Investigations on the physical properties of RTX2 type rare earth based intermetallic compounds

• Main Authors: Jenq-Jong Lu, 呂正中
• Other Authors: Cheng Tien
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/sfkdpf

博士 === 國立成功大學 === 物理學系碩博士班 === 90 === The mixed-valence (MV) and heavy-fermion (HF) phenomena in cerium based intermetallic compounds are the subjects of continuous interest for experimental study of their physical behaviors for decades. It is believed that the hybridization of highly correlated 4f electrons with the itinerant conducting electrons should be responsible for both HF and MV behaviors. New ternary intermetallic compounds of type CeTX2 (where T = transition metals and X= Si, Ge) remain a considerable focus. In this work, we studied on this new type of Ce-based system. The thesis mainly consists of three parts. In the first part, we studied on the physical properties of CePdSi2. We examined x-ray diffraction, dc electrical resistivity, dc and ac magnetic susceptibilities, specific heat and the Ce LIII-edge x-ray absorption spectrum of this cerium based ternary compound. Electrical resistivity ρ(T) indicates the presence of Kondo and crystal-field effects. The results of magnetic susceptibility measurements for CePdSi2 exhibit a spin-glass behavior at Tf ~ 5 K and the antiferromagnetism at TN = 2.7 K. The M(t) measurement and magnetic entropy calculation also indicate the presence of spin-glass phase. The specific-heat measurement shows two peaks in C(T), one is at around 2.7 K, another is at ~ 6.8 K. The former is likely due to an antiferromagnetic transition, the latter might be due to the Schottky anomaly with spin-glass contribution. The linear specific-heat coefficient γ of CePdSi2 is 0.348 Jmole-1K-2, which is much larger than those of normal metals. This large γ value might result from Kondo effect, crystal effect and spin-glass magnetism. From these measurements, in additional to an antiferromagnetic Kondo Lattice, this compound might be magnetically classified as a re-entrant spin glass. The second part of this thesis, we worked on the evolution from heavy-fermion to mixed valence behavior in the series CePt1-xIrxSi2. To study the evolution from heavy-fermion to mixed-valence behavior upon substituting Pt for Ir in the ternary intermetallic compound CePtSi2, we carried out the x-ray diffraction, dc electric resistivity, Ce LIII-edge x-ray absorption spectra (XAS), dc susceptibilities, and specific heat measurements in the series CePt1-xIrxSi2 (x = 0, 0.2, 0.4, 0.6, 0.8. 1.0). The results obtained allowed us to suggest that the evolution from heavy-fermion to mixed-valence behavior is likely due to the enhancement of hybridization between the Ce 4f-electron wave function and sp wave function of the adjacent transition metal ion. Besides, it was found that x ~ 0.6 is the borderline region between heavy-fermion and mixed-valence regimes. The last part of this thesis, we studied on the evolution from mixed-valence to anti-ferromagnetic Kondo-Lattice behavior in the series CeNi(Si1-xGex)2, we carried out the x-ray diffraction, dc electric resistivity, Ce LIII-edge x-ray absorption spectra (XAS), dc and ac susceptibilities, and specific heat measurements in the series CeNi(Si1-xGex)2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8. 1.0). The results show that CeNiSi2 is a mixed-valence compound. On the other hand, CeNiGe2 is a Kondo-Lattice with an anti-ferromagnetic phase transition at ~ 5.8 K. When we substituted Ge for Si, with an increase in x, there is a discernible tendency towards a increasing in unit-cell volume. The increasing in unit cell volume will decrease the hybridization of Ce 4f electron wave function with the sp wave function of the neighboring transition metal atoms and increase the localization of 4f electrons and finally drive the system from MV behavior to anti-ferromagnetism. The results allowed us to suggest that the evolution from mixed-valence to Kondo-lattice behavior is likely due to the dehybridization between the Ce 4f-electron wave function and sp wave function of the adjacent transition metal ion. Besides, from these measurements, it was found that x = 0 ~ 0.2, the series show basically mixed-valence behavior. When x = 0.8 ~ 1.0, the series show Kondo-Lattice behavior. Between x = 0.4 ~ 0.6, the behaviors of the samples can be characterized as single impurity Kondo effect.