Si allotropes and group IV clathrates investigated under high pressures

Recent studies on new materials crystallized with the sp(3) open framework of Si and other group IV elements are reviewed. The synthesis and predicted properties of a new allotrope of silicon, i.e., orthorhombically structured silicon Si-24, are investigated. Si-24 can be formed by heating a Na4Si24 precursor at temperatures as low as 47 degrees C. The quasi-direct band-gap nature with a gap of similar to 1.3 eV is predicted on the basis of a first-principles calculation. We also review investigations on clathrate materials having Si, Ge or Sn framework atoms for which pressure plays an important role. The phenomena and characteristics of clathrates under high pressures, i.e., the volume collapse phase transition and amorphization, are discussed on the basis of a survey over various clathrates with types I, II, III, and VIII structures. (C) 2017 The Japan Society of Applied Physics