First-principles Study Of Ferroelectric, Piezoelectric And Optical Properties Of BiFeO₃

Multiferroic materials with rich physical properties are widely used in various com-munication devices, detectors, frequency control devices and storage devices, etc. It’sfound that multiferroic materials also have huge application potential in the field of pho-tovoltaic （PV） recently and are likely to become a kind of new type of solar cell materialsor photosensitive materials. We study the ferroelectric, piezoelectric and elastic, and op-tical properties of tetragonal BiFeO₃using first-principles density functional theory andwe also discuss its mechanism and potential applications.Firstly, we studied the change of polarization of BiFeO₃under [111] uniaxial stressand the stability of the structure in the process. We found that the polarization P increasesalmost linearly from75.45μC/cm2to97.08μC/cm2during the stress changing from8to-8GPa, due to the relative lager displacement of Bi atoms and the increases of Bornefective charge of Bi atoms. At the same time, we studied the change of elastic constantsof BiFeO₃under uniaxial stress. It is found that the structure is stable in the stress rangeof8to-8GPa, while at σ33=11GPa the structure is unstable.Then, we further studied the change of optical properties of BiFeO₃under uniaxialstress or biaxial stress. We found that the bandgap reduced and the value of bandgapcan be adjusted to the ideal value of1.4eV for PV application （the overlap of the lightabsorption spectrum and the solar spectrum increases）; In addition, we also found that thebandgap of BiFeO₃has become a direct band gap from indirect band gap under uniaxialcompressive stress, which further strengthen the light absorption of the band edge. Thesechanges in the band structure are mainly due to the compression of z-axis in the real space.Since the compression of z-axis in the real space causes the tension of the correspondingaxis in the reciprocal space and it further causes the band broadening and the decrease ofthe band gap, etc.The tetragonal BiFeO₃is found in thin-film with huge c/a ratio and broad applica-tion prospect. We discussed the elastic, piezoelectric and optical properties of BiFeO₃inthe last part of the thesis. When the volume is less than113/atom （or greater than173/atom）, the c44of tetragonal phases tend to zero and the structures become unstable.The tetragonal phases are predicted to be softer than the rhombohedral antiferromagneticphase. Other elastic properties, including bulk modulus, shear modulus, Young’s modu- lus, Poisson’s ratio and elastic anisotropy ratios are also investigated. Besides, we foundthat the dielectric functions are blue-shifted compared with that of rhombohedral BFO,and charge transfer excitations that are～0.3eV higher than those of the rhombohedralcounterpart, caused by the damage of symmetry of FeO6octahedral. We also found thatthe tetragonal BiFeO₃is a negative uniaxial crystal and shows significant birefringencephenomenon.