Phase transformations in materials studied by micro-Raman spectroscopy of indentations

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Abstract
Phase transformations occurring in materials under high pressures are important for a wide range of problems in materials science and solid-state physics. Most of the results in this area have been obtained using various sophisticated high-pressure cells. We studied solid-state phase transformations and amorphisation under high non-hydrostatic pressures in very simple experiments using a combination of hardness indentation tests with micro-Raman spectroscopy. Amorphisation of diamond, that did not occur under hydrostatic loading, has been observed. Shearing and distortion of cubic diamond structure above 100 GPa resulted not only in its amorphisation, but also in the formation of threefold coordinated carbon. A carbon film that was squeezed between a SiC substrate and diamond indenter lost its graphitic structure and produced a Raman band typical of diamond-like carbon (DLC). Even for such a well-studied material as Si, principally new data have been obtained. High spatial resolution of the method allowed us to show that the Raman spectrum that was previously ascribed to a metastable Si-III phase originates from two different high-pressure phases of Si. Up to five different phases of Si were found within a single impression. Studies of reversible transformations that occur upon unloading or heating of samples by the laser beam have also been carried out. Amorphisation and/or phase transformations have been observed for some other materials, such as SiC, quartz, Ge, GaAs and other. The combination of indentation tests with micro-Raman spectroscopy provides a powerful and fast tool for in-situ and ex-situ monitoring of pressure-induced phase transformations in materials.