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Allotropes of group IV semiconducting materials have recently received increased attention because they exhibit unique physical properties, such as narrower bandgaps and lower thermal conductivities t...han the diamond-cubic (dc) structure. Metastable phases, which are representative allotropes, are obtained through high-pressure phase transformations. While many studies have been reported on the high-pressure phase transformations and metastable phases of Si and Ge, there are only a few reports regarding Si_<1−x>Ge_x alloys. Herein, we report on the high-pressure phase transformation and metastable phases at atmospheric pressure in a Si_0.20Ge_0.80 alloy using a diamond anvil cell (DAC). The Si_0.20Ge_0.80 alloy with a uniform composition grown by the traveling liquidus zone method was used to investigate phase transformations. In situ observations of phase transformations by synchrotron X-ray diffraction and Raman spectroscopy using the DAC with a pressure-transmitting medium revealed the phase transformation from dc to β-Sn upon pressurization at ∼12GPa. The β-Sn phase transformed to body-centered-cubic bc8 through rhombohedral r8 upon depressurization. The high-pressure β-Sn phase and the metastable r8 and bc8 phases were also observed in DAC experiments without a pressure-transmitting medium, but the phase-transformation pressure for dc to β-Sn upon pressurization was reduced due to shear strain. High-resolution transmission electron microscopy observations confirmed the presence of bc8 nanograins after depressurization irrespective of hydrostatic or nonhydrostatic conditions. These results indicate that bc8-Si_<1−x>Ge_x with high Ge compositions (x≈0.8) can be obtained by high-pressure phase transformation.続きを見る
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