摘要
多铁材料拥有丰富的物理性质，广泛应用于各种通讯、探测、控频和存储器件中。近来的研究发现它在光伏领域也有巨大的应用潜力，很可能会成为一种新型的太阳能电池材料或光敏材料。本论文采用第一原理方法对多铁BiFeO_3的铁电、压电、弹性和光学性质进行研究，并讨论其中的机制和潜在应用。
首先，我们研究了BiFeO_3的极化强度在[111]单轴应力下的变化以及在这个过程中结构的稳定性。发现在应力从8变化到-8GPa的过程中，BiFeO_3的极化强度近线性地从75.45μC/cm2变化到97.08μC/cm2。这是由在单轴应力下Bi原子出现了较大的相对位移且Bi原子的玻恩有效电荷增大引起。同时，我们研究了BiFeO_3体系的弹性常数在应力下的变化，发现体系在8到-8GPa的单轴应力范围内是稳定的，当应力为11GPa时体系是不稳定的。
然后，我们进一步研究了BiFeO_3体系在应力下光学性质的变化。我们发现在单轴压应力或双轴拉应力下，BiFeO_3的能带隙减小，并且带隙的值可以调节到适合光伏应用的最佳值1.4eV（光吸收谱和太阳光谱的交叠增大BiFeO_3；此外，还发现在单轴压应力下BiFeO_3的带隙从间接带隙变为直接带隙，进一步增强了能带边的光吸收。这些能带结构的变化主要是由于实空间中的z轴的压应变引起了倒空间中相应的轴的拉伸，从而引起了能带的展宽和能带隙的减小等变化。
多铁BiFeO_3可以以四方结构存在于薄膜中，这种四方结构拥有巨大c/a比率和极化率，有广阔的应用前景。我们在论文的最后部分讨论了四方BiFeO_3的弹性、压电和光学性质。发现当体积小于113/atom（或大于173/atomBiFeO_3时，c44趋于零，暗示着结构这时是不稳定的。发现四方相比菱形相软。同时，我们给出了四方BiFeO_3的体弹性模量、剪切模量、杨氏模量、泊松比、弹性各向异性比率和压电系数。此外，我们发现和菱形BiFeO_3相比，四方BiFeO_3的介电函数出现了整体蓝移的现象，并且电荷转移激发比菱形相的高约0.3eV，这是由FeO6氧八面体的对称性被破坏造成的。我们还发现四方BiFeO_3是一个负单轴晶体并且在折射率谱中呈现出明显的双折射现象。
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 BiFeO_3using first-principles density functional theory andwe also discuss its mechanism and potential applications.
Firstly, we studied the change of polarization of BiFeO_3under [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 BiFeO_3under 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 BiFeO_3under 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 BiFeO_3has 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 BiFeO_3is found in thin-film with huge c/a ratio and broad applica-tion prospect. We discussed the elastic, piezoelectric and optical properties of BiFeO_3inthe last part of the thesis. When the volume is less than113/atom (or greater than17
3/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 BiFeO_3is a negative uniaxial crystal and shows significant birefringencephenomenon.
引文
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