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Projekt Druckansicht

Struktur-Eigenschaftsbeziehungen Mullit-ähnlicher Verbindungen mit einsamen Elektronenpaaren

Fachliche Zuordnung Mineralogie, Petrologie und Geochemie
Förderung Förderung von 2010 bis 2018
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 183464377
 
Erstellungsjahr 2017

Zusammenfassung der Projektergebnisse

Within the scope of project the stereochemical activity of lone electron pair (LEP) of some mullite-type compounds have been studied. On doing so, the Wang-Liebau eccentricity hypothesis has been exploited and Liebau Density Vector (LDV) has been coined. The crystal chemical features of LEP containing mullite-types PbMBO4, Bi2M4O9 (M = Al3+, Mn3+, Fe3+ and Ga3+) and Bi2Mn4O10 (and their possible solid solutions such as PbM11-xM2xBO4, (Bi1-xSrx)M4O9, Bi2(M11-xM2x)4O9 and Bi2(M11-xM2x)4O10) have been thoroughly investigated using X-ray, neutron and photon elastic and inelastic scatterings along with SEM/EDX, HTREM, infrared, UV-vis, Mössbauer spectroscopes and density functional theory calculation. Although the crystal chemistry is well established of the mullite-type PbMBO4 (M = Al, Ga, Fe, Mn, Cr) compounds, an enormous works is assumed to be left to further investigate their physical properties in particular with external stimuli such as pressure. For the crystal chemical simplicity PbMBO 4 could serve as model compounds to study soft phonon, axial negative thermal expansion, pressure-induced phase transition, magnetic frustration/glass (e.g., PbFe0.5Mn0.5BO4), unusual and still unclear Hopkinson effect in the antiferromagnetic PbFeBO4, non-linear optical activity of the centrosymmetric system and so on. Beside many potential applications of ferroelectrics there exist only very few multiferroic materials due to the symbiotic condition of ferroelectricity and ferromagnetism, which is rarely met. The partially filled 3d orbit can lead to ferromagnetic order, whereas the spontaneous electric polarization requires empty 3d as a necessary condition for non-centrosymmetry. However, the condition can be resolved by incorporating one element with stereochemically active LEP, for instance in the A-site 3+ of the ABO3 perovskite. For instance in BiFeO3, the ferroelectric displacement arises from the Bi LEP, 3+ while the 3d orbital of Fe lead to magnetic ordering. However, the multiferroic behavior of the mullitetype Bi2Fe4O9 and Bi2Mn4O10 compounds cannot be explained from the similar ground. Moreover, the ferroelectric behaviors of these centrosymmetric compounds are still in dispute. Most of the studied mullite-type compounds show anisotropic thermal expansion even with unconventional elastic constants. This anisotropic expansion behavior is a clear departure of the isotropic elastic concept of Debye. In such cases, the axial modeling of thermal expansion, leading to volume expansion may offer better results. Although taking the anharmonicity parameter in the model the Mie-Grüneisen approximation breaks down a realistic system must take it into account. The applied DEA model excludes electronic, magneto-volume and other contributions (ionic diffusion, defects, Ostwald ripening). Theory seeks test and it’s misfit from the experimental observation essentially requires modifications. As such the DEA model was tested in some mullite-type compounds. Within the vast arena of solid state chemistry this contribution particularly helps to further develop the missing points undoubtedly require to understand the microscopic role of the LEP and its combined role to understand superconductivity.

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