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

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。

First-principles Calculation on Ferroelectricity and Its Coupling Behavior with Mechanical Deformation of Ultrathin PbTiO3 Nanotube

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.