MOSSBAUER-SPECTROSCOPY OF NP3AS4 - INTERPRETATION OF THE SPECTRA AND OF THE MAGNETIC-BEHAVIOR IN A CRYSTAL-FIELD MODEL

Np//3As//4 (Th//3P//4-type structure) is ferromagnetic below T//c equals 81 K. In this work, we present results of Mossbauer experiments performed between 4. 2 and 150 K. The isomer shift with respect to NpAl//2 is 5. 9 mm/s, suggesting a Np**4** plus valence state. The hyperfine field at 4. 2 K is 377 T, corresponding to an ordered moment mu //o//r//d equals 1. 75 mu //B. The quadrupolar interaction is very small both in the ferromagnetic and in the paramagnetic state (e**2qQ less than 3 mm/s). Results are discussed in detail within a crystal field model. Such an interpretation is justified by the fact that the large Np-Np interatomic distance (3. 98 Angstrom) allows one to assume rather well localized 5f states which seemed already to be the case for the isostructural U//3X//4 compounds.