Theoretical investigation of the high-pressure crystal structures of Ce and Th.

Abstract

The high-pressure crystal structures of Ce and Th are studied theoretically by means of a full potential linear muffin-tin orbital technique, where the generalized gradient correction to density-functional theory has been implemented. A crystallographic phase transition from fcc to bct is found for u-Ce at about 10 GPa, in good agreement with experiment. The calculated pressure dependence of the crystallographic c/a ratio (of the bct structure) is in good agreement with experiment. For n-Ce a subsequent transition to the hcp crystal structure is predicted at about 1600 GPa. For Th a continuous transformation from fcc to bct at 45 GPa is found, confirming recent experimental findings. Also for this material the calculated pressure dependence of the crystallographic c/a ratio (of the bct structure) is in good agreement with experiment. For Th we also predict a transformation to the hcp crystal structure at even higher pressures. The metals Ce (Z= 58) and Th (Z= 90) are located at the beginning of the 4f (lanthanides) and the Sf (actinides) transition-metal series, respectively. Ce is the first lanthanide metal that has an appreciable occupation of the 4f states, and is a known subject for controversy. Primarily the debate concerns the localized versus itinerant character of the 4f electron. The electronic structure of Ce has been reported earlier. As regards the 5f population at ambient conditions, calculations show that the actinide metal Th has a smaller f occupation than ct Ce Se-ver.al recent investigations of these two metals, both experimental as well as theoretical, have been motivated by their crystallographic phase transitions under compression. In the lanthanide series localized 4f states are progressively being filled and their inhuence on the chemical bonding is negligible. In the actinide series, however, the earlier metals have delocalized 5f electrons that contribute to the chemical bonding. The increasing 5f occupation as a function of atomic number results in increasingly distorted (complex) crystal structures. This is understood from the increased contribution to the bonding from the narrow 5f bands. ' Particularly interesting in the present context is the occurrence of the bct (c/a =0.825) structure at ambient conditions for Pa (Z=91). The increased 5f occupation in Pa, in comparison to Th, results in a bonding which favorizes the bct crystal structure over the fcc structure (which is found in Th at ambient pressure). The effect of pressure upon an itinerant 5f metal is first a lowering of the energy of the 5f bands relative to the other valence states and second a broadening of the Sf band as well as a broadening of the other bands. The relative lowering of the Sf states results in an increased Sf population. Therefore, in view of the known crystal structure of Pa (bct), it is not surprising to find a pressure-induced onset of a bct crystal structure in Th. Since u-Ce from certain aspects can be regarded as an analog element to Th, a similar behavior (stabilization of the bct structure) of Ce under pressure is not surprising either. However, for Ce, in contrast to Th, there is an intermediate bodycentered-monoclinic (bcm) phase found experimentally in the interval 6— 12 GPa. This bcm structure can be seen as a small distortion of the bct crystal structure, and can therefore be, approximately, described as a bct structure. In fact both the bcm and the bct phases of Ce at moderate compressions can be viewed as more or less distorted fcc structures. We note here that the experimental structure is not quite clear for Ce since it has been reported that an orthorhombic n-U type of structure is stabilized, instead of the bcm structure. In the present report we take the view point that the bcm and the u-U structures are very close in energy and that the u-U