Silicon. 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-III. 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.
Carbon. The most common polytypes of carbon in graphitic (G) form are hexagonal (2H) and rhombohedral (3R) polytypes, which allow quasi-martensitic transformation into diamond (D) forms, such as hexagonal diamond 2H or common cubic diamond 3C, at HP and moderate temperatures (~1000 – 1500 K), e.g. G-2H→D-2H or G-3R→D-3C. High tepmeratures lead to the formation of thermodynamically stable allotropes, G-3R→G-2H at ambient pressure and D-2H→D-3C at high pressures.
Graphite intercalation compounds (GIC) with metals such as Li, Na and K form different compositions (and crystal structures) produced by stacking along c-axis of metal (Me) and n carbon (A, B or C) layers. The n number indicate the stage of intercalation. Typically ordered compounds are obtained for n = 1 to 6 with various stacking sequences depending on metals: n = 1, 2, etc. High pressure studies has been performed principally on 1st stage compounds of Li and K, while formation of 1st stage Na-C compound has been observed only at HPHT conditions.
[1] O. O. Kurakevych, et al., J. Phys.: Conf. Ser. 2017, 950, 042049.
[2] O. O. Kurakevych, et al., Energy Procedia 2016, 92, 839.
[3] D. Y. Kim, et al., Nature Materials 2015, 14, 169.
[4] O. O. Kurakevych, et al., Inorganic Chemistry 2016, 55, 8943.
[5] S. Pandolfi, et al., Nano Letters 2018, 18, 5989.
[6] H. D. Zhang, et al., Physical Review Letters 2017, 118, 146601.