Condensed Matter and Statistical Physics Section: "New High Pressure Layered Phases of CS2 and SiS2"
Starts 9 Sep 2016 11:30
Ends 9 Sep 2016 13:00
Central European Time
CS2 and SiS2 belong to the important family of IV–VI AB2 compounds made of light elements which includes archetypal systems such as CO2 and SiO2. In solid CS2 a series of structural transitions was observed at high pressure upon compression of the Cmca molecular crystal, eventually resulting in a disordered tetrahedral structure  which did not allow an accurate structural determination. We applied an ab initio evolutionary search  and found a new layered tetrahedral P21/c structure, of especially high stability, and characterized by pairs of edge-sharing tetrahedra . Unlike tetrahedrally coordinated CO2 phases, this structure undergoes a semiconductor-metal transition at the relatively low pressure of 30-50 GPa , a transition which is in agreement with experiment . This layered structure appears to be a likely candidate for the phase found experimentally above 30 GPa in Ref.. With that in mind we suggest that by applying high pressure at low temperature one could perhaps prepare this phase with higher crystallinity thus allowing for better structural comparisons with our prediction, and to this end we calculated the Raman and IR spectra of the new phase. Wondering moreover whether similar phases could appear in different compounds of the same family, we extended our interest to SiS2. Strikingly, the very same P21/c layered structure which we predicted for CS2 turned out to be recently observed (and named HP1) in SiS2 , a finding which points to a new link in the high-pressure crystal chemistry of this family of compounds. Actually, the high-pressure phase diagram of SiS2 is currently known only up to cca 6 GPa, featuring several tetrahedrally coordinated phases (NP, HP1, HP2, HP3), but nothing seems to be known about the structural and electronic evolution of SiS2 at higher pressures. By means of ab initio calculations combined with evolutionary structure searching  we now predict three new low-enthalpy phases of SiS2 with space groups P-3m1, P63mc and R-3m . In all three phases, the Si coordination has switched from 4 to 6 and, interestingly, all these new structures are layered, consisting of sheets formed by edge-sharing octahedra (SiS6 units). The most stable P-3m1 phase (with a single SiS2 layer per unit cell) is isostructural to CdI2, where all sheets are directly above each other. It becomes stable above 6 GPa and at low pressures is semiconducting with an indirect band gap. The gap closes with increasing pressure leading to metallization around 30 GPa. New high pressure measurements will be called for to address these predictions. Facile sliding of these high pressure layered structures should also be of interest.
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