Séminaire

Vendredi 7 Octobre 2022 à 11h00.

Mechanisms of silicon allotropes' crystallization in condensed media by in situ diffraction of synchrotron radiation

''
HP research on Si started more than 50 years ago and since then several allotropes, displaying a wide variety of physical properties, have been reported.1-5
The narrow-bandgap semiconductor6 Si-III4 with BC8 structure (originally believed to be semimetal) can be obtained from the high-pressure tetragonal metallic phase, Si-II, formed during compression of common silicon according to Si-I→Si-II (Figure 1). Such a transformation during decompression can be either direct, Si-II→Si-III, or with an intermediate step Si-II→Si-XII→SiIII. Our in situ studies of pure Si in oxygen-free environment indicated that in the absence of pressure medium, Si-I remains metastable at least up to ~14 GPa, while the pressure medium allows reducing the onset pressure of transformation to ~10 GPa.
Upon heating Si-III at ambient pressure a hexagonal structure, named Si-IV, was observed. This allotrope was believed to be a structural analogue of the hexagonal diamond found in meteorites (called also lonsdaleite) with the 2H polytypestructure. Calculations have predicted several hexagonal polytypes of Si and of other Group-IV elements to be metastable, such as 2H (AB), 4H (ABCB) and 6H (ABCACB). Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking),5 in contrast with Si-2H (AB stacking) proposed previously. The sequence of transformations Si-III→Si-IV(4H)→Si-IV(6H) has been observed in situ by powder X-ray diffraction. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab-initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region, for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics.
The study of silicon allotropic transformation in Na-Si and K-Si systems at high pressure, high temperature conditions indicated new interesting results on the second-order character of Si-II→Si-XI transformation and will be discussed in the presentation. The impact of the second order character on the topology of the pressure-temperature phase diagram of silicon will be analyzed.
(a) (b)
Figure 1. (a) HPHT phase transformations in pure Si. (b) In situ observations of Si-I→Si-II→Si-III transformation at ID06-LVP beamline of ESRF (Grenoble, France).
1. Kurakevych, O. O.; Le Godec, Y.; Strobel, T. A.; Kim, D. Y.; Crichton, W. A.; Guignard, J., Exploring silicon allotropy and chemistry by high pressure - high temperature conditions. J. Phys.: Conf. Ser. 2017, 950, 042049.
2. Kurakevych, O. O.; Le Godec, Y.; Crichton, W. A.; Strobel, T. A., Silicon allotropy and chemistry at extreme conditions. Energy Procedia 2016, 92, 839-844.
3. Kim, D. Y.; Stefanoski, S.; Kurakevych, O. O.; Strobel, T. A., Synthesis of an open-framework allotrope of silicon. Nature Materials 2015, 14 (2), 169-173.
4. Kurakevych, O. O.; Le Godec, Y.; Crichton, W. A.; Guignard, J.; Strobel, T. A.; Zhang, H. D.; Liu, H. Y.; Diogo, C. C.; Polian, A.; Menguy, N.; Juhl, S. J.; Gervais, C., Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways. Inorganic Chemistry 2016, 55 (17), 8943-8950.
5. Pandolfi, S.; Renero-Lecuna, C.; Le Godec, Y.; Baptiste, B.; Menguy, N.; Lazzeri, M.; Gervais, C.; Spektor, K.; Crichton, W. A.; Kurakevych, O. O., Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype. Nano Letters 2018, 18 (9), 5989-5995.
6. Zhang, H. D.; Liu, H. Y.; Wei, K. Y.; Kurakevych, O. O.; Le Godec, Y.; Liu, Z. X.; Martin, J.; Guerrette, M.; Nolas, G. S.; Strobel, T. A., BC8 Silicon (Si-III) is a Narrow-Gap Semiconductor. Physical Review Letters 2017, 118 (14).

''