Itinerant 5f electrons in U2T21B6 (T=Ni and Co)

The magnetic and thermal properties of U2Ni21B6 and U2Co21B6 are reported. These compounds crystallize into a cubic C6Cr23-type structure and are isostructural to U3Pd20Si6, which shows a multipolar interaction between localized 5f electrons. The 5f electrons in U2Ni21B6 and U2Co21B6 are itinerant, in contrast to those in U3Pd20Si6. For each compound, a slightly enhanced mass of 5f electrons is confirmed by specific heat measurement. The comparison of the lattice parameter of U3Pd20Si6 with that of U2(Ni or Co)21B6 suggests that a strong chemical pressure induces the itinerancy of the 5f electrons.