Crystal structure and physical properties of CePd4Sn: A new magnetically ordered Kondo lattice

Journal of Alloys and Compounds 577 (2013) 677–682

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Crystal structure and physical properties of CePd4Sn: A new magnetically ordered Kondo lattice A. Tursina a,⇑, S. Nesterenko a, Y. Seropegin a, A.M. Strydom b,c, J.L. Snyman b a

Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia Physics Department, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa c Institute of Physics, Chinese Academy of Sciences, PO Box 603, Beijing, China b

a r t i c l e

i n f o

Article history: Received 13 May 2013 Received in revised form 26 June 2013 Accepted 27 June 2013 Available online 5 July 2013 Keywords: Rare earth alloys and compounds Crystal structure X-ray diffraction Kondo lattice Antiferromagnetic order

a b s t r a c t We report on the synthesis, crystallographic analysis, and physical properties of the new intermetallic compound CePd4Sn. It crystallizes in the orthorhombic Cmce space group with cell dimensions a = 9.003(4) Å, b = 23.613(3) Å, c = 8.265(3) Å, Z = 16, Pearson code oS96 and adopts a site exchange variant of the Dy2Ni7Sn3 type. Chemical bonding is characterized by strong PdPd and PdSn interactions. The structure can be presented as a packing of condensed cerium polyhedra. The 4f-electron part of the electrical resistivity is in evidence of incoherent Kondo scattering near and below room temperature that originates from the Ce3+ local moment. The magnetic susceptibility demonstrates stable moment behavior down to 100 K. Magnetic ordering, presumably of an antiferromagnetic nature, occurs below TN = 0.9 K. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The rare earth (RE) palladium stannides have been intensively investigated in the last thirty years with respect to their rich crystal chemistry and manifold physical properties. The ternary REPdSn system comprises a variety of compounds which belong to different structure types – either well-known or of their own types. Equiatomic stannides REPdSn (RE = La – Lu) have been found to crystallize in two modifications-of the TiNiSi-type [1–4] and of the ZrNiAl-type [5–7]. The Heusler phases REPd2Sn with the MnCu2Al-type structure exist for the late rare earths [8,9]. Stannides REPd1-xSn2 with partially filled CeNiSi2-type were established for RE = La, Ce, Nd, Gd [10,11]. More recently, the new compounds RE3Pd4Sn6 (RE = La, Ce, Pr) (own structure type) were synthesized [12]. Among the stannides of rare earth metals, the compounds with cerium, europium, or ytterbium are of a special interest due to the fact that these lanthanides can exist in the two oxidation states. As a consequence, phase formation and crystal structure studies of the Ce- and Yb-based stannides have been of major interest and especially concentrated efforts have been devoted to exploring the crystal structures of ternary stannides for the CePdSn and YbPdSn systems. In spite of a large volume of studies into crystal chemistry and physics of ternary RE–palladium–stannides, thorough phase ⇑ Corresponding author. Tel.: +7 495 9394354; fax: +7 495 9390171. E-mail address: [email protected] (A. Tursina). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.06.166

equilibrium studies of REPdSn systems are still lacking. A partial isothermal section at 600 °C of the YbPdSn system (from 25 to 100 at.% Yb) is so far the only reported investigation in this regard [13,14 and references therein]. In this system, eight ternaries were found and seven of them were structurally characterized: YbPdSn (ZrNiAl and TiNiSi type), Yb2Pd2Sn (Mo2FeB2 type), YbPd2Sn (MnCu2Al type), YbPdSn2 (MgCuAl2 type), Yb2Pd3Sn5 (Yb2Pt3Sn5 type), Yb3Pd2Sn2 (own type), YbPd0.7Sn1.3 (AlB2 type), Yb35Pd20Sn45 (unknown). The reported existence to date of numerous CePd, CeSn and PdSn binaries in fact suggests the formation of a relatively large number of CePdSn ternaries with a wide set of structure types. However, up to now only a few compounds have been structurally described. The only systematic investigation was undertaken by Gordon and DiSalvo who, in addition to the earlier known compounds CePdSn [2], Ce2Pd2Sn [15], and CePd2Sn2 [16], have synthesized and measured magnetic properties of the stannides CePd0.5Sn2, Ce8Pd24Sn, and ‘‘Ce4Pd7Sn4’’ [11]. The authors have examined the region of the CePdSn diagram that lies near the composition line of 25 at.% Ce. More recently, a new compound Ce3Pd4Sn6 having a composition within the investigated region was synthesized [12]. Thus, taking into account previously obtained crystal data and the lack of the phase equilibria investigations, it was decided to start a systematic analytical study of the CePdSn system. Recently, formation of the new stannides Ce4Pd12Sn25 and CePd2Sn3 was announced [17]. Ce4Pd12Sn25 adopts the Ce4Pt12Sn25 type

