9704428 Vohra The research investigates phase transformations in rare- earth metals and alloys subjected to megabar pressures and develops the scientific base necessary to fabricate gem quality diamond anvils by chemical vapor deposition techniques (CVD). The experimental investigations are carried out at the High Pressure and CVD Laboratory at the University of Alabama at Birmingham (UAB) and the National High Pressure beamlines at CHESS, Cornell University and NSLS, at Brookhaven National Laboratory. The focus of this study is to establish phase diagrams for selected prototype rare-earth metals and alloys in the megabar regime (100-400 Gpa). The focus is on heavy rare-earth metals, lutetium and ytterbium, where phase transitions and dramatic changes in electronic structure and valence are expected at ultra high pressures. Pressure also causes delocalization of the f- shell giving rise to "collapsed - low symmetry crystal structures." A high resolution image plate x-ray diffraction technique is employed in conjunction with a synchrotron source to investigate low symmetry crystal structures as well as the phenomenon of crystal grain growth discovered recently in rare-earth metals. Also examined is a related transition metal, yttrium, which is expected to simulate rare-earth behavior at high pressures, and recently has been predicted to transform to the bcc structure at 280 Gpa. The measured phase boundaries and pressure-volume (P-V) relationships provide stringent tests for various models of f-delocalization under pressure. A second direction in the research is on fundamental studies on CVD diamond growth for fabrication of synthetic diamonds with low levels of nitrogen impurity for applications in diamond anvil cell devices, In addition, high pressure materials research facilities supported by this grant are employed in research experiences for undergraduates (REU) projects during the summer and for undergraduates in the Alabama Alliance for Minori ty Participation (AMP) Program at UAB. %%% This research has the potential for making significant contributions to the understanding of the structural and electronic properties of rare-earth materials. The research on diamond has broad technological applications if successful. ***
