Mixed cerium valence and unusual Ce–Ru bonding in Ce23Ru7Cd4

Mixed cerium valence and unusual Ce–Ru bonding in Ce23Ru7Cd4

Intermetallics 17 (2009) 1035–1040 Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Mixe...

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Intermetallics 17 (2009) 1035–1040

Contents lists available at ScienceDirect

Intermetallics journal homepage: www.elsevier.com/locate/intermet

Mixed cerium valence and unusual Ce–Ru bonding in Ce23Ru7Cd4 Frank Tappe, Wilfried Hermes, Matthias Eul, Rainer Po¨ttgen* ¨t Mu ¨ r Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Universita ¨ nster, Corrensstrasse 30, D-48149 Mu ¨ nster, Germany Institut fu

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 March 2009 Received in revised form 11 May 2009 Accepted 12 May 2009 Available online 8 July 2009

The rare earth-rich compounds Ce23Ru7Cd4 and Pr23Ru7Cd4 were synthesized from the elements in sealed tantalum ampoules in an induction furnace. Both structures were refined on the basis of diffractometer data: P63mc, Z ¼ 2, a ¼ 988.7(3), c ¼ 2241.6(5) pm, wR2 ¼ 0.0439, 1976 F2 values for Ce23Ru7Cd4 and a ¼ 992.7(2), c ¼ 2236.4(7) pm, wR2 ¼ 0.0466, 2528 F2 values for Pr23Ru7Cd4 with 74 variables per refinement. Striking structural motifs are ruthenium centered trigonal prisms RuCe6 and RuPr6 which are condensed via common edges and corners, building rigid three-dimensional networks. Larger voids within these networks are filled by slightly elongated Cd4 tetrahedra. Five of the nine crystallographically independent cerium sites in Ce23Ru7Cd4 show Ce–Ru distances which are shorter than the Pr–Ru distances in Pr23Ru7Cd4. This strong hint for mixed cerium valence is supported by the magnetic behavior. Pr23Ru7Cd4 shows Curie–Weiss behavior above 50 K with an experimental magnetic moment of 3.62 mB/Pr atom, indicating stable trivalent praseodymium. Complex magnetic ordering sets in at 13 K. Ce23Ru7Cd4 shows a reduced magnetic moment of 2.05 mB/Ce atom. The trivalent cerium atoms show ferro- or ferrimagnetic ordering below TC ¼ 3.6 K. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: A. Intermetallics, miscellaneous B. Crystal chemistry of intermetallics B. Magnetic properties C. Mixed valence

1. Introduction Among the rare earth elements, cerium is one of the prominent examples for valence instabilities. Trivalent cerium has a [Xe]4f1 configuration and exhibits paramagnetism, while tetravalent cerium, [Xe]4f0, is diamagnetic. These two extreme situations are realized in the oxides Ce2O3 and CeO2, while intermediate or mixed cerium valence often occurs in intermetallic compounds. A prominent example is homogeneous intermediate-valence CeRhSb [1]. So far more than 80 entries occur for CeRhSb in the current SciFinder Scholar version [2]. The cerium valence can often be influenced by temperature, e.g. CeRhGe [3,4], by pressure, e.g. CeP [5], or upon hydrogenation, e.g. CeRhSb / CeRhSbH0.2 [6], leading to interesting property changes. A peculiar bonding situation occurs for intermediate-valent cerium in combination with ruthenium. Already in the binary phases Ce16Ru9 [7,8] and Ce4Ru3 [9] one observes short Ce–Ru distances which are significantly shorter than the sum of the covalent radii [10] of 289 pm. The magnetic data of Ce16Ru9 [8] have been interpreted by different cerium valences. The Laves phase CeRu2 becomes superconducting below 6.2 K [11–14]. This Ce–Ru

* Corresponding author. Tel.: þ49 251 83 36001; fax: þ49 251 83 36002. E-mail address: [email protected] (R. Po¨ttgen). 0966-9795/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2009.05.002

bonding situation becomes more extreme in the ternary indides Ce3Ru2In3 (238 and 273 pm Ce–Ru) [15], Ce16Ru8In37 (237 pm Ce– Ru) [16], Ce2Ru2In3 (232 and 237 pm Ce–Ru) and Ce3Ru2In2 (223 and 228 pm Ce–Ru) [17], CeRu0.88In2 (253 pm Ce–Ru) [18] and CeRuAl (280–286 pm Ce–Ru) [19]. Unfortunately, no magnetic data are available for these compounds. Similar structural peculiarities have been observed for CeRuSn [20,21] and Ce2RuZn4 [22,23]. Both compounds show static mixed cerium valence with one discrete CeIII and one CewIV site. This is evident from the crystal structures, both of which show very short Ce–Ru distances for the CewIV site. The susceptibility data reveal only a paramagnetic moment on the CeIII site. In Ce2RuZn4, the CeIII site orders antiferromagnetically at TN ¼ 2.1 K [23]. Lattice parameter anomalies have recently been observed within the series of hexagonal RE23Ru7Mg4 (RE ¼ rare earth element) [24] and RE23Ru7Cd4 [25] compounds. To give an example, the lattice parameter a of Ce23Ru7Cd4 is even smaller than the a lattice parameter of the praseodymium compound. In contrast, the c parameter fits in between the lanthanum and the praseodymium compound (Fig. 1). These findings are again strong hints for unusual Ce–Ru bonding and potentially intermediate or mixed cerium valence. In order to understand this peculiar structural behavior in more detail we have grown small crystals of both compounds. Herein we report on the single crystal structure refinements and magnetic properties of Ce23Ru7Cd4 and Pr23Ru7Cd4.