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of structure [18] whereas CePd2Sn3 is of a new type. More recently, magnetic and transport properties of the new compound Ce4Pd12Sn25 were investigated [19]. The incommensurate modulated structure of the Ce4Pd13.6Sn5.9 intermetallic (new structure type) was studied by (3 + 1)D analysis [20]. In this work we turn attention to the synthesis and detailed structural analysis of the new palladium rich stannide CePd4Sn. In the anticipation of cooperative magnetic interactions and correlated electron behavior, we supplement the crystallographic investigation with exploratory studies of the thermal and physical properties of this compound. The variable magnetic exchange that is mostly characteristic of correlated Ce- and Yb-based compounds has its origin in the frequent occurrence of f-orbital valence instabilities in these compounds. When the single f-electron that resides in an extremely narrow atomic orbital in Ce (or similarly for the nearly-filled, 13-electron f-orbital in Yb) comes into quantum-mechanical contact with the broad and degenerate conduction-electron band, a variable degree of hybridization may take place that lends a measure of delocalization to the f electrons, and this electronic dispensation has proven to be the basis of a wealth of new and exotic phenomena in condensed matter physics. For an overview of the current state of research, the reader is referred to literature dedicated to this topic [21].

2. Experimental

Table 1 Crystallographic data for CePd4Sn and LaPd4Sn. Compound Pearson code Space group, Z Lattice parameters (Å)

Cell volume (Å3), Formula weight Calculated density (mg/m3) Range in h, k, l Range in h Reflections collected/unique /Rint Reflections with I > 2r(I) Number of parameters Absorption coefficient (mm1) Extinction coefficient R[F2 > 2r(F2)] wR(F2) GooF on F2 Compound Pearson code Space group, Z Lattice parameters (Å)

Table 2 Atomic coordinates and equivalent isotropic displacement parameters (Å2) for CePd4Sn. Atom

Wyckoff position

x

y

z

Ueq

Ce1 Ce2 Pd1 Pd2 Pd3 Pd4 Pd5 Pd6 Sn1 Sn2

8f 8f 16 g 16 g 8f 8f 8f 8d 8e 8e

0.0 0.0 0.27189(16) 0.34182(16) 0.0 0.0 0.0 0.2576(2) 0.25 0.25

0.19279(6) 0.46244(6) 0.38903(5) 0.19887(5) 0.06263(8) 0.09250(8) 0.33842(8) 0.0 0.08813(7) 0.28832(7)

0.07505(19) 0.22771(19) 0.08713(18) 0.09293(19) 0.0805(3) 0.4149(3) 0.1916(3) 0.0 0.25 0.25

0.0089(3) 0.0100(3) 0.0087(3) 0.0120(3) 0.0099(4) 0.0110(4) 0.0097(4) 0.0120(4) 0.0095(3) 0.0078(3)

2.1. Synthesis The metals used were cerium, lanthanum, palladium, and tin of 99.85, 99.85, 99.99, and 99.999 wt.% purity, respectively. Melting of the elemental components was carried out in an Edmund Bühler MAM-1 compact arc furnace on a watercooled copper hearth under Zr-gettered argon atmosphere. The alloys were melted twice to ensure homogeneity. The weight losses after the melting process were confined to less than 0.5 wt.%. Subsequently, the buttons were sealed in evacuated silica tubes, annealed at 1070 K for one month, and finally quenched in cold water. The LaPd4Sn analogue has been synthesized in the same way. The composition of the polycrystalline sample of CePd4Sn investigated by means of energy dispersive X-ray spectroscopy (EDX) using a scanning electron microscope; Carl Zeiss LEO EVO 50XVP equipped with an EDX-spectrometer INCA Energy 450 (Oxford Instruments), was determined as Ce17(±1)Pd66(±1)Sn17(±1). No impurity phases or unreacted elements were detected.

