THE ROLE OF NON-DEFORMABLE UNITS IN PRESSURE-INDUCED REVERSIBLE AMORPHIZATION OF CLATHRASILS

PRESSURE-induced amorphization of solids has been much studied since i t was first observed in 1984(1). It was found recently(2,3) that some materials can be amorphized reversibly under pressure, reverting back to the original crystalline structure and orientation when the pressur e is decreased. It has been suggested(4) that the presence of non-defo rmable units is essential for this reversibility, these units acting a s templates around which the original structure is reformed. Here we i nvestigate this idea by comparing the effect of pressure on two clathr asils-silica solids with open, microporous structures-with and without guest molecules inside the pores. We have studied pressure-induced am orphization of dodecasil-3C, which has cage-like voids, and dodecasil- 3R, which has two-dimensional channels. For both materials, our experi ments and simulations show that amorphization is fully or partly rever sible only when guest molecules are present suggesting that these do i ndeed act as rigid 'organizing centres' for the reversible transformat ion.