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F. Tappe et al. / Intermetallics 17 (2009) 1035–1040 Table 2 Crystal data and structure refinement for Ce23Ru7Cd4 and Pr23Ru7Cd4 (space group P63mc, Z ¼ 2).

Fig. 1. Course of the lattice parameters of the ternary cadmium compounds RE23Ru7Cd4.

2. Experimental 2.1. Syntheses Starting materials for the preparation of the Ce23Ru7Cd4 and Pr23Ru7Cd4 samples were cerium and praseodymium ingots (smart elements), ruthenium powder (Degussa–Hu¨ls) and a cadmium rod (Johnson–Matthey), all with stated purities better than 99.9%. Pieces of the cerium and praseodymium ingots were first arcmelted [26] to small buttons under an argon atmosphere. The argon was purified before with molecular sieves, silica gel, and titanium sponge (900 K). Next the rare earth metal buttons, ruthenium powder and pieces of the cadmium rod were weighed in the exact 23:7:4 atomic ratio and sealed in tantalum tubes under an argon pressure of ca. 700 mbar. The ampoules were placed in a watercooled sample chamber of a high-frequency furnace (Hu¨ttinger Elektronik, Freiburg, type TIG 1.5/300) under flowing argon [27], first heated at about 1370 K and kept at that temperature for 5 min. The temperature was then lowered within 5 min to 873 K and the tubes were annealed for another 3 h. Finally the tubes were quenched to room temperature. The temperature was controlled through a Sensor Therm Methis MS09 pyrometer with an accuracy of 30 K. Both samples could easily be separated from the ampoule by mechanical fragmentation. No reaction with the container material was observed. Polycrystalline Ce23Ru7Cd4 and Pr23Ru7Cd4 samples deteriorate slowly in air and were kept in Schlenk tubes prior to the investigations. 2.2. EDX analyses The investigated single crystals were studied by energy dispersive analyses of X-rays (EDX) using a Zeiss EVO MA10 scanning Table 1 Lattice parameters (Guinier powder data) of the ternary cadmium compounds RE23Ru7Cd4. The data marked with an asterisk refer to the investigated single crystals. Compound

a/pm

c/pm

V/nm3

Reference

La23Ru7Cd4 Ce23Ru7Cd4 Ce23Ru7Cd4* Pr23Ru7Cd4 Pr23Ru7Cd4* Nd23Ru7Cd4 Sm23Ru7Cd4

1015.0(2) 988.7(3) 988.3(1) 992.7(2) 992.9(1) 990.2(2) 980.9(2)

2282.8(4) 2241.6(5) 2241.7(5) 2236.4(7) 2234.3(5) 2225.5(4) 2204.5(8)

2.0367 1.8977 1.8962 1.9086 1.9076 1.8897 1.8369

[25] [25] This work [25] This work [25] [25]

Ce23Ru7Cd4 Table 1 4379.85 7.67 40  40  60 0.385/0.258 31.8 80 3 0–180 , 1.0 11.0; 1.0; 0.020 3668 2–29 13, 13, 30 18,221 1976/0.0670 1396/0.0784 1976/74 0.689 0.0298, 0.0409 0.0516, 0.0439 0.00(4) 0.000193(7) 5.01/3.57

Empirical formula Unit cell dimensions Molar mass, g/mol Calculated density, g cm3 Crystal size, mm3 Transm. ratio (max/min) Absorption coeff., mm1 Detector distance, mm Exposure time, min u range; increment, deg Integr. param. A, B, EMS F(000) q range, deg Range in hkl Total no. reflections Independent reflections/Rint Reflections with I  2s(I)/Rs Data/parameters Goodness-of-fit on F2 R1, wR2 for I  2s(I) R1, wR2 for all data Flack parameter Extinction coefficient Largest diff. peak and hole, e Å3

