First-principles Study Of Piezoelectric Properties Of Strained Ferroelectric Superlattice Structure

The superlattice material is attracting more attention lately, since it has more excellent properties than the single bulk materials when the composition and proportion are different, such as the performances on piezoelectric, photoelectric and dielectric. We can get many kinds of excellent properties which can be well applied in experiments by changing factors which include the chemical composition, proportion, strain, static electric field effect and the types of polar discontinuity. This thesis is based on the first-principles and reports the properties of multi-layers superlattice structure which include Pb Ti O3, La Al O3 and KNb O3, that is(PT)n/(LA)n and(PT)n/(KN)n superlattice, then a PT/LA/PT/KN structure which include two types of polar discontinuity is built up, in order to investigate the polar properties and piezoelectric behaviors when the external strain is applied.According to the relation curve of energy and the in-plane lattice constant, choose the equilibrium lattice constant when n=1. For these two systems,-1.5%~+1.5% strain was applied through changing the lattice constant. Then we calculate the properties of the two systems. Under different strain, the polarization of the two systems increase and the piezoelectric constant ezz is also enhanced. For PT/LA the ezz is negative and 3.343 C/m2, while for PT/KN the ezz is positive and 6.832 C/m2.The magnitude of n is an important factor that affect the properties of(PT)n/(LA)n and(PT)n/(KN)n superlattice. As n increases, the polarization and the piezoelectric constant ezz decrease, and the band gap becomes narrower, on the other hand, the effect of the electric field caused by the discontinuity interfaces turn out to be weaker as well. When n is fixed, with the process that changing from suppress strain to tensile strain, the polarization increase towards the direction of negative and objective for(PT)n/(LA)n and(PT)n/(KN)n superlattice, respectively, but weaker than the structure of which n is 1.Finally, the PLPK structure, which include two types of discontinuity is investigated. The results shows that the system is still maintain insulate when-1.5%~+1.5% strain applied, while the band gap is very small. According to calculate the polarization properties, it can be seen that its polarization is about 2~6 times smaller than the single(PT)n/(LA)n or(PT)n/(KN)n superlattice, the piezoelectric response along the(001) direction decrease a lot at the same time and the ezz is negative and about 0.91147 C/m2. By analyzing its discontinuity interfaces, the reason is that the direction of electric field generated by Ti O2/La O, Al O2/Pb O layers and Ti O2/KO, Nb O2/Pb O layers are opposite and exsiting a canceling effect.