In order to develop an optical method for pressure determination in the multimegabar region, the first-order Raman spectra of diamond anvils were investigated at pressures up to 310GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the anvil culet due to the normal stress, was calibrated against the sample pressure derived from the equation of state of Pt. The obtained pressure dependence of the edge frequency demonstrates the reliability of this diamond anvil Raman gauge. Up to the maximum pressure of this study, the relation between Raman frequency and normal stress at the diamond anvil culet is formally similar to the equation of state of a hydrostatically compressed isotropic elastic body having a bulk modulus of K0=547(11)GPa and a pressure derivative of the bulk modulus K0′=3.75(20).

The first-order Raman spectra of diamond anvils used in a gasketed high-pressure cell have been measured at pressure up to 250GPa. The high-frequency edge of the Raman band, which corresponds to the Raman shift of the culet face, is represented by a function of pressure in the sample chamber up to 250GPa. The dependence is almost independent on loading conditions. The application of the pressure dependence for pressure determination up to the multimegabars pressure region is proposed.

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Structural phase transitions of a 3d transition element, titanium, have been investigated at pressures up to 220 GPa at room temperature using a monochromatic synchrotron x-ray diffraction technique. At 140 GPa, the hexagonal (omega) phase was transformed into an orthorhombic (delta) phase with a distorted bcc structure via an intermediate (gamma) phase, which has recently been proposed by Vohra and Spencer [Phys. Rev. Lett. 86, 3068 (2001)]. Both the delta and the gamma phases had a unique zigzag-chain-like structure, which resulted from an orthorhombic distortion. The delta-gamma transition could be explained as a rearrangement of the packing between the zigzag chains.

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