Pr23Ru7Cd4 Table 1 4398.02 7.65 20  40  60 0.556/0.312 33.5 80 3 0–180 , 1.0 13.0; 3.5; 0.012 3714 2–32 14, 14, 33 23,581 2528/0.0933 1317/0.1590 2528/74 0.523 0.0297, 0.0420 0.0663, 0.0466 0.01(4) 0.000143(5) 2.19/2.18

electron microscope with CeO2, PrF3, Ru, and Cd as standards. The quartz fibers were coated with a thin carbon film to ensure conductivity. The semi-quantitative analyses showed compositions of 66  2 at% RE: 21  2 at% Ru: 13  2 at% Cd for both crystals, close to the ideal composition of 67.6 at% RE: 20.6 at% Ru: 11.8 at% Cd. The standard uncertainties account for measurements at different points of the irregular crystal surfaces. No impurity elements heavier than sodium (detection limit of the instrument) have been observed.

Table 3 Atomic coordinates and isotropic displacement parameters (pm2) for Ce23Ru7Cd4 and Pr23Ru7Cd4. Ueq is defined as one third of the trace of the orthogonalized Uij tensor. Atom

Wyckoff position

Ce23Ru7Cd4 Ce1 6c Ce2 6c Ce3 6c Ce4 6c Ce5 6c Ce6 6c Ce7 2b Ce8 2a Ce9 6c Ru1 6c Ru2 6c Ru3 2b Cd1 6c Cd2 2a Pr23Ru7Cd4 Pr1 6c Pr2 6c Pr3 6c Pr4 6c Pr5 6c Pr6 6c Pr7 2b Pr8 2a Pr9 6c Ru1 6c Ru2 6c Ru3 2b Cd1 6c Cd2 2a

x

y

z

Ueq

0.12840(8) 0.20146(8) 0.20778(8) 0.20757(9) 0.54017(8) 0.53986(8) 1/3 0 0.79264(8) 0.48655(12) 0.85228(12) 1/3 0.89552(8) 0

x x x x x x 2/3 0 x x x 2/3 x 0

0.13554(5) 0.28379(5) 0.44827(4) 0.99085(5) 0.08434(5) 0.35585(4) 0.14757(11) 0.99488(10) 0.21996(6) 0.21287(9) 0.05391(10) 0.37285(14) 0.36218(7) 0.24964(12)

150(2) 146(3) 132(3) 177(3) 147(3) 150(3) 228(5) 157(5) 136(2) 340(5) 349(5) 231(7) 122(3) 113(5)

0.12700(8) 0.20385(9) 0.20717(9) 0.20787(10) 0.54136(9) 0.54026(8) 1/3 0 0.79341(9) 0.48296(11) 0.85411(11) 1/3 0.89462(9) 0

x x x x x x 2/3 0 x x x 2/3 x 0

0.13589(5) 0.28202(5) 0.44730(5) 0.99120(5) 0.08526(6) 0.35489(5) 0.14631(9) 0.99793(11) 0.21968(7) 0.21080(8) 0.06125(8) 0.36853(14) 0.36264(8) 0.25094(14)

96(2) 96(3) 103(3) 120(3) 95(3) 108(3) 90(4) 98(5) 100(3) 133(4) 139(4) 111(7) 91(3) 91(6)

F. Tappe et al. / Intermetallics 17 (2009) 1035–1040 Table 4 Interatomic distances (pm) of La23Ru6.87(1)Cd4 [25], Ce23Ru7Cd4 and Pr23Ru7Cd4, calculated with the powder lattice parameters. Standard deviations are all equal or smaller than 0.3 pm. La23Ru6.87(1)Cd4 La1

La2

La3

La4

La5

La6

La7

La8

La9

2 1 1 1 1 2 2 2 1 2 2 1 2 2 1 1 1 1 2 2 1 1 2 2 1 1 2 2 2 1 1 2 1 2 2 2 2 1 2 1 2 2 2 2 2 1 1 2 2 1 2 2 2 2 3 3 3 3 3 3 3 3 3 3 2 1 1 2 2 1 2 2 2

Ru2 Cd2 La4 La2 La7 Ru1 La9 La5 La8 La1 Ru1 Ru3 La6 Cd1 La1 Cd2 La3 La7 La9 La2 Ru2 Ru3 Cd1 La4 La2 La8 La6 La3 La5 Cd1 La1 Ru2 La8 La3 La6 La4 La5 La7 Ru2 Ru1 La9 La1 La5 La3 La4 La7 Ru1 Cd1 La2 Ru3 La9 La4 La3 La6 Ru1 La1 La2 La5 La4 Ru2 Cd1 La4 La3 La1 Ru1 Cd2 Cd1 La1 La5 Ru2 La6 La9 La2

