Materials Research Institute and Department of Materials Science and Engineering, Pennsylvania State University, MSC Building, University Park, Pennsylvania 16802, USA.
Department of Material Chemistry, Kyoto University, Nishikyo, Kyoto 615-8510, Japan.
Phys Rev Lett. 2014 May 9;112(18):187602. doi: 10.1103/PhysRevLett.112.187602. Epub 2014 May 7.
Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO4 (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P42(1)m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides.
氧八面体的旋转普遍存在，但它们无法打破简单钙钛矿中的反演对称性。然而，在层状氧化物结构中，这是可能的，正如我们在这里展示的 A 位有序的 Ruddlesden-Popper NaRTiO4（R 表示稀土金属），以前被认为是中心对称的。通过重新研究这一系列材料，我们使用同步辐射 X 射线衍射、光学二次谐波产生、压电力显微镜和第一性原理声子计算，发现低温相属于非中心空间群 P42(1)m，具有压电性和非极性。这种新型非中心层状氧化物大家族的机制很普遍，可能会导致更多的非中心氧化物家族的出现。
