CM Seminar @ICTP Stasi Seminar Room Friday 9 September at 11.30 a.m. - Roman MARTONAK

[Condensed Matter Section]

Condensed Matter and Statistical Physics Seminar
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Friday, 9 September at 11.30 a.m.
Luigi Stasi Seminar Room, first floor, Leonardo building


Roman MARTONAK
Comenius University, Dept. of Experimental Physics
Bratislava, Slovak Republic

Title: New High Pressure Layered Phases of CS2 and SiS2


Abstract:
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 [1] which did not allow 
an accurate structural determination. We applied an ab initio 
evolutionary search [2] and found a new layered tetrahedral P21/c 
structure, of especially high stability, and characterized by pairs of 
edge-sharing tetrahedra [3]. Unlike tetrahedrally coordinated CO2 
phases, this structure undergoes a semiconductor-metal transition at the 
relatively low pressure of 30-50 GPa [3], a transition which is in 
agreement with experiment [1]. This layered structure appears to be a 
likely candidate for the phase found experimentally above 30 GPa in 
Ref.[1]. 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 [4], 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 [5] we now 
predict three new low-enthalpy phases of SiS2 with space groups P-3m1, 
P63mc and R-3m [6]. 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.
[1] R. P. Dias, C.-S. Yoo, M. Kim, J. S. Tse, Phys. Rev. B 84, 144104 
(2011).
[2] A. Oganov , C. Glass, J. Chem. Phys. 124, 244704 (2006).
[3] S. S. Naghavi, Y. Crespo, R. Martoňák, E. Tosatti, Phys. Rev. B 
91, 224108 (2015).
[4] J. Evers, P. Mayer, et al., Inorg. Chem. 54, 1240 (2015).
[5] D. Lonie, E. Zurek, Comput. Phys. Commun. 182, 372 (2011) [6] D. 
Plašienka, R. Martoňák, E. Tosatti, in preparation.