Structure and physical properties of RT2Cd20 (R=rare earth, T=Ni, Pd) compounds with the CeCr2Al20-type structure

Author's Accepted Manuscript

Structure and physical properties of RT2Cd20 (R ¼rare earth, T ¼Ni, Pd) compounds with the CeCr2Al20-type structure V.W. Burnett, D. Yazici, B.D. White, N.R. Dilley, A.J. Friedman, B. Brandom, M.B. Maple

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S0022-4596(14)00138-8 http://dx.doi.org/10.1016/j.jssc.2014.03.035 YJSSC18425

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Journal of Solid State Chemistry

Received date: 23 December 2013 Revised date: 18 March 2014 Accepted date: 23 March 2014 Cite this article as: V.W. Burnett, D. Yazici, B.D. White, N.R. Dilley, A.J. Friedman, B. Brandom, M.B. Maple, Structure and physical properties of RT2Cd20 (R ¼rare earth, T ¼Ni, Pd) compounds with the CeCr2Al20-type structure, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j. jssc.2014.03.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structure and physical properties of RT2Cd20 (R = rare earth, T = Ni, Pd) compounds with the CeCr2Al20-type structure V. W. Burnetta , D. Yazicia , B. D. Whitea , N. R. Dilleyb , A. J. Friedmana , B. Brandoma , M. B. Maplea,∗ a

Department of Physics and Center of Advanced Nanoscience, University of California, San Diego, La Jolla CA 92093, USA b Quantum Design, 6325 Lusk Boulevard, San Diego, California 92121, USA

Abstract Eleven new compounds, RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb) and RPd2 Cd20 (R = Ce, Pr, Sm), were grown as single crystals in high temperature cadmiumrich solutions. They crystallize in the cubic CeCr2 Al20 -type structure (Fd3m, Z =8) as characterized by measurements of powder x-ray diffraction. Electrical resistivity, magnetization, and specific heat measurements were performed on RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb) single crystals. Whereas YNi2 Cd20 and LaNi2 Cd20 exhibit unremarkable metallic behavior, when magnetic moments from localized 4f electron states (Gd3+ - Tb3+ ) are embedded into this host, they exhibit ferromagnetic order with values of the Curie temperature TC for RNi2 Cd20 (R = Gd, and Tb) which scale with the de Gennes factor. Keywords: Cage structure, Rare-earth cadmium based intermetallics, Magnetic order, Specific heat, CeCr2 Al20 1. Introduction Interest has been steadily growing in a class of compounds with a type of “cage structure” with the chemical formula RT 2 X 20 where R is a rare earth element, T is a transition metal, and X = Al or Zn. The compounds in these families exhibit a wide variety of correlated electron phenomena including ∗

Corresponding author Email address: [email protected] (M. B. Maple)

Preprint submitted to Journal of Solid State Chemistry

March 27, 2014

heavy fermion behavior, quadrupolar order, ferromagnetic and antiferromagnetic order, and superconductivity. 1–4 These compounds form with the cubic CeCr2 Al20 -type structure and do not contain any R-R, R-T, or T-T nearest neighbors. However, filling the T site with different transition metal elements can result in significantly different behavior as is observed in GdT 2 Zn20 (T = Fe, Co). 2 A large number of new heavy fermion compounds, YbT 2 Zn20 (T = Fe, Co, Ru, Rh, Os, Ir), were also reported with γ = 500-800 mJ/mol·K2 . 3 These compounds have the unique advantage of being in the same family so that the character of the Yb ion can be studied in different local environments (i.e., by changing the transition metal element). Low-temperature states in the isomorphic PrT 2 Al20 (T = Ti and V) compounds have also been recently investigated to characterize a possible two-channel Kondo effect and quadrupolar order observed in these compounds. Furthermore, PrTi2 Al20 exhibits two distinct superconducting phases, one at ambient pressure and another induced under applied pressures near 6 GPa; each phase is associated with a different mechanism. 4–7 YbCo2 Zn20 is reported to order magnetically under pressure and Y(Co1−x Fex )2 Zn20 is observed to be very near the Stoner criterion for the onset of ferromagnetism. 8,9 Most recently, it was revealed that CeRu2 Zn20 is a Kondo lattice compound with moderately heavy quasiparticles. 10 The wide variety of correlated electron behavior displayed by these compounds can be probed further by exploiting the tunability of these systems through chemical substitution. Motivated by an interest in exploring whether similar compounds could be prepared where the cages are formed with cadmium rather than aluminum or zinc, we were able to successfully synthesize eleven new compounds of RT2 Cd20 with T = Ni, Pd. Single crystals of these compounds were grown in a high temperature cadmium-rich solution. These compounds are found to crystallize in the CeCr2 Al20 -type structure in which the R and T crystallographic sites have cubic and trigonal point symmetry, respectively. The coordination for the R site is a Frank-Kasper polyhedron with coordination number 16 (CN-16) and the T atoms occupy a site with icosahedral Cd coordination (CN-12). The Cd ions have three distinct crystallographic sites (see Fig. 1). Both of the R and T sites are fully enclosed by shells consisting of nearest and next nearest neighbors of Cd. If there is any precedence set by the RT 2 X 20 (X = Al, Zn) compounds, we expect that among these RT 2 Cd20 compounds, there may be a plethora of unexplored correlated electron ground states and phenomena waiting to be characterized. In analogy with the isostructural X = Al, Zn series of compounds, there exists a wide 2

