...
  • 主题
高级搜索  >
历史搜索 清空历史
首页> 外文期刊>Acta Crystallographica, Section B. Structural Science >Structure of incommensurate ammonium tetrafluoroberyllate studied by structure refinements and the maximum entropy method
【24h】

Structure of incommensurate ammonium tetrafluoroberyllate studied by structure refinements and the maximum entropy method

机译：通过结构改进和最大熵方法研究不等价四氟铍酸铵的结构

获取原文
获取原文并翻译 | 示例
           
页面导航
  • 摘要
  • 著录项
  • 相似文献
  • 相关主题

摘要

Incommensurately modulated ammonium tetrafluoroberyllate (AFB) occurs in a narrow temperature interval between the paraelectric room-temperature phase with space group Pnma (T-i = 178 K) and the ferroelectric low-temperature phase with space group Pna2(1) (T-c = 173 K). The structure is determined from accurate single-crystal X-ray diffraction data collected with synchrotron radiation at 175 K. The superspace group of the structure is Pnma(alpha00) 0ss with alpha = 0.4796 (4). Both structure refinements and the maximum entropy method lead to the same structure model, which involves only single harmonic modulations. The building units of the structure are BeF42- and NH4+ complex ions with approximately tetrahedral point symmetry. They are relatively rigid and the modulations consist mainly of translations of the tetrahedra and their rotations around a fixed axis. The modulation is related to changes in the network of the hydrogen bonds. The low-temperature superstructure can be described as a commensurately modulated structure with the same superspace symmetry. The first harmonic modulations of the low-temperature and incommensurate phases are related by a scale factor with a value of approximately two. In addition, the low-temperature phase exhibits a second harmonic modulation that is responsible for shifts along c and the ferroelectricity in this phase. The experimental data of the incommensurate phase do not contain any evidence for the presence of a second harmonic in the modulation functions. This suggests that the development of the second harmonic, i.e. the development of the spontaneous polarization, is responsible for the lock-in transition. [References: 25]
机译：在空间群Pnma（Ti = 178 K）的顺电室温相与空间群Pna2（1）（Tc = 173 K）的铁电低温相之间的狭窄温度区间内，出现了不等调制的四氟铍酸铵（AFB）。 。该结构由在175 K的同步加速器辐射下收集的准确的单晶X射线衍射数据确定。该结构的超空间组为Pnma（alpha00）0ss，alpha = 0.4796（4）。结构的改进和最大熵方法都导致了相同的结构模型，该模型仅涉及单谐波调制。该结构的构建单元是具有近似四面体点对称性的BeF42-和NH4 +复合离子。它们是相对刚性的，并且调制主要包括四面体的平移及其绕固定轴的旋转。调节与氢键网络中的变化有关。低温上部结构可以描述为具有相同超空间对称性的相应调制的结构。低温相和不相称相的一次谐波调制的比例因子约为2。另外，低温相表现出二次谐波调制，该二次谐波引起沿c的偏移以及该相中的铁电。不相称相位的实验数据没有任何证据表明调制函数中存在二次谐波。这表明二次谐波的发展，即自发极化的发展，是锁定过渡的原因。 [参考：25]

著录项

相似文献

  • 外文文献
  • 中文文献
  • 专利
获取原文

客服邮箱：kefu@zhangqiaokeyan.com

京公网安备：11010802029741号 ICP备案号：京ICP备15016152号-6 六维联合信息科技 (北京) 有限公司©版权所有

  • 客服微信

  • 服务号