Time-Resolved in Situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation

Abstract

Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na 8 Si 46 and Na 2 Ba 6 Si 46 . Although their structure and properties are well described, there is little understanding of the formation mechanism. Na 8 Si 46 is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si 4 4– polyanions. In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25–500 °C at 8 °C/min) under vacuum conditions (10 –4 bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state 23 Na NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na 8 Si 46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the clathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.