Transcript Ab initio study of the stability of high pressure phases in II-VI semiconductors Davor Kirin and Igor Lukačević Institut Ruđer Bošković, Zagreb, Croatia and Department.

Ab initio study of the stability of high pressure phases in
II-VI semiconductors
Davor Kirin and Igor Lukačević
Institut Ruđer Bošković, Zagreb, Croatia
and
Department of Physics, University of J. J. Strossmayer, Osijek, Croatia
Introduction
The study of pressure induced phase transitions in semiconductors
has been rapidly expanding field of research during the past few
decades and a number of new phases has been discovered
experimentally [1]. In the present work the stability of the
intermediate rock salt structure with respect to the high pressure
Cmcm structure was investigated for several II-VI semiconductors
(ZnS, ZnSe, CdS and CdSe). The lattice dynamics calculations reveal
that the rock salt structure is unstable (i.e. some normal modes have
imaginary frequency) with the respect to the transversal acoustic
mode at the Brillouin zone boundary (X point) (Fig 1.).
Fig 2
Fig. 1
Fig. 2
Discusion
Fig 1
The stable low pressure structure of all studied crystals is zinc-blende
(zb) which at higher pressure undergoes a transition to the NaCl type of
Method
structure with large volume change. The NaCl structure is stable in a
Ab initio lattice dynamics calculations were performed using the certain pressure range and eventually undergoes transition to the
ABINIT program package [4], which is based on density functional orthorhombic Cmcm structure with very small volume change. The fig 2.
theory (DFT) and the use of pseudopotentials (PP).
shows the phonon frequency close to the phase transition showing that
the NaCl phase is still stable.
Results
The present calculation gives the transition pressure more accurately then classical method, which uses the common tangent method. Since the
volume change at the transition is very small or zero in all studied crystals, transition pressure were estimated from the pressure at which the
frequency of zone boundary transversal acoustic mode goes to zero (Fig 1).
The phase transition is of the second order displacive and is associated with the cell doubling in the high pressure Cmcm phase with frozen in
deformation.
The calculated (experimental) transition presures are 67 (69) GPa for ZnS, 49 (51) GPa for CdS, 29(30) GPa for ZnSe, and 26 (27) GPa for
CdSe crystal.
The observed pressures of the phase transitions [2,3] in structural measurements were reproduced more accurately then in theprevious
calculations [5] and the difference between observed and calculated transition pressure is of the order of 3 to 6 %.
Literature
[1] Mujica, A. Rubio, A. Munoz and R. J. Needs, Rev. Mod. Phys. 75, 863 (2003).
[2] R. J. Nelmes and M. I. McMahon, Semicond. Semimetals. 54, 145 (1998).
[3] M. I. McMahon and R. J. Nelmes, Phys. Status Solidi B, 198, 389 (1996).
[4] The ABINIT code is a common project of Universite Catholique de Louvain, Corning Incorporated and
other contributors (www.abinit.org).
[5] M. Cote, O. Zakharov, A. Rubio, and M. L. Cohen, Phys. Rev. 55, 13025 (1997).