Ce23Ru7Cd4 295.7 345.6 358.9 362.6 364.5 369.4 371.4 381.0 383.9 385.5 295.2 303.2 359.0 360.7 362.6 366.0 378.8 382.4 387.5 393.4 278.1 284.0 371.5 377.8 378.8 382.0 382.0 383.5 385.5 344.9 358.9 362.2 365.8 377.8 381.0 382.1 386.4 420.2 295.8 303.4 377.4 381.0 382.0 385.5 386.4 391.1 342.0 349.4 359.0 365.3 380.2 381.0 382.0 385.1 301.9 364.6 382.4 391.1 420.2 294.7 360.1 365.8 382.0 383.9 286.5 369.2 370.6 371.4 377.3 378.8 380.2 387.3 387.5

Ce1

Ce2

Ce3

Ce4

Ce5

Ce6

Ce7

Ce8

Ce9

2 1 1 1 1 2 2 2 2 1 2 1 2 2 1 1 1 2 1 2 1 1 2 2 1 1 2 2 2 1 2 1 1 2 2 2 2 1 2 1 2 2 2 2 2 1 1 2 2 1 2 2 2 2 3 3 3 3 3 3 3 3 3 3 2 1 2 1 2 2 2 2 1

Table 4 (continued) La23Ru6.87(1)Cd4 Ru1

Pr23Ru7Cd4 Ru2 Cd2 Ce4 Ce7 Ce2 Ru1 Ce9 Ce5 Ce1 Ce8 Ru1 Ru3 Cd1 Ce6 Cd2 Ce1 Ce3 Ce9 Ce7 Ce2 Ru2 Ru3 Cd1 Ce4 Ce2 Ce8 Ce6 Ce3 Ce5 Cd1 Ru2 Ce1 Ce8 Ce3 Ce6 Ce4 Ce5 Ce7 Ru2 Ru1 Ce1 Ce9 Ce4 Ce3 Ce5 Ce7 Ru1 Cd1 Ce2 Ru3 Ce3 Ce4 Ce9 Ce6 Ru1 Ce1 Ce2 Ce5 Ce4 Ru2 Cd1 Ce4 Ce3 Ce1 Ru1 Cd2 Ce1 Cd1 Ce5 Ce9 Ce6 Ce2 Ru2

300.3 337.3 351.5 352.0 355.1 360.5 363.6 370.8 380.8 384.4 293.0 301.4 346.7 349.7 353.4 355.1 368.9 378.3 379.8 391.2 258.2 273.5 363.6 368.2 368.9 370.8 371.7 372.4 373.7 338.2 347.0 351.5 355.6 368.2 372.0 373.0 373.5 412.0 289.4 302.4 370.8 373.0 373.5 373.7 375.2 381.5 333.3 340.4 349.7 355.7 371.7 372.0 373.7 376.1 300.5 352.0 379.8 381.5 412.1 285.5 347.1 355.6 370.8 384.4 274.6 361.3 363.6 364.3 373.0 373.6 373.7 378.3 386.0

Pr1

Pr2

Pr3

Pr4

Pr5

Pr6

Pr7

Pr8

Pr9

2 1 1 1 1 2 2 2 1 2 2 1 2 1 2 1 1 1 2 2 1 1 2 1 2 2 1 2 2 1 1 2 1 2 2 2 2 1 2 1 2 2 2 2 2 1 1 2 2 1 2 2 2 2 3 3 3 3 3 3 3 3 3 3 2 1 2 1 1 2 2 2 2

Ru2 Cd2 Pr4 Pr2 Pr7 Ru1 Pr9 Pr5 Pr8 Pr1 Ru1 Ru3 Pr6 Pr1 Cd1 Cd2 Pr3 Pr7 Pr9 Pr2 Ru2 Ru3 Cd1 Pr2 Pr4 Pr6 Pr8 Pr3 Pr5 Cd1 Pr1 Ru2 Pr8 Pr3 Pr4 Pr6 Pr5 Pr7 Ru2 Ru1 Pr9 Pr5 Pr1 Pr4 Pr3 Pr7 Ru1 Cd1 Pr2 Ru3 Pr9 Pr3 Pr4 Pr6 Ru1 Pr1 Pr2 Pr5 Pr4 Ru2 Cd1 Pr4 Pr3 Pr1 Ru1 Cd2 Pr1 Cd1 Ru2 Pr5 Pr6 Pr9 Pr2

289.3 337.5 352.2 352.5 355.5 359.0 362.5 373.8 378.0 378.2 289.6 295.0 351.4 352.5 353.1 357.3 369.7 376.4 379.4 385.6 275.8 279.4 362.0 369.7 370.1 373.1 373.8 375.7 376.7 337.2 352.2 354.3 357.7 370.1 373.6 373.9 376.3 408.5 289.4 298.2 370.6 373.2 373.8 376.3 376.7 382.8 337.0 340.6 351.4 357.1 372.5 373.1 373.9 376.4 295.0 355.5 376.4 382.8 408.5 288.1 352.7 357.7 373.8 378.0 280.8 362.0 362.5 364.0 369.4 370.6 372.6 377.5 379.4

