5f-electronic states of neptunium compounds: NpGe3, NpRhGa5 and NpCd11

Abstract

We studied the electronic states of three typical neptunium compounds, NpGe3, NpRhGa5 and NpCd11. Among them, NpGe3 and NpCd11 with the cubic crystal structure are paramagnets without magnetic ordering, while NpRhGa5 with the tetragonal structure is an antiferromagnet. At high temperatures, the 5f electrons in NpGe3 are almost localized, and the magnetic susceptibility χ(T) approximately follows the Curie-Weiss law. With decreasing temperature, χ(T) shows a broad peak around 50 K, indicating a moderate heavy fermion state at lower temperatures. In fact, the results of the de Haas-van Alphen (dHvA) experiments for NpGe3 are well explained by the 5f-itinerant band model; namely, the 5f electrons contribute to the Fermi surface and the cyclotron effective mass. The cyclotron mass is thus enhanced, ranging from 2.6 to 16 times the rest mass of the electron m0. In the antiferromagnet NpRhGa5, it was clarified from the results of dHvA experiments and energy band calculations that the 5f electrons contribute to the Fermi surface. The cyclotron mass varies from 8.1 to 11.7 m0, and the magnetic moment is 0.89 μB/Np. In NpCd11, the 5f electrons are localized in the whole temperature range and do not contribute to the Fermi surface. The corresponding cyclotron mass is thus light, below 1 m0, revealing no hybridization between the localized 5f electrons and the conduction electrons. The Fermi surface consists of small closed Fermi surfaces, reflecting the small Brillouin zone associated to the large unit cell of the crystal structure in NpCd11. The long Np-Np nearest-neighbor distance (6.568 Å) results in the well localized 5f 4states. Magnetic susceptibility and magnetization curves are well explained in a crystalline electric field (CEF) scheme assuming a singlet ground state and an excited triplet state separated by a 120 K energy gap.