First single crystal growth of the transuranium filled-skutterudite compound NpFe4P12 and its magnetic and electrical properties

We succeeded in growing single crystals of NpFe4P12 using the Sn-flux method and measured the electrical resistivity, magnetic susceptibility, magnetization, and specific heat. This is the first single crystal growth of transuranium filled-skutterudite compounds. NpFe4P12 is found to order ferromagnetically below T-C = 23 K. The magnetic moment is directed along the (100) direction with the saturated moment of 1.35 mu B/Np. The large resistivity value indicates that NpFe4P12 might be a semimetal, where the 5f electrons are localized with a 5f(3) configuration, indicating Np4+. The electronic specific heat coefficient gamma is found to be small, gamma similar or equal to 10mJ/(K(2.)mol), which is consistent with the low carrier system.