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Ru2

Ru3

Cd1

Cd2

2 2 1 1 1 2 1 1 2 2 2 1 3 3 3 1 2 1 2 1 2 1 2 3 3 3 3

La9 La2 La7 La5 La6 La1 La3 La8 La1 La5 La4 La9 La3 La2 La6 Cd2 Cd1 La4 La6 La8 La2 La9 La3 Cd1 La1 La2 La9

Ce23Ru7Cd4 286.5 295.2 302.0 303.4 342.0 369.4 278.2 294.7 295.7 295.8 362.2 378.8 284.0 303.3 365.3 313.9 318.3 344.9 349.4 360.1 360.7 370.6 371.5 313.9 345.6 366.0 369.2

Ru1

Ru2

Ru3

Cd1

Cd2

2 2 1 1 1 2 1 1 2 2 2 1 3 3 3 1 2 1 2 2 1 2 1 3 3 3 3

Pr23Ru7Cd4 Ce9 Ce2 Ce7 Ce5 Ce6 Ce1 Ce3 Ce8 Ce5 Ce1 Ce4 Ce9 Ce3 Ce2 Ce6 Cd2 Cd1 Ce4 Ce6 Ce2 Ce8 Ce3 Ce9 Cd1 Ce1 Ce2 Ce9

274.6 293.0 300.5 302.4 333.3 360.5 258.2 285.5 289.4 300.3 347.0 386.0 273.5 301.4 355.7 309.3 309.9 338.2 340.4 346.7 347.1 363.6 364.3 309.3 337.3 353.4 361.3

Ru1

Ru2

Ru3

Cd1

Cd2

2 2 1 1 1 2 1 1 2 2 2 1 3 3 3 1 2 1 2 1 2 2 1 3 3 3 3

Pr9 Pr2 Pr7 Pr5 Pr6 Pr1 Pr3 Pr8 Pr1 Pr5 Pr4 Pr9 Pr3 Pr2 Pr6 Cd2 Cd1 Pr4 Pr6 Pr8 Pr2 Pr3 Pr9 Cd1 Pr1 Pr2 Pr9

280.8 289.6 295.0 298.2 337.0 359.0 275.8 288.1 289.3 289.4 354.3 369.4 279.4 295.0 357.1 308.6 313.8 337.2 340.6 352.7 353.1 362.0 364.0 308.6 337.5 357.3 362.0

2.3. X-ray powder diffraction The purity of the Ce23Ru7Cd4 and Pr23Ru7Cd4 samples was checked through Guinier powder patterns (Cu Ka1 radiation, a-quartz: a ¼ 491.30 and c ¼ 540.46 pm as internal standard). The Guinier camera was equipped with an imaging plate technique (Fujifilm, BAS-READER 1800). The hexagonal lattice parameters (Table 1) were obtained from least-squares refinements. To ensure correct indexing, the experimental patterns were compared to calculated ones [28], using the atomic positions obtained from the structure refinements. 2.4. Single crystal X-ray data Small single crystals of Ce23Ru7Cd4 and Pr23Ru7Cd4 were isolated from the crushed samples prepared in the induction furnace. They were glued to quartz fibers and investigated on a Buerger precession camera (white Mo radiation, Fujifilm imaging plate) in order to check the quality for intensity data collection. The data sets were collected at room temperature by use of an IPDS II diffractometer (graphite monochromatized Mo Ka radiation; oscillation mode). Numerical absorption corrections were applied to the data sets. Details on the crystallographic data are given in Table 2. 2.5. Physical property measurements The magnetic and heat capacity measurements were carried out on a Quantum Design Physical Property Measurement System (PPMS) using the VSM and heat capacity options, respectively. For VSM measurements, the samples (16.558 mg for Ce23Ru7Cd4; 21.564 mg for Pr23Ru7Cd4) were packed in kapton foil and attached to the sample holder rod for measuring the magnetic properties in the temperature range 3–305 K with magnetic flux densities up to 80 kOe. For heat capacity (CP) measurements (2.1–70 K) the samples (23.644 mg for Ce23Ru7Cd4; 13.053 mg for Pr23Ru7Cd4) were fixed to the platform of a pre-calibrated heat capacity puck using Apiezon N grease.

