MODELING THE HIGH-PRESSURE BEHAVIOR OF THE ACTINIDE MONOPNICTIDES ANDMONOCHALCOGENIDES

At ambient temperature and pressure, a majority of the actinide monopn ictides and monochalcogenides exhibit a face-centered cubic (f.c.c.) s tructure (NaCl-type, B1). Two exceptions are ThTe and ThBi, which have a body-centered cubic (b.c.c.) structure (CsCl-type, B2). Several of these B1 compounds undergo at least one structural transformation unde r pressure. Frequently, the transition is from the initial NaCl-type t o a CsCl-type cubic structure, although other structural forms are enc ountered. In addition to providing structural information, high pressu re studies reveal important information about chemical bonding. Presen ted here are efforts to generate a pseudo-ionic model and geometrical correlations for these materials, which are useful for predicting the high pressure transition pressures, structures and volume changes acco mpanying the transitions. This model relies on ionic radii and lattice parameters at ambient pressure as the input parameters. One of the co rrelations established is that plots of transition pressures as a func tion of the ratio of the radii (r(Any+)/r(Xz-); An = actinide, X = met alloid), for a specific pnictide or chalcogenide anion, can be fitted by a first-order equation: P(GPa) = -339r(+)/r(-) +C, where C = 174, 1 45 or 136, respectively, for anions of the fourth, fifth or sixth peri ods in the Periodic Table. Correlations were also found for volume col lapses accompanying the phase transitions, but these were more complex in nature.