Modal disorder and phase transition in Rb0.91Nb0.96W1.04O5.98. Interpretation of X-ray diffuse scattering using the group theory approach
| Autor: | Andrzej Majchrowski, Dorota Komornicka, Marek Wołcyrz, Adam Pietraszko, Wieslawa Sikora |
|---|---|
| Rok vydání: | 2015 |
| Předmět: |
Phase transition
Condensed matter physics Chemistry Dielectric Condensed Matter Physics Symmetry (physics) Electronic Optical and Magnetic Materials Inorganic Chemistry Crystal Tetragonal crystal system Phase (matter) Materials Chemistry Ceramics and Composites Symmetry breaking Physical and Theoretical Chemistry Group theory |
| Zdroj: | Journal of Solid State Chemistry. 230:325-336 |
| ISSN: | 0022-4596 |
| DOI: | 10.1016/j.jssc.2015.07.021 |
| Popis: | A complex scheme of atomic displacements (modes) that break the Fd 3 ¯ m symmetry of the high-temperature (Tc>395 K) cubic phase of Rb0.91Nb0.96W1.04O5.98 and ultimately lead to a phase transition to the tetragonal phase was determined using the group theory approach. The resulting set of modes was used to construct a model of the disordered crystal that provides diffuse scattering (particularly characteristic extinctions) that is highly consistent with the experimental results. The resulting solution reveals a disordered structure of cubic Rb0.91Nb0.96W1.04O5.98, which is a system of intersecting {111}-type planes in which Nb/W atoms (statistically occupying the centers of oxygen octahedra) are shifted along three symmetry-equivalent 〈110〉 directions parallel to these planes. Oxygen atoms also move in a characteristic manner, but their shifts are considerably smaller and do not substantially affect the diffuse scattering pattern. The movements of Rb atoms are large but uncorrelated. The obtained picture of the local structure of cubic Rb0.91Nb0.96W1.04O5.98 makes it necessary to change the interpretation of existing physical measurements, particularly dielectric measurements. Furthermore, the determined structure of the low-temperature tetragonal phase that exists below 395 K was found to be non-polar (space group I 4 ¯ 2d). Group theory analysis provides a coherent picture of the phase transition from the disordered cubic phase to the ordered tetragonal phase. At Tc, in a multimodal crystal of the high-temperature phase, mode symmetry breaking occurs, and each of the four displacive modes is decomposed: only 1/4 of the atoms of every mode of k=(x,x,x) retain their 〈110〉-type in-plane displacements; the displacements of the remaining atoms undergo reorientation to fulfill the conditions imposed by the k=(0,0,0) mode. The former group of displacements defines the direction of the appearing tetragonal axis. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|
Full Text from ScienceDirect