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F. Tappe et al. / Intermetallics 17 (2009) 1035–1040

3. Results and discussion 3.1. Structure refinements Analyses of both data sets were compatible with the noncentrosymmetric space group P63mc in agreement with our previous work on Pr23Ir7Mg4 type phases [24,25,29]. The atomic parameters of La23Ru6.87(1)Cd4 [25] were taken as starting values and the two structures were refined using SHELXL-97 (full-matrix least-squares on F2) [30] with anisotropic atomic displacement parameters for all atoms. Separate refinements of the occupancy parameters revealed full occupancy for all sites within two standard deviations. Refinement of the correct absolute structure was ensured through calculation of the Flack parameter [31,32]. The final difference electron-density syntheses were flat. The results of the structure refinements are summarized in Table 2. The atomic coordinates and interatomic distances are listed in Tables 3 and 4, respectively. Further information on the structure refinements is available.1

3.2. Crystal chemistry Ce23Ru7Cd4 and Pr23Ru7Cd4 crystallize with the hexagonal Pr23Ir7Mg4 type structure [29], space group P63mc. The main structural features and chemical bonding peculiarities of the various magnesium and cadmium containing Pr23Ir7Mg4 type compounds have already been discussed in detail in previous work [25,29,33,34]. Herein we focus only on the peculiarities that derive from the strong Ce–Ru bonding caused by the intermediate or mixed valence of cerium. The Ce23Ru7Cd4 structure has two striking structural motifs, i.e. tricapped trigonal prisms of cerium atoms which are centered by ruthenium and slightly elongated Cd4 clusters. Since the main structural difference between Ce23Ru7Cd4 and Pr23Ru7Cd4 concerns the Ce–Ru vs Pr–Ru distances, we focus on this comparison in the following discussion. The trigonal prismatic coordination of the three crystallographically independent ruthenium sites in Ce23Ru7Cd4 and Pr23Ru7Cd4 are shown in Fig. 2 together with relevant interatomic distances. Always the three capping cerium (praseodymium) atoms have longer distances from the central ruthenium atoms. The Ce– Ru distances range from 258 to 386 pm, while the Pr–Ru distances have a much narrower range from 276 to 369 pm (Table 4). Especially for Ce23Ru7Cd4 the short Ce–Ru distances are significantly shorter than the sum of the covalent radii of 289 pm [10]. The Ce23Ru7Cd4 and Pr23Ru7Cd4 structures each contain nine crystallographically independent rare earth sites. If cerium would be purely trivalent in Ce23Ru7Cd4, based on the lanthanide contraction, one would expect slightly longer distances for all nine cerium sites with respect to the corresponding praseodymium sites. This, however, is only the case for Ce1, Ce2, Ce5, and Ce7, while Ce3, Ce4, Ce6, Ce8, and Ce9 show shorter Ce–Ru distances as compared to Pr– Ru. Based on a comparison of the interatomic distances, we can assume that 20 of the 46 cerium atoms are trivalent, while 26 cerium atoms show intermediate or mixed valence. This situation is addressed in more detail in the Magnetic properties section (vide infra). Finally we need to comment on the lattice parameter anomalies. Since the RuCe6 trigonal prisms condense to layers which primarily extend within the ab planes, the strong decrease of a lattice

1 Details may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry Nos. CSD-420530 (Ce23Ru7Cd4) and CSD-420531 (Pr23Ru7Cd4).

Fig. 2. Coordination of the ruthenium atoms in Ce23Ru7Cd4 and Pr23Ru7Cd4. Rare earth and ruthenium atoms are drawn as medium grey and black circles, respectively. Relevant interatomic distances and site symmetries are given. The slightly distorted trigonal prisms are emphasized.

parameter of Ce23Ru7Cd4 is understandable. The influence of the mixed cerium valence on the c parameter is less pronounced (Fig. 1). The mixed cerium valence in Ce23Ru7Cd4 (vide infra) is also reflected in the course of the displacement parameters. The ruthenium coordinations are strongly influenced by the different

Fig. 3. Temperature dependence of the magnetic susceptibility (c and c1 data) of Pr23Ru7Cd4 and Ce23Ru7Cd4 measured at 10 kOe.

F. Tappe et al. / Intermetallics 17 (2009) 1035–1040

Fig. 4. Low-temperature susceptibility measurements (zero field cooled and field cooled states) of Pr23Ru7Cd4 and Ce23Ru7Cd4 measured at 100 Oe.

cerium valences, and thus the effective sizes of the cerium atoms. The latter are certainly smaller if the cerium tends to intermediate valence. Consequently we observe larger displacement parameters for the ruthenium sites in Ce 2 3 Ru 7 Cd 4 as compared to La 2 3 Ru6.87(1)Cd4 [21] and Pr23Ru7Cd4. 3.3. Magnetic properties So far, only the magnetic behavior of Ce23Ni7Mg4 [33] and Ce23Rh7Mg4 [34] has been investigated. Both compounds are Curie– Weiss paramagnets with stable trivalent cerium. Ce23Rh7Mg4 orders antiferromagnetically at 2.9 K. Also Pr23Ru7Cd4 reported herein shows stable trivalent praseodymium. The temperature dependence of the inverse magnetic susceptibility of Pr23Ru7Cd4 and Ce23Ru7Cd4 is

Fig. 5. Magnetization isotherms of Pr23Ru7Cd4 and Ce23Ru7Cd4 measured at various temperatures.