phase space available in which to tune these systems via chemical substitution on several distinct crystallographic sites. By replacing X = Al, Zn with X = Cd, we have primarily modified the cage structures surrounding the R and T crystallographic sites, which will potentially facilitate tuning phenomena of interest in these compounds which are sensitive to the character of the local environment (i.e., bond lengths/strengths, hybridization between localized and itinerant electron states, crystalline electric fields, etc.). More then 200 rare earth-transition metal-cadmium intermetallic compounds have been reported with different crystal structure types; for example, R 2 Ni2 Cd (R = La-Nd, Sm, Tb, Dy), 11–13 R 2 Rh2 Cd (R = La-Nd, Sm), 14 R 2 Pd2 Cd (R = La, Ce, Nd), 13 and R 2 Au2 Cd (R = La-Nd, Sm, Gd) 15–18 adopt the orthorhombic Mn2 AlB2 - type structure with space group P 4/mbm; R 23 Ru7 Cd4 (R = Ce, and Pr) crystallize with the hexagonal Pr23 Ir7 Mg4 -type structure with space group P 63 mc, 11,19 and RT4 Cd (R = Y, La-Nd, Sm and Gd-Tm, Lu; T = Co, Ru, and Rh) form in the cubic Gd4 RhIn-type structure space group F43m. 20 To our knowledge, the space group Fd3m of the CeCr2 Al20 type crystal structure has not been previously reported in any Cd-based intermetallic compounds. Herein, we present the synthesis method and crystal structure, including atomic positions and thermal parameters for each atom as well as the lattice parameters, for the new compounds, RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb) and RPd2 Cd20 (R = Ce, Pr, Sm) synthesized in single crystalline form. In addition, we present electrical resistivity, magnetization, and specific heat results for the solution-grown single crystals of RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb). The physical properties of RPd2 Cd20 (R = Ce, Pr, Sm) will be reported elsewhere. 2. Experimental Details 2.1. Synthesis Starting materials for the preparation of the RT 2 Cd20 compounds were ingots of the rare earth metals (Alfa Aesar 99.9%), nickel slugs (Alfa Aesar 99.995%), palladium wire (Alfa Aesar 99.9%), and cadmium shot (Cominco Electronic Materials 99.9999%). The elements were placed in an alumina crucible in the ratio 1:2:48 and sealed in a quartz tube under a 150 torr argon atmosphere at room temperature. The quartz tubes for RNi2 Cd20 (R = La, Ce−Nd, Sm) samples were reacted at 800◦ C (825◦ C for R = Tb) for 96 hours and cooled at a rate of 2◦ C per hour (1.5◦ C for R = Tb) to 500◦ C. 3

Figure 1: a) Crystal structure of the cubic compounds RT 2 Cd20 . The packing of the T Cd12 (grey with blue center) and RCd16 (red) cages forms a triangular pyramid with rare earth cages (red) residing at the center of each face of the unit cell. b) FrankKasper coordination polyhedra (CN-16) formed from Cd1 and Cd3 sites around the R crystallographic site. c) Icosahedron coordination (CN-12) formed from Cd2 and Cd3 sites surrounding the T crystallographic site.

4

The other RNi2 Cd20 (R = Y and Gd) and RPd2 Cd20 (R = Ce, Pr, and Sm) samples were reacted at 900◦ C for 36 hours and cooled at a rate of 1◦ C per hour to 500◦ C. The bulk of the flux was removed for all samples by centrifugation at 500◦ C. Light cleaning of crystalline surfaces was achieved by polishing and etching with dilute HCl. The single crystals of RT 2 Cd20 were found to be stable in air for long periods of time. X-ray diffraction measurements of some of the powdered single crystals were performed three months after the slides were prepared and no evidence of impurity phases or oxidation could be detected. We were unable to synthesize RNi2 Cd20 compounds with the heavier rare earth elements R = Eu, Dy-Yb and U. Instead of obtaining RNi2 Cd20 crystals, we primarily observed single crystals of RCd13 , NiCd11 , RNi2 , or RNi5 binary phases. We speculate that these phases could be stabilized by synthesizing under applied pressure. We have not attempted to synthesize RPd2 Cd20 , R = La, Nd, and Eu-Lu compounds in single crystalline form. 2.2. Structure determination The crystal structure was characterized with x-ray diffraction of powdered single crystals by using a Bruker D8 Discover x-ray diffractometer with Cu-Kα radiation. Crystallographic parameters were extracted using the GSAS+EXPGUI software package which allowed us to perform a Rietveld refinement for all x-ray diffraction patterns. 21–23 The quality of each refinement was characterized by both the conventional and weighted residual parameters which can be found in Table 1 along with a summary of the lattice parameters and unit cell volumes. Orientation of single crystals was performed using a Bruker D8 Discover x-ray diffractometer. 2.3. Physical properties Four-wire electrical resistivity measurements were performed from ∼1.1 K to 300 K in a He4 Dewar, and down to 0.4 K for NdNi2 Cd20 using the Electrical Transport Option (ETO) on the 3 He insert for the Quantum Design PPMS equipped with a 9 T superconducting magnet. Magnetization from 2 to 300 K was measured by a commercial superconducting quantum interference device (SQUID) magnetometer (MPMS, Quantum Design). Specific heat measurements were performed down to 1.8 K using a PPMS Dynacool. The specific heat measurement employed a standard thermal relaxation technique.

