Ultrahigh pressures

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Abstract
This article reviews mainly the use of the diamond anvil cell (DAC) for ultrahigh-pressure generation (a few hundred GPa) and the various techniques employed in studying the high-pressure behavior of solids. A brief historical introduction to devices used in the prediamond-anvil era is presented in Sec. I. The basic principles of the modern DAC, ultrahigh-pressure generation, and pressure calibration are presented. Among the techniques used, x-ray diffraction, optical spectroscopy including Raman and Brillouin spectroscopy, microscopy, electrical resistance,and Mössbauer measurements, and positron annihilation studies with the DAC are reviewed. High-temperature–high-pressure (HT–HP) and low-temperature–high-pressure (LT–HP) generation in the DAC, and the problems associated with pressure calibration under these conditions are discussed. A brief section is devoted to the sintered diamond-tipped anvil apparatus, for it offers a very convenient way of studying resistance changes and superconductivity to 50-GPa pressure at normal and at liquid-He temperatures. In Sec. IV, Raman studies on solid H2 and solid N2 to about 150 GPa, p-v studies on Xe, CsI to 50 GPa, the metallization of CsI, and superconductivity of Si high-pressure polymorphs are presented. Present trends and future possibilities for ultrahigh-pressure research are briefly set out in Sec. V.