Elsevier

Journal of Solid State Chemistry

Volume 196, December 2012, Pages 274-281
Journal of Solid State Chemistry

Structure and physical properties of single crystal PrCr2Al20 and CeM2Al20 (M=V, Cr): A comparison of compounds adopting the CeCr2Al20 structure type

https://doi.org/10.1016/j.jssc.2012.06.035Get rights and content

Abstract

Crystal growth and full structure determination of compounds adopting the CeCr2Al20 structure type, LnTi2Al20 (Ln=La–Pr, Sm, and Yb), LnV2Al20 (Ln=La–Pr, and Sm), and LnCr2Al20 (Ln=La–Pr, Sm, and Yb), are reported. Resistivity, magnetic susceptibility, and heat capacity of flux grown single crystals of the nonmagnetic CeM2Al20 (Ln=Ce, Yb; M=Ti, V) compounds are compared to PrCr2Al20. Of particular interest is PrCr2Al20 which does not show any phase transition down to the lowest temperature of the measurement (400 mK in resistivity measurement and 1.8 K for magnetic susceptibility measurements) and exhibits Kondo behavior at low temperatures.

Graphical abstract

Crystal structure of SmV2Al20 showing the interpenetrating diamond-like samarium network and pyrochlore-like vanadium network.

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Highlights

► Single crystals of LnM2Al20 were grown from a molten aluminum flux. ► Magnetic, electrical, and specific heat of single crystal LnM2Al20 are presented. ► PrCr2Al20 exhibits evidence of Kondo effect.

Introduction

The study of rare earth intermetallics with competing interactions between electrons has led to the discovery of highly correlated states with interesting magnetic and electrical properties such as superconductivity, heavy fermion behavior [1], [2], [3], Kondo behavior [4], [5], valence instability [6], and quantum critical systems, such as β-YbAlB4, which exhibits quantum critical behavior without doping or the application of pressure or a magnetic field [7], [8]. The Kondo effect, often seen in rare earth intermetallic compounds containing Ce and U, is caused by a coupling of localized electron moments with conduction electrons resulting in an enhancement of the electronic effective mass. The Kondo and long range Ruderman-Kittel-Kasuya-Yoshida (RKKY) effects are competing interactions present in materials with localized magnetic moments. Ce- and Yb-based intermetallics have attracted much interest in the last decade because of the interest in competition between magnetic interactions [9], such as the RKKY and Kondo competition in the structurally related CePdGa6 and Ce2PdGa12 systems [10]. Although the Kondo effect is due to coupling of magnetic ions with conduction electrons, the realization of a related effect, the quadrupolar Kondo effect, in the nonmagnetic ground state of 4f2 and 5f2 systems is a subject of interest in condensed matter. Based on the theoretical work of Cox [11], the electric quadrupole moment in the ground state can couple with conduction electrons in compounds with a cubic site symmetry. To reveal the quadrupolar Kondo effect, we have chosen to study compounds of the CeCr2Al20 structure type to systematically evaluate hybridization strength. It is expected that magnetic ordering temperatures in the RKKY systems and quadrupolar ordering are suppressed by increasing conduction-f-electron hybridization strength.

CeCr2Al20 is a robust structure type that features interpenetrating networks of the lanthanide and transition metal [12]. The family of UM2Zn20 and LnM2Zn20 compounds (Ln=lanthanides, M=Fe, Ru, Os, Co, Ir, Rh, and Ni) has been recently been studied [13], [14], [15], [16], [17], [18], [19], [20]. UIr2Zn20 is a heavy fermion ferromagnet with Tc=2.1 K and γ∼450 mJ/mol K2 [14]. In addition, many Yb analogues (Fe, Ru, Rh, Os, and Ir) have been shown to be heavy fermion compounds with γ>500 mJ/mol K2, while YbCo2Zn20 displays a higher Sommerfeld coefficient (γ∼7400 mJ/mol K2) [15]. Recently, the effective mass of the electron of YbCo2Zn20 was determined to be 100–500 me by the de Haas-van Alphen effect [18]. The rare earth magnetic ordering temperature and ordering state in several GdM2Zn20 analogues have been found to depend on the transition metal. GdM2Zn20 orders ferromagnetically at 86 K, 20 K, and 4.2 K when the transition metal is Fe, Ru, and Os, respectively, while antiferromagnetic ordering near 8 K is observed in the Co triad [16]. The magnetic ordering of Tb compounds also depends heavily on the transition metal. TbCo2Zn20 orders antiferromagnetically at 2.5 K, while TbFe2Zn20 orders ferromagnetically at ∼60 K and is very sensitive to disorder on the Fe site [17]. Additionally, doping Al onto the Zn2 site of GdFe2 (AlxZn1−x)20 decreases the Curie temperature from 86 K to 10 K (for x=0.122) by decreasing the number of electrons at the Fermi level [19].

