Rare-Earth Suboxides With Heavy Fermion Properties: Superconductivity and Dense-Kondo Effect



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This thesis reports on rare-earth-metal based intermetallic suboxides with heavy-fermion properties. The synthesis and properties of a new family of complex reduced lanthanum late-transition-metal germanide suboxide superconductors La26Ge19M5O5 (M = Ag, Cu) were investigated, which are comprised of unprecedented [La18O5] clusters that can be described from the face-shared condensation of five derived oxygen centered [La6O] octahedra and an unreported butterfly-like [Ge4] four-member rings. The [La18O5] clusters are capped by two [Ge4] butterfly rings on the top and bottom of the clusters to form complex infinite chains along the c-axis. The two new superconductors feature resistivity and magnetic susceptibility measurements on bulk samples of the title compounds and show the onset of superconductivity at Tc = 5.4 K and 6.4 K for the Ag and Cu compounds, respectively. By applying pressure up to 1.3 GPa, the superconducting temperature (Tc)s were enhanced by 28% and 19% for Ag and Cu compounds respectively, with no sign of saturation. The filled-Mn5Si3 structure of Ce5Ge3O was determined by a single crystal and further studies on the effect of oxygen interstitial chemistry on Ce5Ge3Ox led to the non-stoichiometric interstitial phase, Ce5Ge3Ox (0 ≤ x ≤ 1). Phase analysis was done by powder X-ray diffraction and confirmed by chemical analysis using EMPA. Fourier map analysis and Raman spectroscopy also confirmed the absence of oxygen in the binary parent compound, Ce5Ge3. The electrical resistivity, magnetic susceptibilities of the series of interstitial phases Ce5Ge3Ox (0 ≤ x ≤ 1) were investigated. The parent compound, Ce5Ge3, exhibits a normal metallic paramagnetic behavior while the oxygen-filled phases with x = 0.6 and 0.7 reveal the resistivity changes and magnetic behavior of dense Kondosystem. Interestingly, the transport properties of the Ce5Ge3Ox compounds are all normal metals, except for the x = 0.6 and 0.7 samples, yet show Kondo-like magnetic behavior. The Nd analogs, Nd5Ge3Ox (0 ≤ x ≤ 1), were synthesized in the same manner. Surprisingly the Nd phases which show dense Kondo effect for all samples except the nominal 0.0 and 0.8 samples. The magnetic behavior of Nd5Ge3Ox (0.0 ≤ x ≤ 1.6 nominal) are more complex. For all samples below 0.6, the magnetic phase transitions happen at the same temperature (27 K and 49 K), above 1.0, the phase transitions are at different temperatures without obvious trends.



Superconductor, Kondo Compounds, Heavy Fermion.