Two High-Pressure Phases of SiS₂ as Missing Links between the Extremes of Only Edge-Sharing and Only Corner-Sharing Tetrahedra

Abstract

The ambient pressure phase of silicon disulfide (NP-SiS2), published in 1935, is orthorhombic and contains chains of distorted, edge-sharing SiS4 tetrahedra. The first high pressure phase, HP3-SiS2, published in 1965 and quenchable to ambient conditions, is tetragonal and contains distorted corner-sharing SiS4 tetrahedra. Here, we report on the crystal structures of two monoclinic phases, HP1-SiS2 and HP2-SiS2, which can be considered as missing links between the orthorhombic and the tetragonal phase. Both monoclinic phases contain edge- as well as corner-sharing SiS4 tetrahedra. With increasing pressure, the volume contraction (-ΔV/V) and the density, compared to the orthorhombic NP-phase, increase from only edge-sharing tetrahedra to only corner-sharing tetrahedra. The lattice and the positional parameters of NP-SiS2, HP1-SiS2, HP2-SiS2, and HP3-SiS2 were derived in good agreement with the experimental data from group-subgroup relationships with the CaF2 structure as aristotype. In addition, the Raman spectra of SiS2 show that the most intense bands of the new phases HP1-SiS2 and HP2-SiS2 (408 and 404 cm(-1), respectively) lie between those of NP-SiS2 (434 cm(-1)) and HP3-SiS2 (324 cm(-1)). Density functional theory (DFT) calculations confirm these observations.

High-pressure layered structure of carbon disulfide

“…Recently the very same structure, named HP1, has been experimentally reported for SiS 2 at much lower pressures. 10 Another tetrahedra-based structure, HP2, is also stabilized between 8 and 30 GPa, but only by a small enthalpy difference with respect to HP1 comparable with our calculation errors.…”

“…10 another monoclinic phase with the space group P 2 1 /c denoted as HP2 was observed to follow HP1 at pressure of 3.5 GPa in SiS 2 . Unlike the HP1, which is layered and involves four CS 2 units, the HP2 phase consists of 12 CS 2 units, arranged in a 3D covalent network.…”

Creating new layered structures at high pressures: SiS2

“…Calculated T  = 0 enthalpies show that the sequence of transitions for tetrahedral phases occurring at 3.5 GPa (HP1 → HP2) and 4 GPa (HP2 → HP3) experimentally32 at room temperature is reasonably well reproduced within our calculations, which predict these two transitions to take place (at zero temperature) at 2.8 and 5.4 GPa, respectively - Fig. 3 (upper panel).…”

“…The ambient-pressure stable phase known as NP-SiS 2 has orthorhombic Ibam structure and consists of distorted edge-sharing tetrahedra forming 1D chains which interact via weak van der Waals forces323334353637383940. At 2.8 GPa, a first high-pressure phase HP1-SiS 2 3236 appears, with monoclinic space group P 2 1 / c , by interconnection of the NP-SiS 2 chains to form 2D layers32. The connectivity pattern changes from each tetrahedron sharing two edges in the NP phase to sharing one edge and two corners in HP132.…”

“…Further increase of pressure leads to the HP2-SiS 2 phase at 3.5 GPa3236, again with P 2 1 / c space group, and again with edge- and corner-sharing tetrahedra, in this case, however, with a 3D connectivity network with large cavities32. Finally, at 4 GPa, a tetragonal structure denoted as HP3-SiS 2 takes over, a phase formed by strictly corner-sharing tetrahedra that are slightly distorted and span the whole three-dimensional space323334363738. The same structure as HP3-SiS 2 is adopted by CO 2 phase V at high pressures5 and it can be viewed as a partially collapsed version of SiO 2 β -cristobalite6.…”