Physics Department, University of Arkansas, Fayetteville, 72701, USA.
Phys Rev Lett. 2012 Aug 3;109(5):057602. doi: 10.1103/PhysRevLett.109.057602. Epub 2012 Aug 2.
First-principles calculations are performed to revisit properties of (001) epitaxial BiFeO(3) (BFO) and PbTiO(3) thin films under tensile strain. While these two films possess different ground states when experiencing no misfit strain, they both exhibit the same, previously unknown phase for tensile strains above ≃5% at T = 0 K. This novel state is of orthorhombic Pmc2(1) symmetry and is macroscopically characterized by a large in-plane polarization coexisting with oxygen octahedra tilting in-phase about the out-of-plane direction. On a microscopic point of view, this Pmc2(1) state exhibits short atomic bonds and zigzag cation displacement patterns, unlike conventional ferroelectric phases and typical domains. Such unusual inhomogeneous patterns originate from the coexistence of polar and antiferroelectric distortions having the same magnitude and lead BFO films to be the first known material for which orbital ordering coexists with a large polarization. Furthermore, this Pmc2(1) state is also found in other perovskite films under tensile strain, which emphasizes its generality.
采用第一性原理计算方法重新研究了拉伸应变下（001）外延 BiFeO3（BFO）和 PbTiO3 薄膜的性质。当没有失配位错应变时，这两种薄膜具有不同的基态，但在 T = 0 K 时，拉伸应变量超过 ≃5%时，它们都表现出相同的、以前未知的相。这种新相具有正交 Pmc2(1)对称性，宏观上表现为大的面内极化与氧八面体沿面外方向同相倾斜共存。从微观角度来看，这种 Pmc2(1)态表现出短的原子键和之字形的阳离子位移模式，与传统铁电相和典型畴不同。这种不寻常的不均匀模式源于具有相同大小的极性和反铁电畸变的共存，使 BFO 薄膜成为第一个已知的同时具有大极化和轨道有序的材料。此外，这种 Pmc2(1)态也在其他钙钛矿薄膜中在拉伸应变下被发现，这强调了它的普遍性。