CePd4Sn oS96 Cmce (No. 64), 16 a = 9.003(4) b = 23.613(3) c = 8.265(3) 1757.0(10), 684.41 10.350 0 6 h 6 13, 0 6 k 6 35, 12 6 l 6 5 1.36 6 h 6 24.97 2567/1658/0.0692 958 65 16.674 0.00017(4) 0.0446 0.1136 0.822 LaPd4Sn oS96 Cmce (No. 64), 16 a = 9.060(3) b = 23.740(6) c = 8.274(3)

3. Results and discussion 3.1. Crystal chemistry

2.2. Single crystal and powder X-ray diffraction X-ray analysis of CePd4Sn was carried out by both powder and single crystal methods. The polycrystalline sample was proved to be single-phase using powder diffraction data obtained with a Stoe Stadi-P transmission diffractometer (Cu Ka1 radiation). A single crystal of CePd4Sn suitable for data collection, was isolated from the crushed annealed polycrystalline sample. Single-crystal intensity data were collected on a CAD4 Enraf Nonius diffractometer (Ag Ka radiation, x/h-scan). An empirical absorption correction was done on the basis of W-scan data [22]. Structure solution and refinement in anisotropic approximation were carried out with use of the SHELX program package [23]. Crystal data and further details of the data collection and refinement are given in Table 1. For the LaPd4Sn compound obtained solely in polycrystalline form the cell parameters were refined from powder data. Atomic positions were standardized with the program STRUCTURE TIDY [24]. Final values of the positional and equivalent displacement parameters are given in Table 2. Selected interatomic distances are listed in Table 3.

2.3. Physical properties Measurements of temperature and magnetic field dependencies of physical properties were performed using a PPMS platform from Quantum Design, San Diego. For electrical resistivity a standard dc-current, 4-probe method was used. Specific heat was measured with a relaxation technique. For low-temperature studies a 3 He insert was used on the PPMS platform. Magnetic properties were measured using a 7 T squid-type magnetometer, also from Quantum Design.

CePd4Sn crystallizes in a new structure type. Each crystallographic site is fully occupied by a unique element. The structure of CePd4Sn presents a site-exchange variant of the Dy2Ni7Sn3 structure [25] (see Fig. 1). Interestingly, instead of a simple Sn M Pd exchange providing a transformation from 273 stoichiometry to that of 282, a two-stage substitution variant is realized. Namely, the Dy2 atom substitutes for Pd6 and Sn1 in turn substitutes for Ce2. The structures of CePd4Sn and Dy2Ni7Sn3 can be presented as built of two types of fragments alternating in [0 1 0] direction: fragments of Ce2Pd6Sn2 and Dy2Ni6Sn2 compositions, respectively (denoted as I) and fragments of Ce2Pd10Sn2 and Dy2Ni8Sn4 compositions, respectively (denoted as II). As shown in Fig. 1, fragment I does not change markedly on going from Dy2Ni7Sn3 to CePd4Sn structure while fragment II, on the other hand, changes noticeably. This is consistent with the above-mentioned two-stage substitution which occurs solely in fragment II. The two crystallographically different cerium atoms have distinctly different coordination (Fig. 2). The first coordination sphere of the Ce1 is built of ten palladium atoms at distances in the range 3.074–3.253 Å. The next-nearest neighbor two Pd and six Sn atoms are at 3.499–3.674 Å. The resulting polyhedron is a strongly distorted hexagonal prism centered on both its basal faces

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A. Tursina et al. / Journal of Alloys and Compounds 577 (2013) 677–682 Table 3 Interatomic distances in the structure of CePd4Sn. Atom

To atom

d (Å)

Atom

To atom

d (Å)

Ce1

Pd3 2 Pd2 2 Pd2 2 Pd1 2 Pd2 Pd5 2 Sn2 2 Sn2 Pd5 2 Sn1 Pd4

3.074(2) 3.084(2) 3.095(2) 3.1221(18) 3.2404(19) 3.253(3) 3.4992(16) 3.5329(16) 3.571(3) 3.6420(17) 3.674(3)

