Abstract
Excellent energy-dispersive X-ray diffraction patterns can be obtained, even above 200 GPa (two megabars), with diamond anvil cells and synchrotron radiation from the Cornell High Energy Synchrotron Source (CHESS). Examples of structural determination and equation-of-state studies for cesium halides, barium and mercury chalcogenides, Group IVB elements and III-V compounds, sodium hydride and metals are given. In several cases comparisons are made with detailed first-principle calculations. The power of the wiggler source at CHESS is also illustrated.