High-pressure phase transformation of the silicon clathrate Si136

The compressional behaviour of a new expanded form of silicon, the cubic Si-136 clathrate, is studied by energy dispersive x-ray diffraction in a diamond anvil cell experiment. The ambient temperature bulk modulus and its pressure derivative are determined to be K-0 = 90(3) GPa and K-0' = 5.2(8), in agreement with LDA theoretical calculations. At pressures between 8 and 10 GPa, the structure transforms into the thermodynamically stable beta-Sn phase. This behaviour is analogous to that of diamond-structured silicon. However, the metastable transition from Si-136 to the beta-Sn structure should occur at a much lower pressure (3-4 GPa), from consideration of free energy-pressure relations. The observation can be related to the absence of a convenient low-energy pathway for the IV- to VI-fold coordination change involved in the transition from Si-136 to beta-Sn.