Moessbauer and magnetic studies on alkaline- and alkaline earth oxo-neptunates

Moessbauer studies from 4 to 77 K and susceptibility measurements from 1.3 to 300 K were performed on several oxo-neptunates of hexavalent (5f1) and tetravalent (5f3) neptunium. BaNpO4 with an orthorhombic structure orders antiferromagnetically at 18.2(3) K with a magnetic moment μ(300 K)=1.1 μB. Moessbauer spectra show an isomeric shift of δ=-54(2) mm s-1 (relative to NpAl2) and a quadrupole splitting of eqVZZ=105.3(4) mm s-1 with an asymmetry coefficient η=0.37(3). This behaviour can be explained by assuming an angle between the electric field gradient and the internal magnetic field of 16.0(5) mm s-1. Li6NpO6 with δ=-58(2) mm s-1; eqVZZ=20(1) mm s-1; η=0.22(2) exhibits a more symmetric coordination of the neptunium ion than in the isomorphic Li5NpO6. No magnetic ordering was observed. NpGeO4 is of tetragonal structure and orders magnetically below 5 K. A very small quadrupole splitting suggests a rather symmetric neptunium coordination. The magnetic transition in SrNpO3 at 30.5(5) K had been explained by a ferrimagnetic sublattice model. The different sublattice magnetization seems to be due to different neptunium (IV) sites in the lattice, which can be discriminated by Moessbauer spectroscopy. (orig.)