Aluminides of the CeCr2Al20 structure type have previously been reported, including UCr2Al20 [21], LnM2Al20 (Ln=La–Nd, and Sm–Yb; M=Ti, V, Cr, Nb, Ta, Mo, and W) [22], [23]. However, the physical properties of these materials have been less extensively studied compared to the zinc analogues. CeM2Al20 (M=Ti, V, Cr, Mo) display weak temperature independent paramagnetism resulting from a tetravalent Ce, while Eu analogues are found to be nearly divalent. Lattice parameters of the Ce and Eu analogues have been shown to deviate from the expected lanthanide contraction trends, complementing the magnetic data [23], [24]. In addition YbCr2Al20 has been reported in a phase diagram but not characterized [25].

The Kondo effect has also been observed in a few Pr and Sm-based intermetallic compounds, such as Pr (Cu, Ga)13−x [26] and SmPt4Ge12 [27]leading to heavy electron states with γ∼100 mJ/mol K2 and γ∼450 mJ/mol K2, respectively. Although 4f electrons in Pr-based compounds are generally well localized, hybridization effects, especially between 4f-quadrupoles and conduction electrons, the nature of competing effects are still not well understood. Recently, the first example of a cubic Γ3 nonmagnetic ground doublet system, demonstrated by resistivity and resonant photoemission spectroscopy measurements, has been shown to exhibit the Kondo effect [28], [29]. The crystal electric field ground state of PrTi2Al20 and PrV2Al20 was determined as a non-magnetic Γ3 doublet, which does not have a dipole degree of freedom. Therefore, the observed phase transitions at 2.0 K (PrTi2Al20) and 0.6 K (PrV2Al20) are attributed to quadrupolar orderings [28], [30], [31], [32]. An enhancement of the f-electron hybridization, manifested as the Kondo effect, is found in the vanadium analogue due to a combination of the compression of the unit cell volume and additional 3d electrons in the conduction band, leading to a possible quadrupolar Kondo effect in PrV2Al20 [28]. SmM2Al20 (M=Ti, V, Cr) exhibits Sm valence fluctuations concomitant with the Kondo effect, and the f-electron hybridization increases from Ti–Cr due the additional 3d conduction electrons and the smaller unit cell volume [33]. Herein we report resistivity, magnetic susceptibility, and heat capacity of flux grown single crystals of CeTi2Al20, CeV2Al20, LnCr2Al20 (Ln=Ce, Pr, and Yb), and compare the results to previously reported SmM2Al20 (M=Ti, V, and Cr) [33] and PrM2Al20 (M=Ti and V) [28], [29], [31], [32]. Additionally we present the full structural determination of LnTi2Al20 (Ln=La–Pr, Sm, and Yb), LnV2Al20 (Ln=La–Pr, and Sm), and LnCr2Al20 (Ln=La–Pr, Sm, and Yb).

Section snippets

Synthesis

Samples were prepared via the flux growth method [34], [35]. The elements Ln (4N), M (3N), and Al (5N) in the atomic ratio of 1:2:45 were placed in alumina crucibles, sealed in evacuated silica tubes, and slowly cooled from 1423 K to 1023 K over 80 h. Excess Al was removed by centrifugation at the final temperature. YbCr2Al20 was grown under similar conditions except the dwell temperature was lowered to 1273 K to limit the vapor pressure of Yb. LnTi2Al20 series were also grown by the reaction ratio

Structure

Fig. 1a shows the crystal structure of SmV2Al20. Compounds adopting the CeCr2Al20 structure type [12] are of particular interest because of the interpenetrating rare earth and transition metal sublattices. The rare earth atom is 16-coordinate (green online) and is surrounded by 12 Al1 and 4 Al3 atoms, and the transition metal is surrounded by 12 Al (6 Al1+6 Al2) atoms, forming a distorted icosahedron, as represented as orange (online) in Fig. 1a. It is worth noting that a diamond-like magnetic

Conclusions

We report the synthesis and structures of LnM2Al20 (Ln=La–Pr, Sm and M=Ti, V, Cr), YbM2Al20 (M=Ti, Cr). Temperature dependent magnetization data suggests non-magnetic tetravalent Ce and divalent Yb, consistent with lanthanide contraction trends. The resistivity data and enhanced Sommerfeld coefficient for PrCr2Al20 indicate significant Kondo interactions at low temperatures. PrCr2Al20 shows an upturn in heat capacity and an anomaly in resistivity, which can be ascribed to quadrupolar ordering

Acknowledgments

J.Y.C. acknowledges an NSF-DMR1063735 for support of this project. S. N. acknowledges Grants-in-Aid (No. 21684019) from JSPS, Grant-in-Aid for Scientific Research on Innovative Areas “Heavy Electrons” of MEXT, Japan, and Grant-in-Aid for JSPS Fellows for support of this project.

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