Stress-induced R-MA-MC-T symmetry changes in BiFeO3 films
Stress-induced R-MA-MC-T symmetry changes in BiFeO3 films
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DOI：
10.1103/physrevb.83.144107
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发表时间：
2011-04-14
影响因子：
3.7
通讯作者：
Spaldin, N. A.
中科院分区：
文献类型：
--
作者：
Christen, H. M.;Nam, J. H.;Spaldin, N. A.
最近对外延BiFeO3薄膜的研究表明，当应变超过约-4.5%时，晶体结构从近菱形（“R形”）转变为近四边形（“T形”），T形结构的特征是c/a比大大增强。虽然R-like相和T-like相都是单斜相，但我们详细的x射线衍射结果显示，在这种R-like到T-like的转变中，M-A型和M-C型分别发生了对称变化。因此，在两相中，铁电极化被限制在不同的（伪）平面上。通过施加额外的应变或通过用Ba代替Bi来改变薄膜的单位胞体积，减少了t型MC相的单斜畸变，即系统接近真正的四方对称。因此，在从体膜到高应变膜的过程中，观察到一个从菱形(R)到单斜（R-like M-A）到单斜（T-like M-C）到四方(T)的相序列。在铅基固溶型钙钛矿中，这一顺序只能在亲形态相边界附近看到（即，在成分诱导的相不稳定性附近），在那里它可以由电场、温度或成分控制。我们现在的研究结果表明，这种演变可以发生在无铅的化学计量材料中，并且可以单独由应力诱导。
Recent reports on epitaxial BiFeO3 films show that the crystal structure changes from nearly rhombohedral ("R like") to nearly tetragonal ("T like") at strains exceeding approximate to-4.5%, with the T-like structure being characterized by a highly enhanced c/a ratio. While both the R-like and the T-like phases are monoclinic, our detailed x-ray diffraction results reveal a symmetry change from M-A and M-C type, respectively, at this R-like-to-T-like transition. Therefore, the ferroelectric polarization is confined to different (pseudocubic) planes in the two phases. By applying additional strain or by modifying the unit-cell volume of the film by substituting Ba for Bi, the monoclinic distortion in the T-like MC phase is reduced, i.e., the system approaches a true tetragonal symmetry. Therefore, in going from bulk to highly strained films, a phase sequence of rhombohedral (R)-to-monoclinic (R-like M-A)-to-monoclinic (T-like M-C)-to-tetragonal (T) is observed. This sequence is otherwise seen only near morphotropic phase boundaries in lead-based solid-solution perovskites (i.e., near a compositionally induced phase instability), where it can be controlled by electric field, temperature, or composition. Our results now show that this evolution can occur in a lead-free, stoichiometric material and can be induced by stress alone.