at Google ScholarTitle data
Palatinus, Lukas ; Amami, Mongi ; van Smaalen, Sander:
Structure of incommensurate ammonium tetrafluoroberyllate studied by structure refinements and the maximum entropy method.
In: Acta Crystallographica Section B.
Vol. 60
(2004)
Issue 2
.
- pp. 127-137.
ISSN 2052-5206
DOI: https://doi.org/10.1107/S0108768104000874
Abstract in another language
Incommensurately modulated ammonium tetrafluoroberyllate (AFB) occurs in a narrow temperature interval between the paraelectric room-temperature phase with space group Pnma (Ti = 178 K) and the ferroelectric low-temperature phase with space group Pna2(1) (Tc = 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 BeF4(2-) 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.
Further data
| Item Type: | Article in a journal |
|---|---|
| Refereed: | Yes |
| Institutions of the University: | Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences > Chair Crystallography Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences > Chair Crystallography > Chair Crystallography - Univ.-Prof. Dr. Sander van Smaalen Faculties Faculties > Faculty of Mathematics, Physics und Computer Science Faculties > Faculty of Mathematics, Physics und Computer Science > Group Material Sciences |
| Result of work at the UBT: | Yes |
| DDC Subjects: | 500 Science > 530 Physics |
| Date Deposited: | 17 Mar 2016 10:22 |
| Last Modified: | 17 Mar 2016 10:22 |
| URI: | https://eref.uni-bayreuth.de/id/eprint/31884 |