5

Intensity (a. u.)

observed calculated background difference

20

30

40

50

60

70

80

90

2θ (deg.)

Figure 2: X-ray diffraction pattern for NdNi2 Cd20 (representative of typical patterns observed for RT 2 Cd20 samples) including fit to the background and pattern, and the difference between the fit and data. The inset shows a NdNi2 Cd20 single crystal where the scale of the squares is 1 mm x 1 mm.

3. Results and discussion 3.1. Structure We performed a careful study of the crystal structure for the RT 2 Cd20 compounds since they have not been previously reported. Powder x-ray diffraction patterns were measured over a large range in 2θ (10 ≤ 2θ ≤ 90) with a step size of 0.01◦ for compounds RT 2 Cd20 (T = Ni and Pd). The positions of Bragg reflections in the patterns are consistent with reported 24 patterns for compounds forming with the CeCr2 Al20 -type crystal structure. The x-ray pattern for NdNi2 Cd20 is shown in Figure 2 and is representative of typical patterns observed for RT 2 Cd20 (T = Ni and Pd) samples. It includes a broad, featureless hump at low angle which comes from the glass slide and petroleum jelly used to mount the powder on the slide. Before refining the pattern, we first carefully fit the background. Assuming that 6

3900

RNi2Cd20

3875

RPd2Cd20

3850

3

V (Å )

3825 3800 3775 3750 3725 3700

Y

La

Ce

Pr

Nd

Pm Sm

Eu

Gd

Tb

Figure 3: Unit cell volumes for RNi2 Cd20 (solid squares) and RPd2 Cd20 (open circles) compounds.

7

Table 1: Summary of lattice parameters, unit cell volume, and residual and weighted residual values which characterize the quality of the refinement for each compound. 21

Compound YNi2 Cd20 LaNi2 Cd20 CeNi2 Cd20 PrNi2 Cd20 NdNi2 Cd20 SmNi2 Cd20 GdNi2 Cd20 TbNi2 Cd20 CePd2 Cd20 PrPd2 Cd20 SmPd2 Cd20 aR

p

a (˚ A) 15.505(1) 15.584(1) 15.561(1) 15.575(1) 15.555(1) 15.530(1) 15.504(1) 15.501(1) 15.668(1) 15.699(1) 15.633(1)

V (˚ A3 ) 3727.5(1) 3784.8(1) 3768.0(1) 3778.2(1) 3763.7(1) 3745.5(1) 3726.8(1) 3724.6(1) 3846.3(1) 3869.2(1) 3820.6(1)

Rp a 0.0322 0.0182 0.0233 0.0236 0.0255 0.0285 0.0222 0.0205 0.0183 0.0246 0.0256

wRb p 0.0483 0.0266 0.0336 0.0347 0.0357 0.0415 0.0339 0.0297 0.0277 0.0381 0.0416

2 − SF 2 | /Σ | F 2 | where S is a scale factor, and F 2 and F 2 are observed and calculated = Σ | FO C O O C

structure factors, respectively. b wR

p

2 − SF 2 )/Σw | F 2 |)1/2 where w is a weight factor. = (Σw(FO C O

the crystal structure is of the CeCr2 Al20 -type, we selected initial values for the positional parameters, Wyckoff sequence, and lattice parameters from earlier investigations 25,26 and proceeded with refining the measured x-ray diffraction patterns. The goodness of fit varies between χ2 = 3.98 - 7.01, while the residual parameters are in the ranges Rp = 0.0182 - 0.0322 and wRp = 0.0266 - 0.0483. Similar values were reported for refinements of RT 2 X20 compounds where X = Al, and Zn. 25,26 The unit cell volumes of the compounds in the cadmium series are shown in Figure 3 and listed in Table 1. They are larger than those for both the zinc and aluminum compounds of this type; Cd has a larger atomic radius than Al and Zn. 25,26 The trend of the volume with lanthanide element follows the expected lanthanide contraction with the exception of R = Ce. The volumes of CeT2 Cd20 compounds are significantly smaller than those of their respective neighbors PrT2 Cd20 , which indicates that the valence of Ce is intermediate between trivalent and tetravalent. This behavior of the volume of the R = Ce compounds is similar to that observed in the X = Al series where the Ce valence is intermediate between trivalent and tetravalent, 27 but

8

Table 2: Atomic parameters of RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, and Tb) corresponding to the space group Fd3m (Z =8). The last column contains the isotropic thermal parameters, Uiso . Atoms YNi2 Cd20 Y Ni Cd1 Cd2 Cd3 LaNi2 Cd20 La Ni Cd1 Cd2 Cd3 CeNi2 Cd20 Ce Ni Cd1 Cd2 Cd3 PrNi2 Cd20 Pr Ni Cd1 Cd2 Cd3 NdNi2 Cd20 Nd Ni Cd1 Cd2 Cd3 SmNi2 Cd20 Sm Ni Cd1 Cd2 Cd3 GdNi2 Cd20 Tb Ni Cd1 Cd2 Cd3 TbNi2 Cd20 Tb Ni Cd1 Cd2 Cd3

Fd3m

x/a

y/a

z/a

Uiso (˚ A2 )

8a 16d 96g 48f 16c

1/8 1/2 0.06032(8) 0.48894(14) 0

1/8 1/2 0.06032(8) 1/8 0

1/8 1/2 0.32435(8) 1/8 0

0.0215(14) 0.050(2) 0.0186(3) 0.0202(4) 0.0265(9)

