First Principle Study On The Multiferroic Bifeo₃ And The Doped Materials

Recent there has been considerable interest in developing multifunctional materials in which two or more useful properties are combined in a single compound. Multiferroic material has simultaneous ferromagnetic, ferroelectric, and/or ferroelastic ordering. Coupling between the magnetic and ferroelectric order parameters can lead to magnetoelectric effects, in which the magnetization can be tuned by an applied electric filed and vice versa. Relatively few multiferroics have been identified, and a single phase material with large and robust magnetization and polarization has not been previously identified, the mechanism underlying their ferroelectricity is often unconventional in those that are known. Bulk BiFeO3 has long been known to be ferroelectric with a Curie temperature of about 1103K and an antiferromagnetic with the Neel temperature of about 643K. We use density functional theory (DFT) to calculated the electric structure, Mulliken charges, optical properties of multiferrotic material of BiFeO3, and the doped-BiFeO3. The study conclusions see:1. We use first-principle to calculate the band structure, density of states and optical properties of multiferrotic material of BiFeO3. The calculated band gap is 2.5eV and the band gap is indirect, consistent with the experimental gap; in the calculated Mulliken charge Fe3+ magnetic moment is 2.13 h; and the calculated static dielectric constant is 6.25 and refractive index is 2.5, also consistent with the experimental data.2. Use the local spin-density approximation (LSDA)+U and Generalized Gradient Approximation (GGA)+U calculate the electronic structure and spin magnetic moment respectively. The calculated conclusions show that the result of GGA+U better consistent with the experimental result. And the band gap is large as U large. That is means there is roust electronic exchange potential in BiFeO3, so in this system the coulomb force of between electrons can not be ignored.3. Use GGA method study doped BiFeO3. The doped element rare earth element La replaced A-site Bi. Calculated the electronic structures and optical properties of this material, and analyzed the energy band structure, indirect band gap. And the static dielectric constant are large than parent phase according to the optical properties schemes. And because of the doped La causes the breakage of spiral magnetic structure, so material shows weak magnetic. And the doped makes the state of Bi6s moves to low energy, this in favor of ferroelectricity. And the loss energy spectrum has tow peaks, different from BFO. 4. B-site Fe3+ replaced by magnetic ions Cr3+, Co3+, and nonmagnetic ions Sc3+, Ti4+, respectively. Calculated the electronic structure and optical properties of doped-BFO. The band gaps all are indirect and only Co doped energy level inserts band gap of BFO. In the doped materials the states of Bi6s also move to low energy.