Electronic Structure of Materials under Pressure

Parameter-free calculations based on the density-functional theory are used to examine high-pressure phases of solids. For the elemental semiconductors, as represented by Si, the high-pressure phases are examined in some detail, and particular attention is paid to the Si-VI orthorhombic (Cmca) structure which was resolved only very recently. For III-V semiconductors the optimization of the structural parameters of the Cmcm and Imm2 phases is described. The structural energy diierences are in several cases very small, and in some too small to allow a safe structure prediction on the basis of the calculations. In that context we also discuss ways to go beyond the local density approximation (LDA). We show that the predicted high-pressure phases may be signiicantly aaected by inclusion of (generalized) gradient corrections (GGA). Elemental Zn (hcp) is further taken as an example where we nd that the simple LDA leads to poor results for the equilibrium volume and axial ratio (c=a). Introducing corrections, for example by GGA, it is, however, possible to achieve an accuracy that allows a study of the structural changes of Zn (and Cd) under pressure, and to analyse these in terms of electronic topological transitions.