超薄钛酸铅纳米管铁电性和力电耦合特性的第一性原理研究

doi:10.3724/SP.J.1077.2014.13301

摘要/Abstract

摘要：

利用基于密度泛函理论的第一性原理的方法研究了超薄钛酸铅(PbTiO₃)纳米管的铁电性及力电耦合特性。研究发现对于钛酸铅铁电纳米管结构, 即使在其特征尺寸小于铁电薄膜的铁电临界尺寸时, 依然存在自发极化。钛酸铅铁电纳米管结构不存在铁电临界尺寸。对纳米管力电耦合效应的研究发现, 轴向应变作用会引起包括极化沿轴向方向的铁电相、顺电相和极化沿周向方向的铁电相在内的丰富的相转变。这种相的转变是由于轴向应力所导致的Pb-O共价键的变化所引起的。另一方面, 研究了钛酸铅纳米管结构的机械强度, 明确了在轴向拉伸和压缩作用下纳米管的临界载荷。

关键词: 铁电纳米管, 钛酸铅, 铁电临界尺寸, 第一性原理计算

Abstract:

Ferroelectric properties and its coupling behavior with mechanical strain of ultrathin PbTiO₃ nanotubes were investigated by first-principles calculations. The spontaneous polarization still exists in the nanotube despite their sidewalls thinner than the critical thickness at which the thin films lose ferroelectricity, which indicates the absence of an intrinsic critical size of ferroelectricity. Moreover, the total energy of nanotube is lower than that of the thin film. This means that the nanotube structure is energetically more stable than the thin film. In addition, the coupling behavior of ferroelectricity and axial strain is also studied. The axial polarization of nanotube is enhanced by the tensile strain. On the other hand, with the increase of compressive strain, the axial polarization becomes weak and disappears, and the nanotube structure becomes paraelectric state. With the further increase of compressive strain, a vortex type of polarization emerges along the circumferential direction, and the nanotube structure becomes ferroelectric state again. These rich phase transitions in the nanotube structure are induced by the change of covalent Pb-O bond due to the applied strain. Finally, the mechanical strength of PbTiO₃ nanotube is evaluated, and the critical stresses under the tension and compression states are obtained.

Key words: ferroelectric nanotube, PbTiO₃, critical size, First-principles calculation

中图分类号:

TM22

王晓媛, 嶋田隆広, 北村隆行. 超薄钛酸铅纳米管铁电性和力电耦合特性的第一性原理研究[J]. 无机材料学报, 2014, 29(3): 309-314.

WANG Xiao-Yuan, SHIMADA Takahiro, KITAMURA Takayuki. First-principles Calculation on Ferroelectricity and Its Coupling Behavior with Mechanical Deformation of Ultrathin PbTiO₃ Nanotube[J]. Journal of Inorganic Materials, 2014, 29(3): 309-314.

图/表 8

图1 PbO外侧(a)和TiO2外侧(b)纳米管模型结构

Fig. 1 Simulation models of (a) PbO-outside and (b) TiO2-outside nanotubes

图2 纳米管结构内聚能Ec 随曲率1/R的变化曲线

Fig. 2 Cohesive energy Ec as a function of nanotube curvature 1/R

图3 轴向极化Pz与曲率1/R之间的关系

Fig. 3 Axial polarization (Pz,) as a function of nanotube curvature 1/R Positive and negative values represent the curvatures of the PbO-outside and TiO2-outside nanotubes, respectively

图4 外侧为PbO层, N=18的纳米管中轴向极化Pz以及偶极矩g随轴向应变εzz的变化曲线

Fig. 4 Axial polarization (Pz) and toroidal moment (g) of the PbO-outside N=18 nanotube as functions of axial strain (εzz)

图5 外侧为PbO层, N=18的纳米管外侧PbO层电荷密度分布随拉伸应力的变化

Fig. 5 Change in charge density distribution on the PbO layer of PbO-outside N=18 nanotube under axial tensile strain The covalent Pb-O bonds are emphasized by white lines

图6 外侧为PbO层, N=18的纳米管外侧PbO层电荷密度分布随压缩应力的变化

Fig. 6 Change in charge density distribution on the PbO layer of PbO-outside N=18 nanotube under axial compressive strain The covalent Pb-O bonds are emphasized by white lines

图7 PbTiO3纳米管中轴向应力σzz随轴向应变εzz的变化曲线

Fig. 7 Relation between the axial stress (σzz) and the axial strain (εzz) in PbTiO3 nanotube

表1 PbTiO3纳米管在拉伸和压缩状态下的临界应力和断裂应变

Table1 Critical stress and fracture strain of PbTiO3 nanotube in tension and compression

	Tension	Compression
Critical stress/GPa	7.09	-8.06
Fracture strain	0.06	-0.04

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