8a 16d 96g 48f 16c

1/8 1/2 0.06010(10) 0.49086(19) 0

1/8 1/2 0.06010(10) 1/8 0

1/8 1/2 0.32476(9) 1/8 0

0.0174(14) 0.0122(18) 0.0199(5) 0.0238(7) 0.0249(11)

8a 16d 96g 48f 16c

1/8 1/2 0.05932(8) 0.48996(15) 0

1/8 1/2 0.05932(8) 1/8 0

1/8 1/2 0.32515(7) 1/8 0

0.0217(10) 0.0204(16) 0.0188(2) 0.0242(5) 0.0313(9)

8a 16d 96g 48f 16c

1/8 1/2 0.05928(10) 0.49154(19) 0

1/8 1/2 0.059288(10) 1/8 0

1/8 1/2 0.32456(9) 1/8 0

0.0151(11) 0.0271(18) 0.0209(3) 0.0275(5) 0.0285(9)

8a 16d 96g 48f 16c

1/8 1/2 0.06003(8) 0.49117(14) 0

1/8 1/2 0.06003(8) 1/8 0

1/8 1/2 0.32354(7) 1/8 0

0.0158(8) 0.0187(14) 0.0198(2) 0.0245(4) 0.0265(8)

8a 16d 96g 48f 16c

1/8 1/2 0.06003(7) 0.48957(12) 0

1/8 1/2 0.06003(7) 1/8 0

1/8 1/2 0.32358(6) 1/8 0

0.0093(7) 0.0031(12) 0.0139(2) 0.0142(4) 0.0229(8)

8a 16d 96g 48f 16c

1/8 1/2 0.05914(12) 0.48713(18) 0

1/8 1/2 0.05914(12) 1/8 0

1/8 1/2 0.32419(11) 1/8 0

0.0117(13) 0.019(2) 0.0231(4) 0.0098(5) 0.0307(12)

8a 16d 96g 48f 16c

1/8 1/2 0.05973(9) 0.48924(16) 0

1/8 1/2 0.05973(9) 1/8 0

1/8 1/2 0.32414(8) 1/8 0

0.0140(9) 0.0107(15) 0.0159(2) 0.0251(5) 0.0297(8)

9

Table 3: Atomic parameters of RPd2 Cd20 (R = Ce, Pr, and Sm) corresponding to the space group Fd3m (Z =8). The last column contains the isotropic thermal parameters, Uiso . Atoms CePd2 Cd20 Ce Pd Cd1 Cd2 Cd3 PrPd2 Cd20 Ce Pd Cd1 Cd2 Cd3 SmPd2 Cd20 Sm Pd Cd1 Cd2 Cd3

Fd3m

x/a

y/a

z/a

Uiso (˚ A2 )

8a 16d 96g 48f 16c

1/8 1/2 0.05997(15) 0.48929(27) 0

1/8 1/2 0.05997(15) 1/8 0

1/8 1/2 0.32206(14) 1/8 0

0.0303(19) 0.0341(14) 0.0226(4) 0.0308(8) 0.0356(11)

8a 16d 96g 48f 16c

1/8 1/2 0.06153(17) 0.49044(29) 0

1/8 1/2 0.06153(17) 1/8 0

1/8 1/2 0.32234(14) 1/8 0

0.0144(19) 0.0133(13) 0.0165(7) 0.0318(12) 0.0332(16)

8a 16d 96g 48f 16c

1/8 1/2 0.06026(13) 0.49164(22) 0

1/8 1/2 0.06026(13) 1/8 0

1/8 1/2 0.32085(11) 1/8 0

0.0203(13) 0.0389(13) 0.0156(3) 0.0297(7) 0.0372(13)

is different from that observed in the Zn series where Ce is tetravalent. 25,26 Furthermore, an intermediate Ce valence has been observed in other Cdbased intermetallic compounds like Ce23 Ru7 Cd4 . 19 The site occupancy is assumed to be full in initial refinement cycles, which concentrate on obtaining optimized atomic parameters. It was only after the atomic parameters and unit cell dimensions were optimized that we refined the site occupancies (not shown). We observed a moderate amount of under and over occupation of crystallographic sites typically on the order of ≤ 5%. More significant were the large occupancy deviations of 9-20% on the Ni sites. This observation does not imply that this site is over occupied by Ni ions, but rather is probably an indication that the scattering power from this site is higher than what is calculated for Ni alone because some amount of R ions have occupied this site. The inclination for these compounds to have a small range of homogeneity has already been recognized. 25 Mixed occupancy of 26% Al on the Mo site occurs for CeMo2 Al20 and approximately 7% Tm was observed on the Zn3 site for TmNi2 Zn20 . 25,26 More recently, the addition of a Zn4 site was required to satisfactorily refine the x-ray diffraction pattern for PrRu2 Zn20 , where the Zn1, Zn2, and Zn4 sites were all found to be underoccupied. 1 These results indicate that deviations from ideal occupancy are common for these compounds and not unique to any particular sites. It is very likely that a small amount of rare-earth ions or Cd may be occupying 10

