Valence fluctuation in Ce2Co3Ge5 and crystal field effect in Pr2Co3Ge5

Abstract

a b s t r a c tPolycrystalline samples of ternary rare-earth germanides R 2 Co 3 Ge 5 (R ¼ La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U 2 Co 3 Si 5 structure (space group Ibam). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce 2 Co 3 Ge 5 deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment (m eff E0.95 m B ) compared to that expected for trivalent Ce ions further supports valence-fluctuating nature of Ce in Ce 2 Co 3 Ge 5 . The observed temperature dependence of magnetic susceptibility is consistent with the ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr 2 Co 3 Ge 5 . An analysis of C mag data suggests a singlet ground state in Pr 2 Co 3 Ge 5 separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.

Magnetic, electric and thermoelectric properties of ternary intermetallics from the Ce–Co–Ge system

“…The compound does not order magnetically, and the temperature dependence of its inverse magnetic susceptibility c À1 (T) is reminiscent of a behavior in intermediate valence systems. Our observation is in line with the previous report [8].…”

“…In particular, the compound CeCoGe (PbFCl-type) exhibits an antiferromagnetic ordering at the Néel temperature T N ¼ 5.0 K [3], CeCo 1-x Ge 2 (CeNiSi 2 -type) is a paramagnetic heavy fermion Kondo system [4e6], and Ce 3 CoGe 2 (La 3 NiGe 2 type) has been classified as a Kondo system exhibiting a ferromagnetic phase transition at the Curie temperature T C ¼ 6 K followed by an antiferromagnetic one at 3.4 K [7]. In turn, Ce 2 Co 3 Ge 5 (U 2 Co 3 Si 5 type) is known as an intermediate valence compound [8], CeCo 2 Ge 2 (CeGa 2 Al 2 -type) is a CurieeWeiss paramagnet [9], and CeCo 9 Ge 4 (LaFe 9 Si 4 -type) has recently been reported as a heavy fermion system with an antiferromagnetic order below T N ¼ 12.5 K [10]. Especially intriguing physical behavior has been reported for CeCoGe 3 (BaNiSn 3 -type), in which complex magnetic behavior coexists with a pressure-induced superconducting state (T sc ¼ 0.7 K, p sc ¼ 5.5 GPa) [11].…”

Valence fluctuation in Ce$_2$Re$_3$Si$_5$ and Ising-type magnetic ordering in Pr$_2$Re$_3$Si$_5$ single crystals

“…This type of mixed valent state is not uncommon in Ce-compounds. For example, Ce 2 Co 3 Ge 5 [26], Ce 2 Ni 3 Si 5 [27], Ce 2 Rh 3 Ge [28] and many other compounds exhibit the intermediate valence state of Ce [29,30]. This type of intermediate valence state of Ce and Yb atoms can be analysed by a two-level ionic interconfiguration fluctuations (ICF) model [31].…”

Influence of chemical composition on the X-ray photoemission, thermopower, specific heat, and magnetic properties of CeNi2(Si1-yGey)2

“…T SF is the spin fluctuation temperature corresponding to the fluctuation between the magnetic and nonmagnetic valence states of Ce and can be expressed via the rate of fluctuations [25] T SF = ℏω SF /k B . It should be noticed that two notations of the effective temperature are used in literature, (T + T SF ) [23] or (T 2 + T SF 2 ) 1/2 [24,26].The latter can be also interpreted as the broadening of the 4f energy levels and in principle can include different contributions to the spin fluctuations (thermal excitations, quantum mechanical fluctuations, the effect of the 4f-conduction electron hybridization).…”