Structural transitions at high pressure and metastable phase in Si0.8Ge0.2

• Published in: Journal of alloys and compounds Vol. 954; p. 170180
• Main Authors: Gerin, M., Machon, D., Radescu, S., Le Floch, S., Le Godec, Y., Gaudisson, T., Alabarse, F., Veber, P., Debord, R., Amans, D., Pischedda, V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 05-09-2023; Elsevier
• Subjects: Condensed Matter; Density functional theory; High-pressure; Metastable phases; Physics; Raman spectroscopy; Si-Ge alloys; X-ray diffraction; X-ray diffraction; Si-Ge alloys; Density functional theory; Metastable phases; Raman spectroscopy; High-pressure
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2023.170180

The high-pressure behaviour of Si0.8Ge0.2 alloy is explored using in situ Raman spectroscopy, X-ray diffraction techniques and density functional theory (DFT) simulations. High pressure experiments revealed a pressure-induced transition from the stable cubic semiconducting phase (dc- Si0.8Ge0.2) to the tetragonal β-tin metallic phase (β-Si0.8Ge0.2) during compression. This sluggish transition is significantly accelerated at moderate temperature (<300 °C). Upon decompression, successive transitions towards metastable phases are observed. A first transition from the metallic β-Si0.8Ge0.2 toward the rhombohedral r8-Si0.8Ge0.2 phase is observed at 10.3 GPa followed by a partial transition to the body-centered cubic bc8-Si0.8Ge0.2 phase at 2.2 GPa. After releasing the pressure, r8 and bc8 phases coexist at ambient conditions. This transition pathway is similar to that followed by pure silicon and is consistent with the ab initio enthalpy calculations. This phase transition sequence is confirmed by in situ Raman spectroscopy, where signatures of r8 and bc8 phases are observed in the Raman spectra at decompression. An ab initio simulation method is proposed to assign the Raman spectrum of Si0.8Ge0.2 alloy using group theory and projection operators. The exploration of metastable states in these alloys is of major interest both in terms of applications (e.g. optoelectronics) and from a fundamental point of view to better understand the effects of alloying on the physical properties (e.g. vibrational).
•pressure-induced transition from cubic semiconducting to tetragonal metallic phase of Si0.8Ge0.2 during compression•Upon decompression, transitions towards metastable rhombohedral and body-centered cubic Si0.8Ge0.2 phases•Enthalpy variations as a function of pressure explains the coexistence of the two metastables phases at ambient conditions•Ab initio calculation method, using group theory and projection operators, to assign the Raman spectrum of Si0.8Ge0.2