the Ni sites. Both have higher atomic scattering factors than Ni because of their greater number of electrons. Less significant deviations from single occupancy were also observed in the Pd containing compounds. Cadmium and palladium have comparable scattering factors which makes it difficult to speculate on the character of site disorder using only results from refined x-ray diffraction patterns. A similar problem is found in the RFe2 Zn20 (R = rare-earth) series because iron and zinc have similar scattering powers. Small deviations in the occupancy of these sites are detected by observing differences in magnetic ordering temperatures between single crystals grown from different starting element concentrations. 28 In the final cycle of Rietveld refinement, the thermal parameters were refined just prior to refining the site occupancies. The thermal parameters resulting from our refinements for RT 2 Cd20 x-ray diffraction patterns show some irregularities. For example, the smallest and the largest displacement parameters in LaNi2 Cd20 are for the Ni and Cd3 sites, respectively, while the displacement parameter for Ni is larger than for Cd3 in YNi2 Cd20 . These thermal parameters reflect the complex interplay of different atomic weights, bond strengths, and coordination numbers. The dependence of Uiso on the coordination numbers is well demonstrated by the Cd1, Cd2, and Cd3 sites. The Cd3 sites have CN-14 and larger displacement parameters than the displacement parameters of Cd1 and Cd2 sites which have smaller CN-12. This indicates that the Cd atoms occupying positions with higher coordination number are more weakly bonded to their neighbors. 3.2. Physical properties Temperature-dependent magnetization data, M, divided by applied magnetic field, H, for single crystals of RNi2 Cd20 (R = Ce, Pr, Nd, Sm, Gd, and Tb) are shown in Figure 4. The three compounds, RNi2 Cd20 with R = Sm, Gd, and Tb, exhibit a large step increase in M/H with decreasing temperature, indicating a phase transition to ferromagnetic order. The corresponding Curie temperatures, TC , of these three compounds are indicated by solid arrows in Figure 4(a). In addition, M/H goes through a maximum at TM 1 ∼ 3.4 K and ∼ 3.5 K for GdNi2 Cd20 and TbNi2 Cd20 , respectively. This suggests that there may be a magnetic phase transition to a ferrimagnetic or antiferromagnetic ordered state at TM 1 , indicated by the dashed arrows in Figure 4(a). For TbNi2 Cd20 , M/H goes through a minimum near TM 2 ∼ 2.3 K followed by an upturn with decreasing temperature, indicating a possible third magnetic phase transition. There is a large increase in M/H 11

below ∼ 5 K for NdNi2 Cd20 , as shown in Figure 4(b), which is indicative of a possible magnetically ordered state at temperatures below 2 K. The remaining compounds with R = Ce and Pr do not show any evidence for magnetic order above 2 K. The increase of M/H below 5 K in CeNi2 Cd20 may be attributed to a very small amount of paramagnetic impurities; although, the powder x-ray diffraction pattern showed no evidence for impurity phases. The saturation of M/H at lower temperatures for PrNi2 Cd20 as shown in Figure 4(b) indicates a strongly-correlated, nonmagnetic ground state in this compound. Due to the large R − R spatial separation, such low temperature magnetic order for the 4f local moments, which are coupled via the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction, is not unexpected. As is shown in Figure 5, the TC values of RNi2 Cd20 (R = Gd and Tb) scale fairly well with the de Gennes factor, dG = (gJ − 1)2 J(J + 1), which is consistent with the RKKY exchange interaction being responsible for magnetic order. The temperature-dependent H/M data, approximately equal to the inverse magnetic susceptibility χ−1 , are linear over an extended temperature range of 20-300 K for RNi2 Cd20 with R = Pr, Nd, Sm, Gd, and Tb. Least-squares fits of the data to a Curie-Weiss law χ = C0 /(T − ΘCW ),

(1)

where C0 = μ2ef f NA /3kB , NA is Avogadro’s number, and kB is the Boltzmann constant, allow us to obtain μef f values which are tabulated in Table 4. These effective magnetic moments are reasonably close to the expected values for trivalent R3+ (R = Pr, Nd, Sm, Gd, and Tb) as calculated using Hund’s rules. The χ−1 data exhibit two distinct linear regions for the CeNi2 Cd20 compound. Fitting the data in the range 20 K ≤ T ≤ 150 K yields ΘCW = 4 K and μef f = 2.18 μB (tabulated in Table 4), and fitting the data in range 150 K ≤ T ≤ 300 K results in best fit values of ΘCW = 26.4 K and μef f = 1.97 μB . These μef f values for CeNi2 Cd20 are lower than the theoretical value of μef f = 2.54 μB for both temperature regions. This result is consistent with an intermediate Ce valence, which we have already inferred from the deviation of the unit cell volume from the lanthanide contraction (see Figure 3). The compounds RNi2 Cd20 (R = Sm, Gd, and Tb) exhibit typical ferromagnetic behavior in which the Curie-Weiss temperature is comparable to the Curie temperature. The ΘCW value we obtain for PrNi2 Cd20 is very close to zero, consistent with the absence of magnetic order down to 2 K. Samarium compounds are known to exhibit interesting but complex mag12

Table 4: Residual electrical resistivity ratio, RRR = R(300 K)/R(1.2 K), Curie-Weiss (CW) temperature ΘCW (with ± 0.2 K uncertainty), effective magnetic moment μef f (from the CW fit of M/H data in the temperature ranges described in the text), saturated moment at 5 T, μsat , Curie TC and N´eel TN temperatures, and Sommerfeld coefficient γ for RNi2 Cd20 compounds (R = Ce - Nd, Sm, Gd, and Tb).

