The Raman experiments were performed in a backscattering geometry at room temperature using 514.5 nm (2.41 eV), 496.8 nm (2.496 eV), and 488 nm (2.541 eV) lines from an argon-ion laser and the 632.8 nm (1.96 eV) line from a He-Ne laser. The Raman spectra were analyzed using a JY T64000 micro-Raman system with a liquid nitrogen cooled CCD detector. The pressure-dependent measurements were carried out using a diamond anvil cell (DAC) with a 4:1 mixture of methanol and ethanol as the pressure-transmitting medium. Figure 12.2 shows a schematic diagram of the high-pressure experimental setup. A small sample with dimensions of ~100 x 100 x 30 |j.m3 was prepared by mechanical polishing and cutting before
loading into the gasket of the DAC. The applied pressure was measured by the shift of the ruby R1 line . Because the frequency of the first-order Si Raman peak shows a blue shift with a pressure coefficient of 0.52 cm-1/kbar , we can use it as an internal pressure calibration. The signal collected from DAC is at least an order of magnitude weaker than that obtained outside the DAC.
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