铁电材料的第一性原理研究
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摘要
本论文采用第一性原理计算了钙钛矿型铁电体和具有铁电性能的掺杂氧化锌的电子结构。
采用第一性原理计算了Pb(Zr0.4Ti0.6)03五层超晶胞的顺电相和铁电相的电子结构,由态密度、电子密度和能带结构的计算结果发现顺电相下的钛氧八面体Ti-06和锆氧八面体Zr-O6在铁电相中分裂为由1个01离子和4个02离子组成的金字塔结构Ti-05和Zr-05；与顺电相相比,铁电相中钛离子的3d电子和氧离子的2p电子存在更强的轨道杂化,这种杂化降低了离子间的短程排斥力,使得具有铁电性的四方结构更为稳定；由电子密度的分布可推断立方结构的Pb-O键呈现离子键特征,而铁电相下Pb-O键则有较大的共价成分,铅离子与氧离子的这种轨道杂化对Pb(Zro.4Tio.6)03的铁电性起着重要作用。对比计算了不同Zr/Ti比例的Pb(ZrxTi1-x)03中Zr离子和Ti离子对铁电畸变的贡献,发现钛离子与氧离子的相互作用对于铁电相Pb(ZrxTil.x)03沿c轴自发极化的贡献大于锆离子与氧离子的相互作用。
采用第一性原理计算了PbTiO3和Bi4Ti3O12两种铁电体在从顺电相到铁电相的相变过程中的氢阻碍机制。对于不掺氢的铁电相PbTiO3,Ti离子沿c轴偏离平衡位置有两个能量最低点；对于掺氢的铁电相PbTiO3,这两个能量最低点不复存在,而在平衡位置处能量最低,而且高于顺电相的能量最低点。这样,在立方结构的PbTiO3从高于居里温度的高温冷却到室温的过程中,Ti离子很难沿c轴发生位移从而产生铁电畸变。应用这一规律同样可以解释高于居里温度的铁电体Bi4Ti3O12在Fonning gas退火后铁电性能下降的现象。
针对具有铁电性能的Li掺杂ZnO和V掺杂ZnO,计算其电子结构,研究其铁电性的产生机制。发现两种掺杂ZnO的铁电性均是由于掺杂离子偏离平衡位置引起的,并且计算了Li掺杂ZnO的极化强度。
The electronic structures of both ferroelectric and doped ZnO which show ferroelectric property have been investigated using First principles plane-wave pseudopotential density functional theory.
The calculated results including density of states, band structure and electron density show that the Ti-O6 and Zr-O6 octahedron in cubic structure change into the Ti-O5 and Zr-O5 pyramid in ferroelectric Pb(Zro.4Tio.6)03. A stronger hybridization between the titanium 3d states or the zirconium 4d states and the oxygen 2p states in ferroelectric phase than that in paraelectric phase can reduce short-range repulsion in atoms and enhance the stability of the ferroelectric structure of Pb(Zr0.4Ti0.6)03. The major contribution to the spontaneous polarization along the c axis comes from the TiO5 pyramid unit rather than from the ZrO5. The Pb-O bonds in tetragonal Pb(Zr0.4Ti0.6)03 demonstrate rather strong covalency, while those in cubic Pb(Zr0.4Ti0.6)03 are ionic. Our results indicate that the Pb-O bonding interaction is also a key factor of the stability of the ferroelectric structure. The electronic structure of the Pb(ZrxTi1-x)O3with different atomic ratio of Zr/Ti has also been calculated.
Hydrogen-hindered phase transition of ferroelectric ceramics PbTiO3 and Bi4Ti3O12 from paraelectric to ferroelectric have been studied by first principles calculation. The calculation shows that for hydrogenated tetragonal PbTiO3, double-lowest-energy sites of Ti along the c axis exist no longer and the only lowest energy site locates at the center of the cell. Therefore, for a cubic PbTiO3 above its Curie temperature, Ti cannot move along the c axis to be a tetragonal structure during cooling to room temperature. This can also explain the experiment the hydrogen charged above its Curie temperature can hinder phase transition of Bi4Ti3O12 from paraelectricity to ferroelectricity.
The electronic structures of V-doped ZnO and Li-doped ZnO have been investigated by first principles calculation. The results show that V is in the 5+ state replacing Zn and Li is in the 1+ state replacing Zn. The calculations also show that a Li1+ ionic displacement of 0.1 A is responsible for the ferroelectric behavior. For V5+, the displacement is 0.06 A. It is estimated that the polarization of V-doped ZnO is smaller than that of Li-doped ZnO.
采用第一性原理计算了Pb(Zr0.4Ti0.6)03五层超晶胞的顺电相和铁电相的电子结构,由态密度、电子密度和能带结构的计算结果发现顺电相下的钛氧八面体Ti-06和锆氧八面体Zr-O6在铁电相中分裂为由1个01离子和4个02离子组成的金字塔结构Ti-05和Zr-05；与顺电相相比,铁电相中钛离子的3d电子和氧离子的2p电子存在更强的轨道杂化,这种杂化降低了离子间的短程排斥力,使得具有铁电性的四方结构更为稳定；由电子密度的分布可推断立方结构的Pb-O键呈现离子键特征,而铁电相下Pb-O键则有较大的共价成分,铅离子与氧离子的这种轨道杂化对Pb(Zro.4Tio.6)03的铁电性起着重要作用。对比计算了不同Zr/Ti比例的Pb(ZrxTi1-x)03中Zr离子和Ti离子对铁电畸变的贡献,发现钛离子与氧离子的相互作用对于铁电相Pb(ZrxTil.x)03沿c轴自发极化的贡献大于锆离子与氧离子的相互作用。
采用第一性原理计算了PbTiO3和Bi4Ti3O12两种铁电体在从顺电相到铁电相的相变过程中的氢阻碍机制。对于不掺氢的铁电相PbTiO3,Ti离子沿c轴偏离平衡位置有两个能量最低点；对于掺氢的铁电相PbTiO3,这两个能量最低点不复存在,而在平衡位置处能量最低,而且高于顺电相的能量最低点。这样,在立方结构的PbTiO3从高于居里温度的高温冷却到室温的过程中,Ti离子很难沿c轴发生位移从而产生铁电畸变。应用这一规律同样可以解释高于居里温度的铁电体Bi4Ti3O12在Fonning gas退火后铁电性能下降的现象。
针对具有铁电性能的Li掺杂ZnO和V掺杂ZnO,计算其电子结构,研究其铁电性的产生机制。发现两种掺杂ZnO的铁电性均是由于掺杂离子偏离平衡位置引起的,并且计算了Li掺杂ZnO的极化强度。
The electronic structures of both ferroelectric and doped ZnO which show ferroelectric property have been investigated using First principles plane-wave pseudopotential density functional theory.