RRR ΘCW (K) μef f (μB ) μsat (μB ) TC (K) TN (K) γ (mJ mol−1 K−2 )

Ce 33 4.0 2.18

Pr 18 0.4 3.50

Nd 20 -1.6 3.79

– – 71

– – 621

– 1.5 99

Sm 62 7.7 0.92 0.52 7.2 – 165

Gd Tb 39 30 12.7 9.6 7.47 9.77 5.84 9.37 13.8 7.5 3.4 3.5 59 66

netic behavior owing to the closely spaced energy levels of Sm3+ . 14,28,29 Unlike the SmCo2 Zn20 and SmT2 Al20 compounds, H/M for SmNi2 Cd20 is observed to deviate from Curie-Weiss behavior (see inset of Figure 4(c)) and displays a clear ferromagnetic transition at TC = 7.2 K. The magnetic ordering temperatures TC and TN for RNi2 Cd20 (R = Nd, Sm, Gd, and Tb) compounds were determined from the temperatures of the extrema in d(M/H)/dT (not shown) and are listed in Table 4. These magnetic ordering temperatures are consistent with results obtained from electrical resistivity and specific heat measurements which are discussed below. The nature of the magnetic order is further characterized by performing isothermal magnetization measurements at low temperature, which are presented in Figure 4(d). The M vs H data were measured at 2 K with H parallel to the [100] direction. For RNi2 Cd20 (R = Sm, Gd, and Tb), the data are consistent with a FM-ordered state in which M increases rapidly with increasing H, saturating with large ordered magnetic moments. The magnetization along the [100] direction for GdNi2 Cd20 appears to exhibit a metamagnetic phase transition near 0.04 T. By extrapolating the high magnetic field slope of the magnetization curves to zero magnetic field, the saturation moments μsat were determined to be 0.52 μB /f.u., 5.84 μB /f.u., and 9.37μB /f.u. for R = Sm, Gd, and Tb, respectively. These values are slightly lower than the theoretical values of μsat = gJ JμB = 0.71 μB /f.u. and 7.00 μB /f.u. for SmNi2 Cd20 and GdNi2 Cd20 , respectively, and slightly higher than 13

80 SmNi2Cd20

70

Tm1 ~ 3.4 K

NdNi2Cd20 0.8

50 40

PrNi2Cd20

TbNi2Cd20

M/H (emu/mol)

M/H (emu/mol)

60

CeNi2Cd20

1.0

GdNi2Cd20

Tm1 ~ 3.5 K Tm2~ 2.3 K

30

0.6

0.4

20 0.2 10 (b)

(a) 0 0

5

H/M (mol/emu)

400

3

5

10 T (K)

15

20

10 (c)

SmNi2Cd20

(d)

9.37 μB/ f.u.

8

1 100

200

300

T (K)

200

6 5.84 μB/ f.u. 4 5xM 2

100

0 0

0 0

20

2

0 0

300

15

M (B per f.u.)

3

500

H/M (10 mol/emu)

4

10 T (K)

100

200

300

0 0

5 x 0.52 = 2.6 μB/ f.u.

1

2

3

4

5

H (T)

T (K)

Figure 4: (a) Temperature-dependent magnetization M data for RNi2 Cd20 divided by applied magnetic field H (H = 0.1 T for R = Ce-Nd and H = 0.03 T for R = Sm, Gd, and Tb). Magnetic ordering temperatures are indicated by arrows. (b) M/H vs. T for R = Ce, Pr, and Nd. (c) H/M vs. T for RNi2 Cd20 (R = Ce-Nd, Gd, and Tb). Inset: H/M vs. T in an applied magnetic field of H = 0.03 T for SmNi2 Cd20 . Solid lines represent Curie-Weiss fits to the data using equation (1) as described in the text. (d) M vs. H at 2 K for the compounds RNi2 Cd20 (R = Ce, Pr, Sm, Gd, and Tb). The M vs. H data for SmNi2 Cd20 were scaled by a factor of 5 for clarity.

.

14

15

RNi2Cd20 12

TC TN

2

dG = (g - 1) J(J + 1)

0.9(dG)

TM (K)

9

6

3

0 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

R Element

Figure 5: Curie temperatures TC (open squares) and N´eel temperatures TN (solid squares) plotted vs. R element. The values of TC appear to scale with the de Gennes factor, dG, which is normalized to the value of TM for Gd (dashed line). The arrows for R = Ce, and Pr indicate no magnetic order down to 1.2 K, magnetic order may occur below that temperature.

15

0 40

50

CeNi2Cd20 ρ (μΩ-cm)

NdNi2Cd20 SmNi2Cd20 TbNi2Cd20

10 0 12 ρ (μΩ-cm)

4

20

ρ (μΩ-cm)

8

10

(a) 50

100

300

YNi2Cd20

(b)

0

250

20

30 ρ (μΩ-cm)

200

30

GdNi2Cd20

0

T (K) 150

100

RRR ~ 110

PrNi2Cd20 40

50

150 T (K)

200

250

300

H=5T

3

H=0T

2 1 0

4

LaNi2Cd20

1

Tm1 ~ 3.4 K

0 0

2 3 T (K)

4

5

(c)

Tm1 ~ 3.5 K

5

10

15

T (K)

Figure 6: (a) Temperature-dependent electrical resistivity ρ of single crystals of RNi2 Cd20 (R = Ce-Nd, Gd, and Tb). (b) ρ vs. temperature T of YNi2 Cd20 and LaNi2 Cd20 . (c) At low temperatures ρ(T )exhibits anomalies at the magnetic ordering temperatures (indicated by arrows) of SmNi2 Cd20 , GdNi2 Cd20 , and TbNi2 Cd20 . Inset: ρ vs. T for NdNi2 Cd20 measured in zero-field and under an applied magnetic field of H = 5 T.

