Proceedings of the 29th International Conference on Low Temperature Physics (LT29)
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Proc. 29th Int. Conf. Low Temperature Physics (LT29)
JPS Conf. Proc. 38, 011078 (2023)
https://doi.org/10.7566/JPSCP.38.011078

Y-Substitution Effects on Electrical Resistivity of(Ce1-yYy)PtGe2
Hayato MUTO1*, Shoma ONUMA1, Tatsuya WATANABE1, Ryota TAKAHASHI1,Tomohito NAKANO1, Naoya TAKEDA1, Jun GOUCHI2, Yoshiya UWATOKO2,Klára UHLÍŘOVÁ3, Jan PROKLEŠKA3, and Vladimír SECHOVSKÝ3
1

Niigata University, Ikarashi, Niigata 950-2181, JapanInstitute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan
3
Faculty of Mathematics and Physics, Charles University, Prague 121-16, Czech Republic2*E-mail: f22k002k@mail.cc.niigata-u.ac.jp(Received July 15, 2022)
In an antiferromagnetic CePtGe2 (TN = 3.8 K), with two inequivalent Ce sites, the coexistenceof a localized f-electron state at the one Ce site and an itinerant heavy-electron state at the otherCe site has been suggested. We have prepared polycrystalline samples of (Ce1-yYy)PtGe2(y = 0 ~ 1) and have measured the electrical resistivity of y = 0, 0.1, and 0.2. For y = 0, theresistivity shows the maximum at Tmax ~ 54 K, and below Tmax, it decreases with decreasing
temperature. With increasing y, Tmax decreases, and the resistivity for y = 0.2 increases withdecreasing temperature down to 4.5 K. TN evidenced by the downward bend in the resistivity isrobust to Y substitution. From these results, a possible shift from the coherent Kondo state tothe incoherent Kondo state and the preferential Y substitution to the Ce site with the itinerant
heavy-electron state are suggested.

1. Introduction
Recently, multisite Ce compounds, which have multiple crystallographicallyinequivalent Ce sites, attract much attention. For example, the coexistence of different
f-electron states due to the different local environments of each Ce site has been reported
in Ce3Pd20Si61), Ce7Ni32), and Ce3PtIn113). In such multisite Ce compounds, newly
complex states are expected by cooperative/competitive interaction between inequivalent
Ce sites. CePtGe2 is one of the Ce-based heavy-fermion compounds which has two
inequivalent Ce sites.
The crystal structure of CePtGe2 is orthorhombic YIrGe2-type in the space group
Immm with lattice constants: a = 4.428, b = 16.48, and c = 8.795 Å4), and there are two
crystallographically inequivalent Ce sites: 4h site with local inversion symmetry and 4i
site without local inversion symmetry. Antiferromagnetic-like transition in CePtGe2 at
T = 3.8 K was reported first by Pecharsky et al. in 19955). In contrast, ferromagnetictransition in CePtGe2 at T = 5.1 K was reported by Kirita et al.6). Then, Nakano et al.
showed the antiferromagnetism in CePtGe2 with TN = 3.8 K, suggesting a possible crystal
polymorphism in CePtGe2, and they proposed the scenario of the coexistence of thelocalized f-electron state at one Ce site and the itinerant heavy f-electron state at the otherCe site7). Moreover, the direct transition from the antiferromagnetic state to aferromagnetic-like polarized paramagnetic state was observed at a very weak magnetic
field 0H = 0.2 T applied in the easy magnetization direction7). Furthermore,011078-1©2023 The Author(s)This article is published by the Physical Society of Japan under the terms of the Creative Commons Attribution 4.0 License. Any further distribution of this work
must maintain attribution to the author(s) and the title of the article, journal citation, and DOI.

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JPS Conf. Proc. 38, 011078 (2023)

preferential La-substitution to the Ce site with the localized f-electron state wassuggested on(Ce1-xLax)PtGe28).
In this study, we have prepared polycrystalline samples of Y-substituted CePtGe2,(Ce1-yYy)PtGe2, and measured their electrical resistivity to investigate a preferentialsubstitution.
2. Experimental Methods
Polycrystalline samples of (Ce1-yYy)PtGe2 (y = 0 ~ 1) were synthesized byarc-melting in the stoichiometric ratio Ce : Y : Pt : Ge = (1−y) : y : 1 : 2, under argon
atmosphere. The obtained ingots were annealed in an evacuated quartz tube at 900 ℃ forone week, and subsequently, they were quenched using iced water. The single-phase
YIrGe2-type crystal structure was confirmed by the powder X-ray diffraction pattern.
Electrical resistivity was measured by the four-probe method using a 4He cryostat.
3. Results and Discussion
Figure 1 shows the unit cell volume V of (Ce1-yYy)PtGe2 as a function of Y
concentration y. The V decreases with increasing y, and it obeys Vegard’s law, indicating
a positive pressure effect.

