Molecular Dynamics Study Of BTO, BIT Ferroelectrics

Ferroelectric materials are extremely important information functional materials with promising applications in many fields, such as ferroelectric random access memories. Recently, considerable attention has been devoted to the development of ferroelectric perovskitic materials due to their many important applications, such as nonvolatile ferroelectric thin-film memories, micro-electromechanical systems and tunable microwave devices. In this thesis, the radiation displacement effect of simple perovskitic ferroelectrics BaTiO₃ (BTO), and pressure-induced phase transition of bismuth layer-structured perovskitic ferroelectrics Bi₄Ti₃O₁₂ (BIT) were studied using a shell model via molecular dynamics method.(1) The radiation displacement effect of BaTiO₃ (BTO) ferroelectrics was studied using shell model molecular dynamics simulations. Using O as the primary knock-on atom, in different incident direction, the creation and evolution of various defects in the system corresponding to the PKA energy of 1 keV were studied. The results show that the largest numbers of defects were created when the incident direction is along [001]. Among all the defect species, O defects were dominant with a concentration of 80%. The creation of defects does not change the spontaneous polarization of the system significantly, and the polarization reversal also changes little. However, we cannot underestimate the influence of radiation displacement effect, because defect migration is observed under an applied electric field.(2) Pressure-induced ferroelectric phase transition of Bi₄Ti₃O₁₂ (BIT) was studied using a shell model via molecular dynamics method. First the interatomic potentials were derived from pertinent literature, the calculated spontaneous polarizations of ferroelectric orthorhombic B2cb phase BIT single crystal at room temperature were not agree with the experimental values. The Ti-Ti short range interaction potential was added in an effort to make interatomic potentials integrated and to increase the accuracy of the simulation. The calculated spontaneous polarizations were in reasonable agreement with the experimental values. To determine whether the model provided a good reproduction of the experimental data, the lattice parameters of BIT were examined, the calculations were in good accord with the experimental data. The pressure-induced phase transition of BIT was also calculated. It was observed that BIT single crystal undergoes two structural transformations with increasing pressure from -2 GPa to 24 GPa. The first transition may be ferroelectric-ferroelectric phase transition, occurs around 6 GPa; the second transition occurs around 20 GPa, may be ferroelectric-paraelectric phase transition, the crystal lattice symmetry has changed. The accompanying symmetry changes of BIT ferroelectrics may be the same as those observed at ambient pressure at elevated temperature. These results thus provide a theoretical prediction of the pressure-induced phase transition in BIT.