Pd3

2 Sn1 2 Pd1 2 Pd6 Ce2 Pd4 Ce2 Ce1 Pd3

2.7188(16) 2.727(2) 2.830(2) 2.848(3) 2.853(3) 2.975(3) 3.074(2) 3.243(4)

2 Sn1 Pd5 Pd3

2.6332(15) 2.809(3) 2.853(3)

Pd3 Pd4 Pd5 Pd3 2 Pd6 2 Pd1 2 Pd1 Pd4 2 Pd6 2 Sn1 Ce2

2.848(3) 2.893(3) 2.944(3) 2.975(3) 3.015(2) 3.0925(19) 3.2165(19) 3.290(3) 3.351(2) 3.730(2) 4.161(3)

2 Pd1 2 Pd2 Ce2 2 Pd6 Ce2

2.865(2) 2.888(2) 2.893(3) 3.167(2) 3.290(3)

2 Sn2 Pd4 2 Pd1 2 Pd2 Ce2

2.5881(15) 2.809(3) 2.858(2) 2.887(3) 2.944(3)

Pd1 Pd3 Pd6 Sn2 Pd2 Sn1 Pd5 Pd4 Pd5 Ce2 Ce1 Ce2

2.721(3) 2.727(2) 2.7304(14) 2.7398(18) 2.751(2) 2.8448(18) 2.858(2) 2.865(2) 2.998(2) 3.0925(19) 3.1221(18) 3.2165(19)

2 Pd1 Ce1 Ce1

2.998(2) 3.253(3) 3.571(3)

2 Pd1 2 Pd3 Sn1 2 Ce2 Pd4 2 Ce2

2.7304(14) 2.830(2) 2.9334(14) 3.015(2) 3.167(2) 3.351(2)

2 Pd4 2 Pd3

2.6332(15) 2.7188(16)

Sn2 Pd1 Pd2 Pd5 Pd4 Sn2 Sn1 Pd2 Ce1 Ce1 Ce1 Pd2

2.6134(19) 2.751(2) 2.848(3) 2.887(3) 2.888(2) 2.9678(19) 3.034(2) 3.078(3) 3.084(2) 3.095(2) 3.2404(19) 3.305(3)

2 Pd1 2 Pd6 2 Pd2

2.8448(18) 2.9334(14) 3.034(2)

2 2 2 2

2.5881(15) 2.6134(19) 2.7398(18) 2.9678(19)

Ce2

Pd1

Pd2

Pd4

Pd5

Pd6

Sn1

Sn2

Pd5 Pd2 Pd1 Pd2

and on the four side faces Ce1[Pd12Sn6]. Compared to the situation with Ce1, the Ce2 polyhedron is much more regular, being a distorted pentagonal prism centered on all side faces Ce2[Pd13Sn2]. Here, cerium–palladium distances range from 2.848 to 3.351 Å in the immediate Ce2 environment, with the next two Sn atoms further away at a distance of 3.730 Å. The structure of CePd4Sn can be presented as a packing of condensed Ce1[Pd12Sn6] and Ce2[Pd13Sn2] polyhedra completely filling the space (Fig. 3). Compared with the sum of the atomic radii of Ce and Pd (3.20 Å [26]), the shortest CePd distances are 3.074 Å (4% contraction) for Ce1 and 2.848 Å (11% contraction) for Ce2. Moreover, the latter contact is even below the sum of the pair of covalent radii of 2.93 Å. This indicates fairly strong CePd interactions. It is noteworthy that the upper values for CePd contacts are approximately equal for the Ce1 and Ce2 whereas the lower values as well as the mean values vary significantly. In contrast to the essential CePd bonding, CeSn contacts in the range 3.499–3.730 Å are significantly longer than the sum of atomic radii for cerium and tin of 3.23 Å [26] and hence cannot be considered as bonding distances.

