High-pressure clathrate chemistry of silicon: New compounds and allotrope - Oleksandr Kurakevych - 5 janvier 2015

Oleksandr Kurakevych, IMPMC

Lundi 5 janvier 2015 à 10 h 30

IMPMC, Université P. et M. Curie, 4, Place Jussieu, 75005 Paris

Salle de conférence, 4e étage, Tour 22-23, Salle 1

Abstract

Three different sodium-silicon clathrate compounds–Na8Si46 (sI), Na24+xSi136 (sII) and a new structure, NaSi6–were obtained for the first time using high-pressure techniques [1]. Experimental and theoretical results unambiguously indicate that Naintercalated  clathrates are only thermodynamically stable under high-pressure conditions, and equilibrium synthesis provides significant improvements in the electrical ,properties of bulk clathrate materials samples.
The synthesized NaSi6 was then used as a starting material for chemical processing, and sodium was completely removed from the precursor by a thermal “degassing” process [2]. The Cmcm structure of o-Si24, which has 24 Si atoms per unit cell (oC24), contains nanoporous channels along the crystallographic a-axis that are formed from six- and eight-membered sp3 silicon rings. The new o-Si24 allotrope has a zeolite structure and possesses a quasidirect band gap near 1.3 eV, close to the theoretically suggested optimal value for photovoltaics.

[1] Kurakevych, O. O.; et al., Crystal Growth & Design 13, 303-­‐307 (2013).
[2] Kim, D. Y.; Stefanovski, S.; Kurakevych, O. O.; et al., Nature Materials, doi:10.1038/NMAT4140 (2014).