the theoretical value of μsat = 9.00 μB /f.u. for TbNi2 Cd20 . The ratio of the effective magnetic moment to the saturation magnetic moment for these compounds is μef f /μsat ∼ 1, suggesting that the f electrons that participate in magnetic order are localized. 30 The M (H) isotherms at temperatures greater than their respective TC values (not shown here), are approximately linear. For CeNi2 Cd20 and PrNi2 Cd20 , the M (H) curves vary continuously with H (i.e., there is no spontaneous magnetization) and are more consistent with a paramagnetic state at 2 K. Figure 6(a) shows electrical resistivity ρ as a function of temperature T for RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb). The zero-field residual resistivity ratio RRR ≡ ρ(300 K)/ρ(1.2 K) of ∼ 18-110 indicates that the single crystals studied are of good metallurgical quality (i.e., low impurity scattering). The RNi2 Cd20 compounds exhibit metallic behavior and a kink in the ρ(T ) curves is observed at the magnetic ordering temperatures TC and TN 16

for R = Nd, Sm, Gd, and Tb. There are no anomalies observed in ρ(T ) data for the R = Ce and Pr compounds. The ρ(T ) curves for YNi2 Cd20 and LaNi2 Cd20 are shown in Figure 6(b); they can be used to estimate the phonon contribution to ρ(T ) for RNi2 Cd20 (R = Ce-Nd, Sm, Gd, Tb). The low-temperature electrical resistivity data for these two compounds exhibit a quadratic temperature dependence (ρ(T ) = ρ0 + AT 2 ), which is consistent with Fermi liquid behavior. Figure 6(c) shows a feature in ρ(T ) related to magnetic order for RNi2 Cd20 (R = Nd, Sm, Gd, Tb). The inflection point of the first derivative dρ/dT (not shown), indicated by solid arrows in Figure 6(c), is located at temperatures TC = 7.2 K, 13.5 K, and 7.5 K for RNi2 Cd20 (R = Sm, Gd, and Tb), respectively, which are in agreement with TC values that were determined from magnetization measurements. There is another feature, indicated by dashed arrows, which can be seen for GdNi2 Cd20 and TbNi2 Cd20 at ∼ 3.4 K and 3.5 K, respectively. This feature corresponds to the ferrimagnetic or antiferromagnetic order (Tm1 ) observed in magnetization measurements. The inset of Figure 6(c) shows ρ(T ) data for NdNi2 Cd20 below 5 K, which were measured in zero-field and in an applied magnetic field of H = 5 T. There is a clear kink in the zero-field ρ(T ) curve (indicated by an arrow) which disappears in an applied magnetic field of H = 5 T. This behavior confirms the presence of antiferromagnetic order at TN = 1.5 K for NdNi2 Cd20 . The electrical resistivity of RNi2 Cd20 (R = Gd and Tb) at temperatures below TC is consistent with the ρ(T ) ∼ T 2 behavior expected for electron-magnon scattering. Data for RNi2 Cd20 compounds with R = Ce, Pr and Sm will be discussed in more detail elsewhere. 31 Specific heat divided by temperature, C/T , versus T data for RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb) are displayed in Figure 7. The similar slopes of the data for T ≥ 20 K indicate that the lattice contributions for R = Ce-Nd, Sm, Gd, and Tb are comparable with their nonmagnetic analogues YNi2 Cd20 and LaNi2 Cd20 . Therefore, we anticipate that all of these compounds have approximately the same Debye temperatures; however, significant differences in C/T at low temperatures indicate that they exhibit different values of the electronic specific heat coefficient γ ≡ C/T . Least-squares fits of C/T = γ + βT 2 to the data, where γ is the coefficient of the electronic contribution to specific heat, β = 12π 4 rR/(5Θ3D ) is the coefficient of the phonon contribution, r = 23 is the number of atoms per formula unit, R is the universal gas constant, and ΘD is the Debye temperature, yield the values to obtain ΘD  214 K for all samples and values of γ = 22.9 mJ mol−1 K−2 and 25.9 mJ mol−1 K−2 for YNi2 Cd20 and LaNi2 Cd20 , respectively. The 17

7 -2

C/T (Jmol K )

0.15

-1

6 5

0.10 0.05

5

10 2

4

15

20

25

5

YNi2Cd20

LaNi2Cd20

CeNi2Cd20

PrNi2Cd20

NdNi2Cd20

SmNi2Cd20

GdNi2Cd20

TbNi2Cd20

Tm1 ~ 3.4 K

2

T (K )

4 T ~ 2.5 K m2

-1

-2

C/T (Jmol K )

0 0

LaNi2Cd20

3

Tm1 ~ 3.5 K

-1

-2

C/T (Jmol K )

3 2

2 1

1 0 0

0

0

10

20

5

30

T (K)

10

40

15

50

T (K) Figure 7: Specific heat divided by temperature C/T vs. T for single crystals of RNi2 Cd20 (R = Y, La-Nd, Gd, and Tb). Left Inset: Low temperature C/T vs. T 2 for LaNi2 Cd20 . The solid line represents a linear fit of the data. Right Inset: Low-temperature C/T data vs. T for R = Ce-Nd, Gd, and Tb; magnetic ordering temperatures are indicated by arrows.

