Superconductivity and structural phase transitions in caged compounds RT₂Zn₂₀ (R=La, Pr, T = Ru, Ir)

Electrical resistivity ρ specific heat C, magnetization M measurements are reported on four compounds, LaRu₂Zn₂₀, PrRu ₂Zn₂₀, LaIr₂Zn₂₀, and PrIr2Zn20, which crystallize in a cubic CeCr₂Al₂₀- type structure. LaRu2Zn20, LaIr₂Zn₂₀, and PrIr2Zn20 show superconducting transitions at TC = 0:2, 0.6, and ∼0:05 K, respectively, whereas PrRu ₂Zn₂₀ remains a normal state down to 0.04 K. This is the first observation of superconductivity in the family of RT₂X ₂₀ (T = transition metal; X = Al and Zn). Furthermore, structural phase transitions manifest themselves at T_s = 150, 138, and 200K for LaRu₂Zn₂₀, PrRu₂Zn₂₀, and LaIr ₂Zn₂₀, respectively. No magnetic transition is found in PrRu₂Zn₂₀ and PrIr₂Zn₂₀ down to 1.8 K. On cooling PrIr₂Zn₂₀ below 2K, the specific heat divided by temperature, C=T, continuously increases and reaches 5 J/(K ² •mol) at 0.4 K, suggesting that Pr 4f₂ electrons are involved in the heavy-fermion state, as observed in a related compound YbCo₂Zn₂₀.