Dy2

Pd6

Sn1

Ce2

I

II

Dy2Ni7Sn3

CePd 4Sn

Fig. 1. Projection of the structures CePd4Sn and Dy2Ni7Sn3 onto XY-plane. Cerium and dysprosium atoms are drawn as big green circles, palladium and nickel atoms as blue circles, and tin atoms as pink circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The Ce1 atom has one cerium neighbor at 4.656 Å and two cerium neighbors at 4.937 Å. Ce2 atom has one cerium neighbor at 4.161 Å and the next four neighbors considerably more distant at 4.517, 4.517, 4.656 and 4.838 Å, respectively. The Ce2Ce2 separation of 4.161 Å is within a typical bonding distance for the compounds with low rare-earth and high transition metal content and is comparable to CeCe distances of 4.256 Å in Ce2Pd9Sb3 [27], 4.379 Å in Ce3Pd6Sb5 [28], and 4.199 Å in Ce8Pd24Sn [11]. Therefore, CeCe interactions in the structure of CePd4Sn are likely to be limited to the Ce2–Ce2 pairs 4.161 Å apart (Ce2 having also the shortest CePd distances, as discussed further above). Summing up the crystal data on CeCe, CePd, and CeSn contacts, we note the well-marked difference in the coordination environment of the Ce1 and Ce2 atoms, suggesting different valence state of the two cerium sites with Ce2 valence state higher than 3. Six crystallographically independent palladium sites in the CePd4Sn structure have identical coordination environment – severely distorted icosahedra (Pd1, Pd2, Pd4, Pd6) or a polyhedron derived from icosahedra (Pd3, Pd5). Such coordination is characteristic of the compounds with high content of the transition element and was observed earlier among the crystal structures of the rareearth stannides Eu3Cu8Sn4 [29], Yb4Cu23Sn11 [30], CeCu5In [31], CeNi5Sn [32], and Yb2Cu8Sn3 [33]. The PdPd and PdSn contacts being comparable to the sums of the respective atomic radii (2.75 Å for PdPd and 2.88 Å for PdSn [26]) indicate strong bonding interactions. Both crystallografically different tin atoms are surrounded exclusively by Pd neighbors. A severely distorted trigonal prism with additional atoms is characteristic of the Sn1 atom coordination, and a distorted tetragonal antiprism presents the nearest environment of the Sn2 atom (Fig. 2). 3.2. Physical properties The magnetic properties of CePd4Sn are presented in Fig. 4. A small measuring field of 0.02 T was used for the temperature

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Fig. 2. Coordination polyhedra of atoms in the structure of CePd4Sn.

range from 400 K to 120 K. This is ascribed to local-moment magnetism arising from ionic Ce3+. There are two points to note regarding this observation, however. First, the obtained effective magnetic moment amounts to leff = 2.28(1) lB, which is approximately 10% reduced compared to the unperturbed Ce3+ free-ion value. In view of the fact that there are two distinctly different Ce sites provided as elaborated in the previous section on Crystal Chemistry, the situation to assign the high-temperature magnetic state to CePd4Sn is not entirely straight-forward. Taking the heuristic view that one of the two cerium atoms bears the full free-ion effective moment of lCe1 eff ¼ 2:54 lB , then in terms of the nett effective moment value the second Ce atom may be expected to contribqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ute lCe2 ½ð2  ð2:282 Þ  ð2:542 Þ ¼ 1:99 lB . We denote Ce2 as eff ¼ the atom with this strongly reduced magnetic moment value, on account of the fact that Ce2 is characterized by the shortest Ce–Pd atomic distances. The situation for Ce2 is conducive to strong admixing of the 4f-electron orbital with its environment that would result in a valency intermediate between magnetic Ce3+ and the nonmagnetic Ce4+ state. A second point to note from the v1(T) graph in Fig. 4 is the deviation from Curie–Weiss behavior noticeable below 120 K. A loss of magnetic susceptibility (increase in

Fig. 3. Cerium polyhedra presentation of the structure of CePd4Sn.

dependence of susceptibility v(T), main panel showing inverse susceptibility, and this field was verified to be situated in a region of field-independent susceptibility. It is evident that v(T) follows Curie–Weiss behavior (dashed line) over an extended temperature

Fig. 4. Inverse magnetic susceptibility of CePd4Sn (main panel) with a least-squares fit to the v1(T) / T Curie–Weiss law as discussed in the text. Inset: isothermal magnetization measured at two different temperatures.