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displayed in Fig. 3. For Pr23Ru7Cd4 we observe a Curie–Weiss behavior above 50 K with an experimental effective magnetic moment of 3.62(1) mB/Pr atom, in good agreement with the free ion value of 3.58 mB for Pr3þ. However the reciprocal magnetic susceptibility of Ce23Ru7Cd4 had to be fitted with a modified Curie–Weiss law c1 ¼ ðc0 þ c=ðT  qp ÞÞ1 above 50 K, leading to an effective paramagnetic moment of meff ¼ (8C/23)1/2 ¼ 2.05(1) mB/Ce atom. This value is significantly lower than the free ion value of 2.54 mB for Ce3þ and can be attributed to not all cerium atoms being in a trivalent state. The effective moment is equal to 65% of Ce3þ in the compound and is in accordance with the crystal data which concludes 43% of the cerium atoms being in a stable trivalent state, while the remaining cerium atoms show intermediate or mixed valence (vide supra). Due to the nine independent cerium sites with different site symmetries, we are not able to assign the oxidation state for each site. The fits lead to Weiss constants of qp ¼ 5.6(5) K for Pr23Ru7Cd4, indicative of ferromagnetic interactions, and qp ¼ 13.6(5) K for Ce23Ru7Cd4, indicative of antiferromagnetic interactions, as well as a temperature independent term c0 of 7.6(1)  103 emu/mol for the latter compound. The low field susceptibility (H ¼ 100 Oe) measured in the zero field cooled (ZFC) and field cooled (FC) states of the samples is shown in Fig. 4. Pr23Ru7Cd4 shows complex magnetic ordering at around 13 K. Below this temperature bifurcation between the ZFC and FC states is clearly visible. Although the sample is pure on the level of X-ray powder diffraction, at 25 K we observe an additional small anomaly which is not visible in the heat capacity measurement. This anomaly is not evident in the 10 kOe measurement. We attribute this anomaly to a trace amount of ferromagnetic Pr3Ru (TC ¼ 25 K) [35]. A kink-point measurement of Ce23Ru7Cd4 displays ferro- or ferrimagnetic ordering at 3.6 K. This is in line with an observed anomaly in the heat capacity measurement (Fig. 6). The presence of long-range magnetic ordering underlines that some cerium sites are in a trivalent state. The magnetization isotherms taken at 5, 10 and 50 K for Pr23Ru7Cd4 as well as those taken at 3, 5, 10 and 25 K for Ce23Ru7Cd4 are shown in Fig. 5. We observe an almost linear increase of the magnetization with the applied field at 50 and 25 K, respectively for each compound, as expected for a paramagnetic material. At 3 K the magnetization isotherm of Ce23Ru7Cd4 is indicative of parallel oriented magnetic moments, with no hysteresis being noted. The saturation magnetization at 80 kOe and 3 K is 0.61 mB/Ce atom, which is significantly smaller than the theoretical value of 2.14 mB/Ce atom (g  J). The reduced moment is due to the mixed valence and could additionally be ascribed to crystal field splitting of the J ¼ 5/2 ground state.

Fig. 6. Heat capacity measurements of Pr23Ru7Cd4 and Ce23Ru7Cd4 without an external applied field.

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At 5 K the magnetization isotherm of Pr23Ru7Cd4 increases linearly with the applied field up to around 30 kOe and then begins to saturate, with a saturation magnetization at 80 kOe of 1.6 mB/Pr atom. This is almost half of the theoretical value. A small hysteresis was observed between 20 and 30 kOe, associated with a small jump in the magnetization curve. This may be associated with a spin-flip transition (spin reorientation). In view of the nine crystallographically independent praseodymium sites, however, the magnetic structure is most likely complex. The characterization of the two ruthenium-based compounds belonging to the 23-7-4 family shows that the cerium compound is in a mixed valence state, whereas the praseodymium atoms are purely trivalent. The structure refinement reveals 43% trivalent Ce atoms, of which all or some order ferro- or ferrimagnetically. The other 57% are in an intermediate-valence state. The other cerium based 23-7-4 compounds known in literature, show a purely trivalent state for all cerium sites. Our findings for Ce23Ru7Cd4 appear to be due to the strong cerium–ruthenium interactions, which has also been found and verified for CeRuSn [20,21] and Ce2RuZn4 [22,23] by magnetic measurements, band structure calculations and crystal structure refinements. Other rutheniumbased intermetallic compounds, mentioned in the introduction, also display an unusual behavior. A more systematic study of the structure–property relationships of such cerium–ruthenium intermetallics is underway. Acknowledgments We thank Dipl.-Ing. U.Ch. Rodewald and Dr. R.-D. Hoffmann for the single crystal data collection. This work was financially supported by the Deutsche Forschungsgemeinschaft. W.H. is indebted to the Fonds der Chemischen Industrie and to the NRW Graduate School of Chemistry for a Ph.D. stipend. References [1] Malik SK, Adroja DT. Phys Rev B 1991;43:6277. [2] 88 entries occur for the formula CeRhSb in the SciFinder Scholar version 2009: http://www.cas.org/SCIFINDER/SCHOLAR/.

