Ricinschi Dan, Yun Kwi-Young, Okuyama Masanori
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, 560-8531 Osaka, Japan.
J Phys Condens Matter. 2006 Feb 15;18(6):L97-L105. doi: 10.1088/0953-8984/18/6/L03. Epub 2006 Jan 27.
Following our experimental report of a giant ferroelectric polarization in the region of 150 µC cm(-2) in BiFeO(3) (BFO) films, we have performed first-principles calculations based on the local density approximation to density functional theory, aiming to clarify its mechanism. Upon optimization of lattice constants we have shown that the natural tetragonal structure of BFO has a giant tetragonality ratio of 1.26 and large ionic off-centring. Experimentally this structure has been detected in BFO films deposited on La-doped SrTiO(3) substrates. The spontaneous polarization calculated ab initio for this structure is 143.5 µC cm(-2), in agreement with the remanent polarization of hysteresis loops measured at 90 K. These results suggest that the giant polarization of our BFO films may occur upon stabilization of the optimal tetragonal phase with giant tetragonality. Future experimental effort aiming to routinely obtain such values of spontaneous polarization should concentrate on how to isolate this phase without compromising the insulating and switching properties of BFO.
继我们报道了BiFeO₃（BFO）薄膜中出现150 μC cm⁻²量级的巨大铁电极化的实验报告后，我们基于密度泛函理论的局域密度近似进行了第一性原理计算，旨在阐明其机制。在优化晶格常数后，我们发现BFO的天然四方结构具有1.26的巨大四方度比和较大的离子偏离中心。实验上，这种结构已在沉积于La掺杂SrTiO₃衬底上的BFO薄膜中被检测到。针对该结构从头算得到的自发极化强度为143.5 μC cm⁻²，与在90 K下测量的磁滞回线的剩余极化强度一致。这些结果表明，我们的BFO薄膜的巨大极化可能是在具有巨大四方度的最佳四方相稳定时出现的。未来旨在常规获得这种自发极化强度值的实验工作应集中于如何在不损害BFO的绝缘和开关特性的情况下分离出该相。