A. Tursina et al. / Journal of Alloys and Compounds 577 (2013) 677–682

v1(T)) in a local-moment system in this manner typically denotes the action of a crystal-electric field that achieves a splitting of, in this case, the 6-fold multiplet of the J = 5/2 Hund’s rule ground state. Finally, we calculate a Weiss temperature hP = +5.0(5) K for CePd4Sn according to the Curie–Weiss fit, which suggests a nett ferromagnetic exchange. In a 3-dimensional crystal structure the magnetic exchange is probably subject to a measure of magnetocrystalline anisotropy, and from polycrystalline data the disentanglement of contributions to the nett exchange, ferromagnetic or otherwise, is not feasible in a quantitative manner. A further in-depth analysis of especially the nature of the magnetic exchange prevalent in CePd4Sn is best left to the availability of bulk single-crystal sample material. Further to the point of different magnetic states in CePd4Sn, we note that the magnetization (inset in Fig. 4) measured at 2 K achieves a high-field value of 0.9 lB. The full saturated moment of free Ce3+ is 2.14 lB, and the measured value thus achieves somewhat less than half of the theoretical value. We ascribe this to a combined effect of crystal-electric fields, and half of the Ce ions in CePd4Sn being in a reduced-moment state. The electrical resistivity of both CePd4Sn and LaPd4Sn are metallic at first sight and display a steady positive temperature coefficient over the entire range below room temperature down to 2 K, see Fig. 5. For CePd4Sn the measured room-temperature resistivity amounts to nearly double that of LaPd4Sn, and the reason for this can be found by isolating the 4f-electron derived magnetic resistivity, i.e., by subtracting the temperature-dependent part, qLa(T)qLa(T ? 0), of the resistivity of LaPd4Sn from that of CePd4Sn. This yields the data displayed in blue symbols in Fig. 5. The magnetic resistivity thus obtained exhibits a weak upturn upon cooling from 300 K to 130 K. This is the hallmark of incoherent electron scattering from a local moment, in this case most probably provided by the higher-moment Ce3+ ionic state of the Ce1 atom. We therefore classify CePd4Sn as a new cerium-based Kondo lattice. The specific heat Cp(T) as function of temperature below 20 K of CePd4Sn and LaPd4Sn are displayed in the main panel of Fig. 6. Spin magnetic entropy associated with Ce3+ produces a heat capacity that is systematically higher than that of LaPd4Sn, but this difference becomes most prominent below 5 K where Cp(T) of CePd4Sn rises to achieve a peak of 4 J/mol K. We ascribe this to long-range

Fig. 5. Electrical resistivity of LaPd4Sn (black), CePd4Sn (red), and the 4f-electron derived magnetic resistivity of CePd4Sn (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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magnetic ordering taking place among cerium atoms. In the usual sense, we set the phase transition temperature at TN = 0.90(5) K, which is where Cp(T) achieves a maximum slope on the high-temperature side of the peak. The electronic behavior between the two compounds is further distinguished by the Cp(T)/T vs. T2 plots illustrated in the inset of Fig. 6. The Sommerfield or c-exponent of a metallic solid is indicative of the electronic density of states at the Fermi energy N(EF) through the relation c = 1/3p2kB2N(EF), in which kB is Boltzmann’s constant. For LaPd4Sn we obtain c = 5(1) mJ/mol K2, while for CePd4Sn, c = 50(5) mJ/mol K2. For CePd4Sn we confine this analysis to T > 6 K in order to avoid the region of upturn in CP(T) towards its phase transition. The density of states value thus calculated for CePd4Sn represents a moderate enhancement, even in the paramagnetic state, over the normal metallic situation prevailing in nonmagnetic LaPd4Sn. This is a consequence of the Kondo interaction which gathers part of the 4f-electron spectral weight into the Fermi energy to result, in this case, in a moderately correlated electron state. We have furthermore calculated the magnetic entropy S4f(T) of CePd4Sn, see Fig. 7. Here the blue symbols depict C4f (T)/T, obtained by subtraction of CP(T) of LaPd4Sn from that of CePd4Sn. This achieves isolation of the electron (and thus magnetic) part of the specific heat of CePd4Sn, based on the assumption that the lattice specific heats of the two compounds are alike to within the ratio of their Debye temperatures which, in turn, scale with the respective molar masses of Ce and La. The solid line in Fig. 7 illustrates the magnetic entropy. The orthorhombic crystal environment of Ce in this system is expected to heighten the entropy and to create splitting of the 6-fold electronic multiplet of Ce3+ into 3 doublet states. In case the ground state multiplet happens to be suitably separated in energy from the first excited state, the melting of magnetic order within a sublattice of Ce spins upon warming through the magnetic phase transition is expected to release an amount of entropy equal to S = R‘n 2, where R is the universal gas constant. We have indicated in Fig. 7 that S4f reaches, however, only about 1/2R‘n 2 at TN. Note that this plot is in terms of Joule per mole of compound of CePd4Sn, whereas the Crystal Chemistry discussed further above indicates that there are two distinct Ce sites in this structure for every formula unit. The entropy deficiency therefore indicates that only half of the Ce atoms per formula unit participate to any meaningful extent in the magnetic ordering process at TN. The conclusion is that the Ce1 sublattice is responsible for magnetic order in CePd4Sn, whereas the Ce2