The calculated results including density of states, band structure and electron density show that the Ti-O6 and Zr-O6 octahedron in cubic structure change into the Ti-O5 and Zr-O5 pyramid in ferroelectric Pb(Zro.4Tio.6)03. A stronger hybridization between the titanium 3d states or the zirconium 4d states and the oxygen 2p states in ferroelectric phase than that in paraelectric phase can reduce short-range repulsion in atoms and enhance the stability of the ferroelectric structure of Pb(Zr0.4Ti0.6)03. The major contribution to the spontaneous polarization along the c axis comes from the TiO5 pyramid unit rather than from the ZrO5. The Pb-O bonds in tetragonal Pb(Zr0.4Ti0.6)03 demonstrate rather strong covalency, while those in cubic Pb(Zr0.4Ti0.6)03 are ionic. Our results indicate that the Pb-O bonding interaction is also a key factor of the stability of the ferroelectric structure. The electronic structure of the Pb(ZrxTi1-x)O3with different atomic ratio of Zr/Ti has also been calculated.
Hydrogen-hindered phase transition of ferroelectric ceramics PbTiO3 and Bi4Ti3O12 from paraelectric to ferroelectric have been studied by first principles calculation. The calculation shows that for hydrogenated tetragonal PbTiO3, double-lowest-energy sites of Ti along the c axis exist no longer and the only lowest energy site locates at the center of the cell. Therefore, for a cubic PbTiO3 above its Curie temperature, Ti cannot move along the c axis to be a tetragonal structure during cooling to room temperature. This can also explain the experiment the hydrogen charged above its Curie temperature can hinder phase transition of Bi4Ti3O12 from paraelectricity to ferroelectricity.
The electronic structures of V-doped ZnO and Li-doped ZnO have been investigated by first principles calculation. The results show that V is in the 5+ state replacing Zn and Li is in the 1+ state replacing Zn. The calculations also show that a Li1+ ionic displacement of 0.1 A is responsible for the ferroelectric behavior. For V5+, the displacement is 0.06 A. It is estimated that the polarization of V-doped ZnO is smaller than that of Li-doped ZnO.
引文
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[2]Shim S I, Kim S I, Kim Y T, et al. Operation of single transistor type ferroelectric random access memory [J]. Electron. Lett.,2004,40(22):1397.
[3]Ryoo Sung-Nam, Yoon Soon-Gil and Kim Seung-Hyun, Improvement in ferroelectric properties of Pb(Zr0.35Ti0.65)O3 thin films using a Pb2Ru2O7-x conductive interfacial layer for ferroelectric random access memory application[J]. Appl. Phys. Lett.,2003,83(14):2880.
[4]Kojima Takashi, Sakai Tomohiro and Watanabe Takayuki. Large remanent polarization of (Bi, Nd)4Ti3O12 epitaxial thin films grown by metalorganic chemical vapor deposition[J]. Appl. Phys. Lett., 2002 80(15):2746.
[5]钟维烈.铁电体物理学[M].北京.科学出版社.1996.
[6]Lee S H, Esashi M. Characteristics on PZT (Pb(ZrxTi1-x)O3) films for piezoelectric angular rate sensor [J].Sensors and Actuators,2004,114(1):88.
[7]Beeby S P, Ross N, White N M. Thick film PZT/micromachined silicon accelerometer [J]. Electron. Lett.,1999,35(23):2060.
[8]HU S H, MENG X J, HU G J, et al. Preparation and optical waveguide property of metal alkoxide solution-derived Pb(Zro.5Tio.5)03 thick films [J]. Appl. Phys Lett,2004,84(18):3609.
[9]Roemer A, Millon E, Vincent B, et al. Epitaxial PbTiO3 thin films grown on (100) MgO by pulsed-laser deposition for optical waveguiding properties [J].J. Appl. Phys.,2004,95(6):3041.
[10]Hu G J, Hu S H, Meng X J, et al. Fabrication of ferroelectric PbZrxTi1-xO3 thick films and their optical waveguide properties[J].J. Vac. Sci. Tech.,2004,A22(2):422.
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[12]H.Cao, J.Y.Wu and S.T.Ho, Random laser action in semiconductor powder, Phys. Rev. Lett.,1999, 82:2278.
[13]K.Vanheusden,C.H.seager,W.L.Warren,et al. Correlation between photoluminescence and oxygen vacancies in ZnO phosphors, Appl. Phys. Lett.1996,68:403.
[14]Groenen R, Linden J L. An expanding thermal plasma for deposition of surface textured ZnO:Al with focus on thin film solar cell applications,Appl Surf Sci,2001,173(1-2):40.
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