X-ray and Gamma-ray Spectroscopy of Solids under Pressure

This report describes our recent synchrotrons x-ray absorption fine structure (XAFS) measurements on a number of systems that undergo pressure induced changes in local structure at high pressure. The reader should also refer to the accompanying renewal proposal for a more in-depth discussion of the general scope of this program, and its relevance to condensed matter science. We merely state that the methods here are aimed at using XAFS to probe the various phenomena that are caused by high pressure, especially including various structural, and/or electronic, changes or transitions. Our general technique is based upon a pressure cell which utilizes scintered boron carbide anvils, since diamond anvils generally produce Bragg glitches which spoil the high quality XAFS necessary for precision structural measurements. Sample pressure is determined at the beam-line by measuring and analyzing, via XAFS, the compression of some cubic material contained within the sample chamber. Recently we have extended this work to 77 K using helium gas for the applied force, rather than hydraulic oil. This report period has been productive. The increased flux available at the Stanford Synchrotrons Radiation Laboratory (SSRL) has permitted our going to smaller beams, on the order of 300 pm in diameter, for precision probing of the sample region. At the same time we have received ample amounts of beam time at SSRL, in part because of the high rating of our latest user proposal. We also were invited to share some of the beam time at the National Synchrotron Light Source (NSLS) assigned to the group of our collaborator, E. A. Stern. Below we describe in some detail our recent work. Some of the pending papers are reports on systems that have been under study for some time and have been described in past progress reports and, as such, need not be described again here