Superconductivity and Structural Phase Transitions in Caged Compounds RT2Zn20 ($\text{R} = \text{La}$, Pr, $\text{T} = \text{Ru}$, Ir)

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Electrical resistivity $\rho$, specific heat $C$, magnetization $M$ measurements are reported on four compounds, LaRu2Zn20, PrRu2Zn20, LaIr2Zn20, and PrIr2Zn20, which crystallize in a cubic CeCr2Al20-type structure. LaRu2Zn20, LaIr2Zn20, and PrIr2Zn20 show superconducting transitions at $T_{\text{C}} = 0.2$, 0.6, and ${\sim}0.05$ K, respectively, whereas PrRu2Zn20 remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT2X20 ($\text{T} = \text{transition metal}$; $\text{X} = \text{Al}$ and Zn). Furthermore, structural phase transitions manifest themselves at $T_{\text{s}} = 150$, 138, and 200 K for LaRu2Zn20, PrRu2Zn20, and LaIr2Zn20, respectively. No magnetic transition is found in PrRu2Zn20 and PrIr2Zn20 down to 1.8 K. On cooling PrIr2Zn20 below 2 K, the specific heat divided by temperature, $C/T$, continuously increases and reaches 5 J/(K2$\cdot$mol) at 0.4 K, suggesting that Pr 4f2 electrons are involved in the heavy-fermion state, as observed in a related compound YbCo2Zn20.
2010-03-15