High-pressure Mossbauer data

NpAli (Cl5 Laves phase) orders ferromagnetically at Tq = 56K with a moment Wnp = 1.5 Ub. That value is considerably below the free-ion value (of either Np" + or Np " ") and has been taken in connection with high-pressure Mossbauer data as evidence for Np moment delocalization because of 5f-5f electron overlap. A summary of bulk properties is given by Foiunier and Troc (1985). The Mossbauer results are discussed in detail by Potzel et al. (1993). 

The spin and valence states of iron in the lower-mantle minerals have been investigated bya number of synchrotron X-ray and optical laser spectroscopic techniques in a high-pressure diamond anvil cell (DAC). Mossbauer spectroscopy has been instrumental in our understanding of this topic because this method is specific to Fe-containing minerals and provides hyperfine QS and CS parameters [5,19-22]. Specifically, synchrotron Mossbauer spectroscopy (SMS) with a highly intense and focused beam coupled with the X-ray transparent DAC permits in situ observations of the Mossbauer spectra with reasonable data collection times at extreme P-T conditions [20-22]. 

Mossbauer spectroscopy is a powerful tool to gain essential information on the difference in electron structure between intermetallic compounds of 4f and 5f elements. In the latter series the work concentrates on Np materials. For this actinide bulk magnetic and transport data are still scarce and even neutron diffraction results are not plentiful. Mossbauer spectroscopy is thus in some sense a forerunner. It has been shown that measurements under high pressures are pivotal for a deeper insight of electronic properties, in particular with the question of 3d-like itinerant versus 4f-like localized electron behavior of the 5f electrons in Np. It becomes more and more apparent that U and Np behave often quite differently mainly because of the somewhat more pronounced tendency of the latter towards f localization. Nevertheless, the formation of 5f bands via hybridization plays a central role also in Np metallic compounds. 

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