Solids under Pressure. Ab Initio Theory.

physica status solidi (b) (Impact Factor: 1.49). 09/1998; DOI: 10.1002/(SICI)1521-3951(199901)211:13.0.CO;2-N
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ABSTRACT Parameter-free calculations based on the density-functional theory are used to examine high-pressure phases of solids, mainly semiconductors. For the elemental semiconductors, as represented by Si, the diamond!fi-tin!Imma sequence is examined, and for III-V semiconductors the optimization of the structural parameters of the Cmcm and Imm2 phases is described. The structural energy differences 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 significantly affected by inclusion of (generalized) gradient corrections (GGA). Elemental Zn (hcp) is further taken as an example where we find that the simple LDA leads to poor results. 1 I. INTRODUCTION Theoretical studies of cohesive, structural and vibrational properties of semiconductors under pressure are now routinely bein...

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    ABSTRACT: The phase transition between the cubic diamond (cd) and beta-tin (β-Sn) phases of Si under pressure and the region of interaction of the two phases are studied by first-principles total energy calculations. For a non-vibrating crystal we determine the pressure of the thermodynamic phase transition p(t) = 96 kbar, the Gibbs free energy barrier at p(t) of ΔG = 19.6 mRyd/atom that stabilizes the phases against a phase transition and the finite pressure range in which both phases are stable. We show that the phases in that pressure range are completely described by three equilibrium lines of states along which the structure, the total energy E, the hydrostatic pressure p that would stabilize the structure and the values of G all vary. Two equilibrium lines describe the two phases (denoted the ph-eq line, ph is cd or β-Sn phase); a third line is a line of saddle points of G with respect to structure (denoted the sp-eq line) that forms a barrier of larger G against instability of the metastable ranges of the phase lines. An important conclusion is that the sp-eq line merges with the two ph-eq lines: one end of the sp-eq line merges with the cd-eq line at high pressure, the other end merges with the β-Sn-eq line at low pressure. The mergers end the barrier protecting the metastable ranges of the two ph-eq lines, hence the lines go unstable beyond the mergers. The mergers thus simplify the phase diagram by providing a natural termination to the stable parts of all metastable ranges of the ph-eq lines. Although 96 kbar is lower than the experimental transition pressure, we note that phonon pressure raises the observed transition pressure.
    Journal of Physics Condensed Matter 05/2012; 24(22):225501. · 2.22 Impact Factor
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    ABSTRACT: First principles calculations of the total energy of Imma states have found instabilities in states near the β-Sn phase and in states near the simple hexagonal (sh) phase of Si crystal. In agreement with experiment the two instability ranges narrow the stable range between them and also in agreement with experiment the instabilities force first-order transitions to both the β–Sn and sh phases when the pressure is held constant, the experimental condition. The transition pressures to the β-Sn and sh phases for a non-vibrating crystal model are found to be 96 and 110 kbar respectively. These pressure values are considerably lower than the experimental values, but we show that lattice vibrations will increase the equilibrium-state pressures. We find widespread occurrence of instability in the equilibrium states of the three phases and show the presence of three kinds of instability. Near and up to the sh phase structure we find the unusual case of stability at constant volume, but, as observed, instability at constant pressure p. Two special computational procedures are discussed, which locate the unstable ranges of structure. One is based on finding phases from minima of total energy E at constant V and the other finds phases from minima of the Gibbs free energy G at constant p. When the minima cease to exist the Imma phase is unstable.
    Physics of Condensed Matter 01/2011; 81(4):411-418. · 1.28 Impact Factor
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    ABSTRACT: The Imma phase of Si is found by first-principles calculations to have unstable states at the two ends of its structural range. These instabilities force first-order transitions to both the β-Sn and the simple hexagonal phase. These transitions occur for the non-vibrating crystal at 96 and 110 kbar, respectively. Two special computational procedures are described that have been developed to find the instabilities.
    EPL (Europhysics Letters) 11/2010; 92(2):27002. · 2.26 Impact Factor

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