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

Peter T. Hutchins (1, 2) , Olivier Leynaud (3) , Luke A. O'Dell (4) , Mark E. Smith (4) , Paul Barnes (1, 2, 5) , Paul Mcmillan (1, 2)

1 Department of Chemistry, UCL
2 Centre for Materials Research, UCL
3 NEEL - X'Press - X'Press : diffraction et hautes pressions
4 Department of Physics [Coventry]
5 Birkbeck College [University of London]

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 degrees C at 8 degrees 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 Na8Si46 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 dathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.

Mots clés: silicon clathrate; Na(8)Si(46); Zintl phase; NaSi; solid-state synthesis; synchrotron radiation; X-ray diffraction; NMR spectroscopy; in situ studies

Domaines: Matériaux

Peter T. Hutchins, Olivier Leynaud, Luke A. O'Dell, Mark E. Smith, Paul Barnes, et al.. Time-Resolved in situ Synchrotron X-ray Diffraction Studies of Type 1 Silicon Clathrate Formation. Chemistry of Materials, 2011, 23 (23), pp.5160-5167. ⟨10.1021/cm2018136⟩. ⟨hal-00993017⟩