Ab Initio Studies Of The Pressure-indcued Phase Transitions In Binary Semicondcutors

One of the interesting phenomena that may occur under applied pressure is a sudden change in the arrangement of the atoms, i.e., a structure phase transition. The occurrence of the new structure means the new physical characters. Therefore, the studying of the pressure-induced phase transition is very interesting to find new materials and probe the new characters in them. Since the pressure-induced structural behaviors in binary semiconductors are very rich, they are often studied by theorists and experimentalists. However, the works about the physical mechanisms and the rules of the phase transition in the binary semiconductors are still not complete. For instance, there is no study about the physical mechanism of the pressure-induced phase transition in silver halides. The knowledge of the similarity of the mechanisms of the phase transition between the silver halides and copper halides is not clear although these two kinds of objects are all I-VII compounds. Therefore, the studying of the mechanism of phase transition of the silver halides is very important for us to complete the pictures of the phase transitions in I-VII compounds.On the other hand, much research is still needed to obtain the rules of the transition sequence for some certain binary compounds although much attention has also been focused on the general rules of the structural behaviors these compounds under high pressure. To our best knowledge, the rules of the phase transition sequences still remains unclear about two kinds of compounds: B8(P63/mmc, Z = 2) and B2 (Pm-3m, Z = 1) phases.Under different pressures, some semiconductors (BeS, BeSe, BeTe, MgTe, AlAs, AlP, BaO) all crystallize in B8(P63/mmc, Z = 2) phase. The post-B8 phases in these compounds under higher pressure are still not found in experiment. Recently, some structures hase been proposed theoretically. However, there are two shortcomings about these theoretical calculations: (i) it may miss unsuspected, yet more stable structures and/or (ii) the predicted phases may be dynamically unstable. Therefore, the studying of the phase transition sequences in the B8 phase of semiconductors will contribute to the full pictures of the phase transitions in all the binary compounds. On the other hand, almost 100 compounds containing transition metals crystallize in B8 phase under ambient pressure. In this kind of compounds, iron sulfide (FeS) is a representative compound which has been thought to be one of the major constituents of the cores of terrestrial planets such as Mars. Room temperature compression of FeS causes the phase transition of B8 phase into a orthorhombic phase. Accordingly, the study of the phase transition sequence in the B8 phase of semiconductors might contribute to the understanding of the mechanism of the structural behaviors of the binary compounds containing transition metals under pressure.The B2 (Pm-3m, Z = 1) phase has long been empirically considered to be a candidate structure for most binary semiconductors under pressure. However, B1 phase (Fm-3m, Z = 2)of MgO and CdO, are not observed to transform to B2 phase under very high pressure. In addition, the preivious theoretical works has proved the absence of the B2 phase in GaP, GaAs, InAs under pressure. Therefore, not all the binary semiconductors would crystallize B2 phase under compression. The reason for the absence of B2 phase for these two compounds under pressure remains unclear. Therefore, to study the possibility of the existence of the B2 phase in binary semiconductors under high pressure is interesting to fulfill the understanding of the phase transition rules in this kind of compounds. Moreover, whether those compounds which have no B2 phase under pressure would have another candidate structure is also interesting to be considered.Lattice dynamics has often been used to understand the mechanism of the phase transition sequence and predict the phase transition sequence. Many previous works have also proved the feasibility of this method. Therefore, we will make use of the calculation of the lattice dynamics to study the mechanisms of the phase transitions in silver halides and predict the general rules of the structural behaviors in binary semiconductors.Ab initio calculations within the framework of density-functional perturbation theory employing the generalized gradient approximation have been performed to study the lattice dynamics and elastic properties for several silver halides (AgX; X=F, Cl, Br, I) under pressure. In AgF, the softening of the longitudinal and transverse acoustic (TA) phonon modes at the zone boundary X (0.0 0.0 1.0) point dominates the pressure-induced structural phase transition from B1 to B2 phase. For AgCl, a TA phonon branch softens to zero pressure at 6.5 GPa along [ξ00] direction results in transition from the B1 structure to the monoclinic structure. The softening of TA phonon mode at X point in AgBr deduced transition from B1 to monoclinic phase. The dynamical instabilities of AgI are extensively studied to probe the mechanism of the phase transitions. The electronic properties of AgCl under pressure are also extensively studied. The location of the valence band maximum and the conduction band minimum in the entire Brillouin zone are predicted for both structures. The rocksalt and monoclinic phase of AgCl are both semiconductors with indirect gap and the variations of the band gaps with pressure indicate that they do not become metallization before structural instability. Furthermore, the bonding between Ag and Cl atoms becomes less ionic or more covalent under compression.Analysis of the phonon calculations suggested that the pressure-induced instabilities of the TA modes at the M/X points of zone boundary for B4/B3 AgI are responsible for the phase transitions of B4/B3→tetragonal. While the tetragonal→B1 transition is not induced by phonon instability, but likely by energetic. Moreover, it is found that B1 AgI transforms to the monoclinic phase is driven by the softening of the TA(X) mode. The elastic constants calculations suggest that although the predicted elastic instabilities in B4 and B3 might couple with the softening phonon to lower the transition pressure. Pressure-induced metallization has been predicted for B2-AgI by evidence of the significant band gap overlap. However, the B2-AgI is predicted to be dynamically unstable.The general rules about the phase transition sequence for the B8 phase of (BeS, BeSe, BeTe, MgTe, AlAs, AlP, BaO) have also been studied in terms of the frozen phonon method. MgTe, AlP, and AlAs would undergo the second order phase transition driven by the soft mode and the high pressure phases are Pnma for MgTe and Pbcm for AlP and AlAs, respectively. For BeX (X=S, Se, Te), the dynamical instabilities were not found in BeS and BeSe but in BeTe. In BeTe, the softening of the transtic phonon mode at K point at the boundary of the brillouin zone leads to the phase transition from B8 to Pmmn phase. BaO might have the same high pressure phase as BeTe because of the similar lattice dynamical behavior. Therefore, except for BeS and BeSe, all the B8 phase of semiconductors will transform to an orthorhombic phases under pressure.Many calculations demonstrated that not all the binaryd semiconductors will crystallize in B2 phase under very high pressure because they are dynmical instable although energy favored. This is atributed to the instability of M 2? mode. In addition, the corresponding eigenvectors of the soft mode are same for all the above semiconductors. The anion at (0.5,0.5,0.5) move along the <001> direction while the cations at (0,0,0) remains quiet. By freezing the softening mode, a tetragonal phase (P4/nmm) will replace the B2 phase as the high pressure structure. All the calculated results proved that those semiconductors whose ionic scale arranged from 0.327 to 0.780 will not have a B2 pahse but a tetragonal phase under high pressure.