Guo Yuan-Yuan, Gibbs Alexandra S, Perez-Mato J Manuel, Lightfoot Philip
School of Chemistry and EaStCHEM, University of St Andrews, St Andrews KY16 9ST, Scotland.
ISIS Facility, Rutherford Appleton Laboratory, Harwell Campus, Harwell OX11 0QX, UK.
IUCrJ. 2019 Apr 9;6(Pt 3):438-446. doi: 10.1107/S2052252519003804. eCollection 2019 May 1.
The high-temperature phase behaviour of the ferroelectric layered perovskite BiTiO has been re-examined by high-resolution powder neutron diffraction. Previous studies, both experimental and theoretical, had suggested conflicting structural models and phase transition sequences, exacerbated by the complex interplay of several competing structural instabilities. This study confirms that BiTiO undergoes two separate structural transitions from the aristotype tetragonal phase (space group 4) to the ambient-temperature ferroelectric phase (confirmed as monoclinic, 11). An unusual, and previously unconsidered, intermediate paraelectric phase is suggested to exist above with tetragonal symmetry, space group 4. This phase is peculiar in displaying a unique type of octahedral tilting, in which the triple perovskite blocks of the layered structure alternate between tilted and untilted. This is rationalized in terms of the bonding requirements of the Bi cations within the perovskite blocks.
通过高分辨率粉末中子衍射对铁电层状钙钛矿BiTiO的高温相行为进行了重新研究。先前的实验和理论研究提出了相互矛盾的结构模型和相变序列，几种相互竞争的结构不稳定性之间的复杂相互作用加剧了这种情况。本研究证实，BiTiO经历了从原型四方相（空间群4）到室温铁电相（确定为单斜，11）的两个独立结构转变。有人认为在具有四方对称性、空间群4的温度以上存在一个不寻常的、以前未被考虑的中间顺电相。这个相的特殊之处在于显示出一种独特的八面体倾斜类型，其中层状结构的三重钙钛矿块在倾斜和未倾斜之间交替。这可以根据钙钛矿块内Bi阳离子的键合要求来解释。
