(237)Np Mössbauer effect study on NpFeAsO.

We report (237)Np Mössbauer measurements on NpFeAsO. The Np atoms were found to occupy only one crystallographic site. The value of the isomer shift (δ âˆ¼ 9.1 mm s(-1) versus NpAl2) indicates a 5f(4) electronic configuration (Np(3+) ions). The magnetic ordering of the Np sublattice below 60 K is established and the saturated ordered magnetic moment is determined to be 1.73µB at 3.6 K. The unique set of hyperfine parameters exclude a modulated magnetic structure or spin waves on the Np site. The neptunium magnetic moments µNp are found to lie along the tetragonal c-axis.

Magnetic and electronic properties of NpFeGa5.

NpFeGa(5) single crystals prepared by the Ga-flux method have been investigated by magnetisation, specific heat and (237)Np Mössbauer spectroscopy measurements. An antiferromagnetic (AF) transition has been observed at T(N) ~114 K. The second AF transition which takes place at T* ~74 K has only been detected in the magnetisation data. A saturated ordered Np moment of 1.00(5) µ(B) and the occurrence of an Np(3 + ) charge state were inferred from the Mössbauer data. The angle between the Np ordered moment and the basal plane was shown to be ~17°, in good agreement with the value observed by neutron diffraction in the low temperature AF phase. The absence of change of the quadrupole interaction parameters at T* did not allow us to gain information on the Np moment reorientation at T*. The trend of the isomer shift along the NpTGa(5) series and the discrepancy between the Np moments determined by neutron diffraction and Mössbauer spectroscopy indicated a partial spatial delocalisation of the 5f electrons in NpFeGa(5).