Moessbauer effect measurements on NpAs at high pressures and compressibility determination by means of angular dispersive X-ray diffraction

The intermetallic compound NpAs was studied with the help of the 60 keV Mossbauer resonance in 237Np within a temperature range between 4.2 K and room temperature, and under quasi-hydrostatic pressures (of up to 85 kbar). Furthermore, the modification of the lattice parameter with the pressure of up to 99 kbar at room temperature was determined with the help of a Guinier X-ray diffractometer. In both experimental methods, the pressure in Cu-Be forming pliers was transmitted to the pulverized sample material via two B4C punches. At low temperatures (4.2 K) and under zero pressure, NpAs crystallizes with a cubic NaCl structure and an antiferromagnetic order. Between T=154 K and T=173 K we find a tetragonal crystalline structure with several complex magnetic phases. If the external pressure is raised at a temperature of 4.2 K, then a considerable reduction of the magnetic hyperfine field Bhf, the isomeric shift S and the Neel temperature TN is observed. Under pressures above 26 kbar, a new magnetic phase emerges in which two magnetic hyperfine fields (B1 and B2) are measured. The surprisingly strong pressure dependence of the hyperfine field parameters Bhf and S as well as of the Neel temperature and the low value of the bulk modulus of NpAs by comparison with the alloys DyAl2, NpCo2Si2, NpAl2 and NpOS2 is a strong indication of the fact that the 5f electrons of neptunium are far less strongly localized than has so far been assumed, and that they hybridize to a considerable extent with the 4s, 4p and possibly with the 3d electrons of the As ligands. (orig./RB)