mats'kiv, Vestn. L'vov. Univ. Ser. Khim. 21, 46 (1979).

[9] N. B. M anyako, T. I. Yanson, O. S. Zarechnyuk, Izv. A kad. Nauk SSSR, Met. 2, 202 (1990).

[10] O. S. Zarechnyuk, T. I. Yanson. O. I. Ostrovskaya, P. L. Shevchuk, Vestn. L'vov. Univ. Ser. Khim. 29, 44 (1988 ).

[11] K. Yvon. W. Jeitschko, E. Parthe, J. Appl. C rystal­logr. 10, 73 (1977).

[12] D. T. Cromer, J. B. Mann, A cta Crystallogr. A 24, 321 (1968).

[13] D. T. Cromer, D. Liberm an, J. Chem . Phys. 53, 1891(1970).

[14] T. Nasch, doctoral thesis. U niversität M ünster (1997).

[15] W. Jeitschko, G. Block, G. E. Kahnert, R. K. Behrens, J. Solid State Chem. 89, 191 (1990).

[16] E. Teatum, K. G schneidner, J. Waber, U. S. D epart­m ent of Com m erce, report LA-2345. W ashington,D. C. (1960).

[17] W. B. Pearson, The Crystal Chem istry and Physics o f M etals and A lloys. Wiley, New York (1972).

[18] J. D onohue, The Structures o f the Elements. Wiley, New York (1974).

[19] S. Samson, A cta Crystallogr. 14, 1229 (1961).[20] X ue-an Chen, W. Jeitschko, J. Solid State Chem.

121,95(1996).