Density driven structural transformations in amorphous semiconductor clathrates

The pressure induced crystalline collapse at 14.7 GPa of the semiconductor clathrate Sr₈Ga₁₆Ge₃₀ and its polyamorphic structures are reported up to 35 GPa. In-situ total scattering measurements under pressure allow the direct microscopic inspection of the mechanisms associated with the pressure induced amorphization in these systems, as well as the structure of the recovered phase. It is observed that, between 14.7 and 35 GPa, the second peak in the structure factor function gradually disappears. Analysis of the radial distribution function extracted from those data indicates a systematic lengthening of the nearest-neighbor framework bonds. This feature is associated with gradual cage collapse and breakdown of the tetrahedral structure. This suggests a change in the local bonding in the high density amorphous form, similarly to that observed in other semiconductor clathrates and elemental silicon. Upon recovery from high pressure, the sample remains amorphous and, while there is some indication of the guest-host cage reforming, it does not seem that the tetrahedral coordination is fully reestablished. As such, the compression-decompression process in these systems gives rise to three distinct amorphous forms.