(Ce Y )PtGe

3V (Å )

640

1-yy2630620

61000.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Y Concentration y1

Fig. 1. Unit cell volume V of (Ce1-yYy)PtGe2.

Figure 2 shows the temperature dependence of normalized electrical resistivity
RN(T) of (Ce1-yYy)PtGe2, where they are normalized at 250 K, and RN(T) of CePtGe2 (y= 0) is obtained from the resistivity in ref.8). For y = 0, RN(T) increases with decreasingProceedings of the 29th International Conference on Low Temperature Physics (LT29)
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JPS Conf. Proc. 38, 011078 (2023)(Ce Y )PtGe

1.21-yy2

y = 0.2
2781TN276

y = 0.1

y = 0.2274

 (·cm)

0.80.6200y = 0.1
190

NR (T) = R (T) / R (250 K)

temperature due to the Kondo effect, and it attains a maximum at Tmax ~ 54 K. Below
Tmax, RN(T) decreases and shows a downward bend at TN = 3.8 K as shown in the insetof Fig. 2. For y = 0.1, Tmax decreases to ~ 37 K, and the decrease of RN(T) below Tmax
becomes weaker than that of y = 0, and TN increases slightly to 4.0 K. For y = 0.2, theincrease in RN(T) with decreasing temperature due to the Kondo effect remained downto Tmax ~ 4.4 K and, TN decreases slightly to 3.8 K. Since applying pressure to usual Ce
compounds enhances Kondo temperature TK with an increase of Tmax in resistivity, thedecrease of Tmax in (Ce1-yYy)PtGe2 with decreasing volume is unusual, suggesting theCe-dilution effect rather than chemical pressure effect by Y substitution. In addition, thedevelopment of the Kondo-increase in the resistivity by Y substitution implies that thecoherent Kondo state for y = 0 shifts to the incoherent Kondo state for y = 0.2. A similar
increase in resistivity has been reported on (CexLa1-x)Cu69), and it is understood as thetypical process from the coherent Kondo state to the incoherent Kondo state. On theother hand, this is different behavior from that on (Ce1-xLax)PtGe2 in which theresistivity keeps metallic temperature dependence in spite of La substitution8).

180

y=00.41602110y=034T (K)56100T (K)
Fig. 2. Temperature dependence of normalized electrical resistivity RN(T)of (Ce1-yYy)PtGe2. Inset is expansion around transition temperature TN.

Figure 3 shows the phase diagram of (Ce1-yYy)PtGe2. TN does not show significant
change although the TN shows a very small maximum at y = 0.1. This contrasts with thatof (Ce1-xLax)PtGe2 in which TN decrease to 2.3 K at x = 0.28). It indicates that themagnetic Ce ions with localized 4f moments are not greatly affected by Y substitution
since Y ions preferentially replace the Ce ions with the itinerant heavy-electron. This isconsistent with the fact that the Y ions are much smaller than the Ce ions.

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JPS Conf. Proc. 38, 011078 (2023)

6NT (K)

4AFM
2(Ce Y )PtGe1-y000.1

y20.2

Y concentration yFig. 3. Phase diagram of (Ce1-yYy)PtGe2.

4. Summary
In this study, we have prepared polycrystalline samples of (Ce1-yYy)PtGe2 (y = 0 ~ 1)and have measured the electrical resistivity for y = 0, 0.1, and 0.2. The unit cell volume
decreases with increasing y, indicating a positive pressure effect of Y substitution. The
resistivity maximum simultaneously shifts to lower temperatures. As a result, increaseof the resistivity was observed down to 4.5 K for y = 0.2. On the other hand, TN = 3.8 K
for
y = 0 is robust to the Y substitution. This contrasts with the La substitution inwhich TN decreases to 2.3 K at (Ce0.8La0.2)PtGe2. These results suggest a possible shift
from the coherent Kondo state for y = 0 to the incoherent Kondo state for y = 0.2, andthe preferential Y-substitution to the Ce site with the itinerant heavy-electron state.
Acknowledgment
This work was supported by JSPS KAKENHI Grant Numbers JP15K05162,JP24740226, JP15KK0156, and JP22K04669. Klára Uhlířová, Jan Prokleška, andVladimír Sechovský were supported by MGML (https://mgml.eu) within the program ofCzech Research Infrastructures (project no. LM2018096).
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