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Pressure - induced phase transition mechanism of ZnO materials research

Author: ChenChangBin
Tutor: JiangZhenYi
School: Northwestern University
Course: Condensed Matter Physics
Keywords: ZnO First-principles Phonon spectrum Molecular dynamics simulation Phase change
CLC: O521.23
Type: Master's thesis
Year: 2011
Downloads: 60
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Abstract


First-principles simulation method based on density functional theory, combined with the plane-wave pseudopotential method, using the local density approximation study of ZnO. Systematic study of the ZnO crystal hexagonal structure (B4), sphalerite structure (B3), the CsCl structures (B2), and cubic NaCl structure (B1) phase lattice dynamics, equilibrium lattice parameters, bulk shells modulus and other properties associated with the atomic structure. The total energy relationship with the volume, the change trend of the four different structures of the order of phase stability and possible volume phase. Further calculation of the T = 0 K system Gibbs free energy with the pressure curve, and may be a phase transition pressure calculated results show that: we calculate is consistent with the experimental values ??and other theoretical value. Obviously, the study of the phase-change mechanism is our focus. In the paper, we use two different theories to study the the ZnO phase transition mechanism, one is the soft mode theory, a molecular dynamics simulation. By analyzing the calculated results of the two methods, we can draw this conclusion: (1) the wurtzite The phonon spectrum of the transverse acoustic mode but decreases with increasing pressure, about 10GPa about the point M and H near the imaginary frequency, the intrinsic displacement of the M point vibration analysis, starting from the point of view of the soft mode theory further evidence of the wurtzite to rock salt structure phase transition path is the path of the \However, for the B2 phase is yet another phenomenon. With the increase in the pressure of the phonon spectrum imaginary frequency is getting smaller and smaller, when the pressure increased 340GPa when imaginary frequency disappeared. This shows that the B2 structure is only possible in the presence of more than 340GPa. (2) WZ → RS, ZB → RS transition pressure were 33GPa, 27GPa, its value in the 30-35GPa ,25-30GPa. Furthermore wurtzite, sphalerite and rock-salt structure enthalpy calculations show that: wurtzite and sphalerite, respectively, to the rock-salt structure phase transition pressure is 10 GPa, the wurtzite transition pressure with the experimental values ??and theoretical values ??are very close to; sphalerite value to be larger than the experimental values, but smaller than any other theory. Between wurtzite and zinc blende phase change occurs in the case of pressure alone is impossible because as the pressure increase both the enthalpy change trend is consistent. However, if the phase change may have occurred under the joint action of pressure and temperature both. (3) atmospheric first-principles molecular dynamics method to study the pressure-induced phase transition of ZnO wurtzite structure. Found that this phase change process consists of three consecutive steps: First, WZ phase along the c-axis direction is compressed into the space group P63/mmc the hexagonal structure; Secondly, under the effect of shear deformation \into symmetry is middle of the Fmmm orthogonal state; Finally, when the atoms move to the \This phase transition mechanism from this point of view is the path of the \(4) at the conclusion can be drawn at low pressure WZ → RS-phase phase-change path is the path of the \(5) use the same molecular dynamics method described ZB → RS phase phase change mechanism from the atomic point of departure. ZB → RS phase change mechanism analysis, we predict that the path of the phase transition can be described as: cubic → tetragonal → monoclinic, wherein the intermediate structure is a monoclinic crystal structure of the space group Cm, this conclusion and SiC, ZnS ZB → RS the prediction phase transition with molecular dynamics simulations and classical dynamics of mechanisms to maintain a high degree of agreement.

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CLC: > Mathematical sciences and chemical > Physics > High pressure and high temperature physics > High Pressure Physics > The physical properties of the material under high pressure > Phase transition under high pressure
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