Fig. 6. Main panel: specific heat of LaPd4Sn (black), and CePd4Sn (blue). Inset: Electronic specific heat, Cp(T)/T vs. T2, with lines depicting Cp(T)/T / T 2 behavior in the T ? 0 limit as discussed in the text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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eventually achieved among the Ce atoms in the higher-moment magnetic sublattice in Ce3Pd20Si6. Low-temperature magnetic and electric transport studies are underway in order to further characterize aspects of the magnetic phase transition and the electronic ground state in CePd4Sn. Single crystals are highly desirable in order to determine anisotropy effects produced by putative crystal-electric field excitations. Acknowledgments This work was supported by the Russian Foundation for Basic Research under Project No. 11-03-00957a. AMS thanks the URC of UJ, and the SA-NRF (78832) for funding support. JLS thanks the URC of UJ for a NGS Doctoral Fellowship. The structural part of this study was performed at the User Facilities Center of M.V. Lomonosov Moscow State University under support of Ministry of Education and Science of Russia, Contract N16.552.11.7081. Fig. 7. 4f-Electron derived magnetic specific heat of CePd4Sn as function of temperature (left-hand axis), and magnetic entropy S4f (right-hand axis). At the magnetic phase transition TN, S4f achieves only about half of the doublet electronic entropy.

atoms probably remain paramagnetic in the ground state. The Kondo effect that is evident from the electrical resistivity in CePd4Sn (Fig. 5) can be expected to achieve partial Kondo screening of 4f-electron spins in CePd4Sn, and this will also decrease the available magnetic entropy that collects at TN. Upon warming through TN, we note furthermore in Fig. 7 that a considerable amount of entropy gain continues to occur well above TN. We ascribe this to the precursor effect of short-range order among predominantly Ce1 atoms. However, against the situation of strong bonding between Ce2 and Pd as noted earlier, we cannot rule out a valence fluctuating effect among these atoms and which may also add to magnetic disorder and an excess of entropy essentially above TN. 4. Conclusion

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

The pair of new compounds CePd4Sn and LaPd4Sn form in a peculiar new orthorhombic crystal structure type. There are two distinct sites occupied by the rare-earth atom in the unit cell. The difference between the two sites is found in their respective nearest-neighbor distances, and this in turn produces very different magnetic outcomes between the Ce3+ ions occupying the two sites. A comparison of the nearest-neighbor CePd distances permits us to conclude that the Ce2 atoms, which are confined to 15-membered pentagonal prisms in CePd4Sn, have a higher valence state due to strong CePd interaction, compared to the 18-member distorted hexagonal prism in which Ce1 is located. This results in a loss of magnetic moment on the Ce3+ single 4f-electron, an observation which has been confirmed by our magnetism and specific heat studies. CePd4Sn is a new example among a relatively small class of systems in which different valence and consequently magnetic states prevail, counter-intuitively, in a metal. A recent extreme case of such a situation was found in CeRuSn [34,35], where charge ordering is produced by the periodicity between a magnetic and a nonmagnetic Ce site in the lattice. An example of coexistence between Kondo and magnetic ordering among two distinct Ce sites is found in Ce3Pd20Si6 [36]. The different point symmetries of Ce sites lead to dipolar (magnetic) and quadrupolar (electric) exchange interactions among the two respective sites. As is the case with CeRuSn and the present CePd4Sn as well, magnetic ordering is

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