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 diam
ond anvil cell experiment. The ambient temperature bulk modulus and its pre
ssure 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 ph
ase. This behaviour is analogous to that of diamond-structured silicon. How
ever, 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 energ
y-pressure relations. The observation can be related to the absence of a co
nvenient low-energy pathway for the IV- to VI-fold coordination change invo
lved in the transition from Si-136 to beta-Sn.