Mössbauer effect and magnetization studies of mixed valence solid solution NpOH·NpH2−x

Abstract By dissolving Np metal in a mixed solution of NaCl, MgCl 2 and Na 2 SO 4 in water, a new compound NpOH precipitates together with NpH 2− x forming a solid solution (double salt) NpOH·NpH 2− x . 237 Np Mossbauer spectroscopy, d.c. magnetization and X-ray studies of the precipitate are reported. NpOH crystallizes in the CaF 2 ( Fm3m ) structure with a lattice parameter of 5.375 A. The Mossbauer absorption spectrum of NpOH is very well resolved from that of NpH 2− x (single absorption line) owing to their large difference in Mossbauer isomer shift (IS; 28.1 mm s −1 ). The Mossbauer studies indicate that the Np ion in NpOH is tetravalent whereas in NpH 2− x it is trivalent. Both behave paramagnetically down to 2.5 K; however, NpOH exhibits paramagnetic spin relaxation. The Mossbauer spectra can be fitted within a relaxation model assuming an isolated Kramers doublet and, even better, assuming a quartet ground state ( T 8 (2) of 4 I 9 2 in cubic symmetry). The saturation hyperfine field derived is 350(50) T. NpH 2− x in the solid solution exhibits a positive IS relative to NpH 2.0 as a result of volume contraction. The IS of the neptunium dihydride system depends on the hydrogen concentration. From this result we derive for NpH 2− x stabilized in the solid solution the value x ≈0.4.