18

values of γ for RNi2 Cd20 (R = Ce-Nd, Sm, Gd, Tb) are tabulated in Table 4. An enhanced value of γ  621 mJ mol−1 K−2 is observed for PrNi2 Cd20 . This value is a factor of 30 times larger than the values obtained for the non-magnetic reference compounds YNi2 Cd20 and LaNi2 Cd20 . The combination of such a large value of γ, which certainly cannot be attributed to d-electron states from Ni, and the cross over from local magnetic moment behavior to an enhanced Pauli susceptibility at low temperature (see Figure 4(b)) provides strong evidence for a heavy fermion ground state in PrNi2 Cd20 . Features are observed in C/T for SmNi2 Cd20 , GdNi2 Cd20 , and TbNi2 Cd20 at ∼ 6.4 K, 13.5 K, and 7.4 K, respectively, (indicated by solid arrows) which coincide with the magnetic phase transitions in these compounds. There are additional peaks observed for GdNi2 Cd20 and TbNi2 Cd20 at ∼ 3.44 K and 3.49 K, respectively, which are similar to the transition temperatures obtained from magnetization and electrical resistivity measurements. Below 2 K, C/T data for NdNi2 Cd20 show an upturn in C/T , coinciding with the feature in electrical resistivity, which is related to antiferromagnetic ordering at TN = 1.5 K. CeNi2 Cd20 shows an upturn below 2.6 K but does not show any evidence for a phase transition down to the lowest temperatures of the electrical resistivity and magnetization measurements. This upturn may be due to magnetic order at lower temperature or a Schottky anomaly due to the splitting of the Ce multiplet by the crystalline electrical field (CEF). A low temperature divergence of the specific heat of the form C/T ∼ -InT is a signature of the breakdown of Fermi liquid theory as seen in numerous Ce, Yb, and U systems. 32,33 However, we rule out the possibility of non-Fermi liquid behavior in CeNi2 Cd20 because no evidence for it was observed in our measurements of ρ and χ. Measurements to lower temperature will be required to resolve the nature of this upturn. C/T data for both PrNi2 Cd20 and NdNi2 Cd20 exhibit a broad hump around ∼ 3.5 K and 5.8 K, respectively, which is most likely a Schottky anomaly due to CEF splitting of the Hund’s rule ground state of Pr3+ and Nd3+ . 4. Conclusion We report the synthesis and structures of eleven new compounds of RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb) and RPd2 Cd20 (R = Ce, Pr, and Sm) in single crystalline form. They crystallize in the cubic CeCr2 Al20 -type structure (Fd3m, Z =8) as characterized by measurements of powder x-ray diffraction. Additionally, we present measurements of electrical resistivity, 19

magnetization, and specific heat on the single crystals of RNi2 Cd20 (R = Y, La-Nd, Sm, Gd, Tb). Our results show that NdNi2 Cd20 orders antiferromagnetically at TN = 1.5 K, while SmNi2 Cd20 , GdNi2 Cd20 , and TbNi2 Cd20 order ferromagnetically with Curie temperatures of TC = 7.2 K, 13.8 K, and 7.5 K, respectively. The samples where R = Ce and Pr do not show any evidence for magnetic order above 2 K. The new series of compounds generated by filling the X position in RT 2 X 20 with Cd opens this family to further investigations of correlated electron behavior that may not be present in the X = Al, Zn series of compounds. 5. Acknowledgements Sample synthesis and physical properties measurements were supported by the US Department of Energy under grant No. DE-FG02-04-ER46105. Helpful discussion with Prof. S. K. Sinha are gratefully acknowledged. [1] T. Onimaru, K. T. Matsumoto, Y. F. Inoue, K. Umeo, Y. Saiga, Y. Matsushita, R. Tamura, K. Nashimoto, I. Ishii, T. Suzuki, T. Takabatake, Superconductivity and structural phase transitions in caged compounds RT2 Zn20 (R = La, Pr, T = Ru, Ir), J. Phys. Soc. Jpn. 79 (2010) 033704. [2] S. Jia, S. L. Bud’ko, G. D. Samolyuk, P. C. Canfield, Nearly ferromagnetic Fermi-liquid behaviour in YFe2 Zn20 and high-temperature ferromagnetism of GdFe2 Zn20 , Nat. Phys. 3 (2007) 334. [3] M. S. Torikachvili, S. Jia, E. D. Mun, S. T. Hannahs, R. C. Black, W. K. Neils, D. Martien, S. L. Bud’ko, P. C. Canfield, Six closely related YbT2 Zn20 (T = Fe, Co, Ru, Rh, Os, Ir) heavy fermion compounds with large local moment degeneracy, Proc. Natl. Acad. Sci. U. S. A. 104 (2007) 9960. [4] A. Sakai, S. Nakatsuji, Kondo effects and multipolar order in the cubic PrTr2 Al20 (Tr = Ti,V), J. Phys. Soc. Jpn. 80 (2011) 063701. [5] A. Sakai, K. Kuga, S. Nakatsuji, Superconductivity in the ferroquadrupolar state in the quadrupolar Kondo lattice PrTi2 Al20 , J. Phys. Soc. Jpn. 81 (2012) 083702. [6] T. Sato, S. Ibuka, Y. Nambu, T. Yamazaki, T. Hong, A. Sakai, S. Nakatsuji, Ferroquadrupolar ordering in PrTi2 Al20 , Phys. Rev. B 80 (2012) 184419. 20

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