Experimental studies at Karlsruhe and Oak Ridge of transplutonium metals and compounds under pressure

Abstract

At Karlsruhe, the structural behavior of the first four transplutonium metals and two Bk-Cf alloys has been studied as a function of pressure in diamond anvil cells using X-ray diffraction. The sequence of structures exhibited as pressure is increased is d.h.c.p. → c.c.p. → orthorhombic. In addition a distorted c.c.p. phase is observed in americium, Bk 0.40Cf 0.60 and californium between the c.c.p. and orthorhombic phases. The volume collapse and transformation to the lower symmetry orthorhombic structure are indicative of the onset of delocalization of the 5f electrons in these materials. The highest delocalization pressure and largest volume collapse occur in curium metal, reflecting its stable half-filled 5f 7 electron configuration. Diamond anvil cells have also been used at Oak Ridge to contain AmI 3, CfBr 3 and CfCl 3 under pressure for investigation by absorption spectro-photometry. Both AmI 3 and CfBr 3 exhibit pressure-induced, irreversible phase transformations to the PuBr 3-type orthorhombic structure, a denser form of these compounds. Thus the driving force for these transformations is more efficient crystal packing. Both hexagonal (to 22 GPa) and orthorhombic (to 35 GPa) CfCl 3 exhibit only reversible spectral changes with pressure. This probably reflects their nearly identical room temperature and pressure (RTF) unit cell volumes. In both cases the spectra obtained are consistent with a continuous alteration of the RTF structure with pressure; physical compression seems to make a given f → f transition easier. Additional data are being sought to elucidate more completely the behavior of CfCl 3 under pressure.