[3] Ueda T, Honda D, Shiromoto T, Metoki N, Honda F, Kaneko K, et al. J Phys Soc Jpn 2005;74:2836. [4] Gaudin E, Chevalier B, Heying B, Rodewald UCh, Po¨ttgen R. Chem Mater 2005;17:2693. [5] Jayaraman A, Lowe W, Longinotti LD, Bucher E. Phys Rev Lett 1976;36:366. [6] Chevalier B, Decourt R, Heying B, Schappacher FM, Rodewald UCh, Hoffmann R-D, et al. Chem Mater 2007;19:28. [7] Fornasini ML, Palenzona A. Z Kristallogr 1991;196:105. [8] Canepa F, Palenzona A, Eggenhoffner R. J Alloys Compd 1994;215:105. [9] Fornasini ML, Palenzona A. Z Kristallogr 1992;200:57. [10] Emsley J. The elements. Oxford: Clarendon Press; 1989. [11] Wilhelm M, Hillenbrand B. J Phys Chem Solids 1970;31:559. [12] Tessema GX, Peyrard J, Nemoz A, Senateur JP, Rouault A, Fruchart R. Z Phys Chem 1979;116:209. [13] Wuilloud E, Baer Y, Maple MB. Phys Lett 1983;97A:65. [14] Huxley AD, Paulsen C, Laborde O, Tholence JL, Sanchez D, Junod A, et al. J Phys Condens Matter 1993;5:7709. [15] Kurenbaeva ZhM, Tursina AI, Murashova EV, Nesterenko SN, Gribanov AV, Seropegin YuD, et al. J Alloys Compd 2007;442:86. [16] Murashova EV, Kurenbaeva ZhM, Tursina AI, Noe¨l H, Rogl P, Grytsiv AV, et al. J Alloys Compd 2007;442:89. [17] Tursina AI, Kurenbaeva ZhM, Gribanov AV, Noe¨l H, Roisnel T, Seropegin YD. J Alloys Compd 2007;442:100. [18] Murashova EV, Tursina AI, Kurenbaeva ZhM, Gribanov AV, Seropegin YuD. J Alloys Compd 2008;454:206. [19] Gribanov AV, Tursina AI, Grytsiv AV, Murashova EV, Bukhan’ko NG, Rogl P, et al. J Alloys Compd 2008;454:164. [20] Riecken JF, Hermes W, Chevalier B, Hoffmann R-D, Schappacher FM, Po¨ttgen R. Z Anorg Allg Chem 2007;633:1094. [21] Matar SF, Riecken JF, Chevalier B, Po¨ttgen R, Eyert V. Phys Rev B 2007;76:174434. [22] Mishra R, Hermes W, Rodewald UCh, Hoffmann R-D, Po¨ttgen R. Z Anorg Allg Chem 2008;634:470. [23] Eyert V, Scheidt E-W, Scherer W, Hermes W, Po¨ttgen R. Phys Rev B 2008;78:214420. [24] Linsinger S, Po¨ttgen R, unpublished results. [25] Tappe F, Po¨ttgen R. Z Naturforsch 2009;64b:184. [26] Po¨ttgen R, Gulden Th, Simon A. GIT Labor-Fachzeitschrift 1999;43:133. [27] Kußmann D, Hoffmann R-D, Po¨ttgen R. Z Anorg Allg Chem 1998;624:1727. [28] Yvon K, Jeitschko W, Parthe´ E. J Appl Crystallogr 1977;10:73. [29] Rodewald UCh, Tuncel S, Chevalier B, Po¨ttgen R. Z Anorg Allg Chem 2008;634:1011. [30] Sheldrick GM. SHELXL-97, Program for crystal structure refinement. Germany: University of Go¨ttingen; 1997. [31] Flack HD, Bernadinelli G. Acta Crystallogr 1999;A55:908. [32] Flack HD, Bernadinelli G. J Appl Crystallogr 2000;33:1143. [33] Tuncel S, Hermes W, Chevalier B, Rodewald UCh, Po¨ttgen R. Z Anorg Allg Chem 2008;634:2140. [34] Linsinger S, Tuncel S, Hermes W, Eul M, Chevalier B, Po¨ttgen R. Z Anorg Allg Chem 2009;635:282. [35] Garde CS, Ray J. J Magn Magn Mater 1998;189:293.