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Author: search.filters.author.Lambrecht, Walter R.L.Has files: No

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    Pressure-dependent elastic constants and sound velocities of wurtzite SiC, GaN, InN, ZnO, and CdSe, and their relation to the high-pressure phase transition: A first-principles study
    (2010-07-01)
    Sarasamak, Kanoknan
    ;
    Limpijumnong, Sukit
    ;
    Lambrecht, Walter R.L.
    Elastic constants and sound velocities calculated from first principles as function of pressure are presented for wurtzite SiC, GaN, InN, ZnO, and CdSe. The C<inf>11</inf> and C<inf>33</inf> elastic constants, which are involved in longitudinal sound waves along symmetry directions, are found to monotonically increase with pressure. The shear moduli C<inf>44</inf> and C<inf>66</inf>, which are involved in transverse sound waves along symmetry directions, either decrease with increasing pressure or initially increase from zero pressure but then turn over and start decreasing. Of special interest is the pressure at which the C<inf>44</inf> and C<inf>66</inf> elastic constants cross. At this pressure, the transverse acoustic waves in the basal plane, which are shown to be closely related to the symmetry breaking strain component that leads to the phase transition, become easier to excite than the ones with displacement along the c axis. It is found that this crossover pressure is an upper limit to the actual phase transition pressure. The average of the calculated equilibrium transition pressure and the crossover pressure is proposed as a good estimate for the actual transition pressure in cases where the transition is strongly kinetically hindered by an enthalpy barrier between the two phases. This occurs for SiC and GaN and is confirmed with literature data for AlN. For the remaining materials, all these pressures are close to each other. The trends of the elastic constants and sound velocities with the materials' Phillips scale ionicity are also reported. © 2010 The American Physical Society.
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    First principles calculations of elastic properties under pressure in SiC
    (2010-04-20)
    Sarasamak, Kanoknan
    ;
    Limpijumnong, Sukit
    ;
    Lambrecht, Walter R.L.
    First principles density functional theory is employed to study the elastic constants and sound velocities in wurtzite-SiC under pressure. The focus is on the behavior of the elastic constants (C<inf>ij</inf>) and sound velocities (Cij/ρ) as a function of pressure. The pressure dependences of the transverse acoustic modes in sound velocities which correspond to the wurtzite to rocksalt phase transformation is analyzed. We found that the phase transformation from wurtzite to rocksalt structure relates to an orthorhombic strain with two components: (1) a traceless compression along the [0 0 0 1] axis which corresponds to the sound velocity C44/ρ and (2) a traceless compression along [011̄0] direction which corresponds to the sound velocity C66/ρ. © 2010 Elsevier B.V. All rights reserved.