MOSSBAUER-EFFECT AND MAGNETIZATION STUDIES OF MIXED-VALENCE SOLID-SOLUTION NPOH-CENTER-DOT-NPH2-X

By dissolving Np metal in a mixed solution of NaCl, MgCl2 and Na2SO4 i n water, a new compound NpOH precipitates together with NpH2-x, formin g a solid solution (double salt) NpOH . NpH2-x. Np-237 Mossbauer spect roscopy, d.c. magnetization and X-ray studies of the precipitate are r eported. NpOH crystallizes in the CaF2 (Fm3m) structure with a lattice parameter of 5.375 Angstrom. The Mossbauer absorption spectrum of NpO H is very well resolved from that of NpH2-x (single absorption line) o wing 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 tetr avalent whereas in NpH2-x it is trivalent. Both behave paramagneticall y 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 grou nd state (Gamma(8)((2)) of I-4(9/2) in cubic symmetry). The saturation hyperfine field derived is 350(50) T. NpH2-x in the solid solution ex hibits a positive IS relative to NpH2.0 as a result of volume contract ion. The IS of the neptunium dihydride system depends on the hydrogen concentration. From this result we derive for NpH2-x stabilized in the solid solution the value x approximate to 0.4.