High-pressure properties of group IV clathrates

We review the investigations on clathrate materials having C, Si, Ge or Sn framework atoms in which the pressure parameter plays an important role. In this article, we studied the synthesis of clathrates, superconducting properties and those studies relative to the structural cohesivity and phase diagrams. We have tried to extensively review these subjects. Key references on other important properties of the group IV clathrates are provided in the introductory sections. As a main result of this review, we note that pressure appears as a key parameter for the elaboration of present and future clathrates in particular those exhibiting superconducting properties. We show how high-pressure research has also played an important role in the understanding of the parameters governing clathrate superconductivity. In contrast, the study of the structural evolution of group IV clathrates (bulk modulus and stability) gives abundant clues for tailoring new materials with improved mechanical properties. Finally, practically all investigations converge to point out that today hypothetical carbon clathrates are candidates for extraordinary superconducting and mechanical properties.

Keywords:

• Clathrates
• High pressure
• Synthesis
• Phase diagram
• Phase transitions
• Superconductivity

Acknowledgements

We are indebted to C. Cros (ICMCB, Bordeaux, France) and S. Yamanaka (University of Hiroshima, Japan) who provided many of the samples used in our experiments. ASM is indebted to P. Mélinon (LPMCN, Lyon, France) for having attracted his interest on the physical properties of clathrate materials and for a very fruitful collaboration. We are indebted to the members of the LPMCN (Lyon, France) working on the clathrate systems and in particular to X. Blase for many discussions and a very rich collaboration on the theoretical understanding of clathrate materials and to A. Perez for his constant encouragement and support. We also thank T. Kume and H. Shimizu (University of Hiroshima, Japan) and P.F. McMillan (University College of London, UK) for many interesting discussions and fruitful collaborations. We also thank the many PhD students and post-doctoral fellows having participated in one way or the other to the different studies on the clathrate systems. We are also indebted to the staff of ESRF (Grenoble, France), LURE (Orsay, France) and ILL (Grenoble, France) for their assistance on many of the experiments we have participated on the study of clathrate materials.

Notes

^†Alder thickets confined the Odyssey's hero in the oracular island by growing around its shores.

^†The clathrate I₈Si₄₆ does not exist; it was found experimentally that iodine occupies 11% of the silicon 16i site 61,62. The correct composition is thus I₈(Si₄₄Si₂). It was measured that this iodine-doped clathrate is not superconducting down to 2 K. It seems that the presence of iodine atoms on the silicon sp³ network destroys superconductivity expected around 6 K.