Mossbauer effect and magnetization studies of mixed valence solid solution NpOHmultNpH sub 2-x

By dissolving Np metal in a mixed solution of NaCl, MgCl sub 2 and Na sub 2 SO sub 4 in water, a new compound NpOH precipitates together with NpH sub 2-x , forming a solid solution (double salt) NpOHmultNpH sub 2-x . exp 237 Np Mossbauer spectroscopy, d.c. magnetization and x-ray studies of the precipitate are reported. NpOH crystallizes in the CaF sub 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 sub 2-x (single absorption line) owing to their large difference in Mossbauer isomer shift (IS; 28.1 mm s exp -1 ). The Mossbauer studies indicate that the Np ion in NpOH is tetravalent whereas in NpH sub 2-x it is trivalent. Both behave paramagnetically down to 2.5K; 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 ( Gamma sub 8 exp (2) ) of exp 4 I sub 9/2 in cubic systems.