Stereochemically active lone-electron pairs as driving force for new mullite-type O8 structures

• Applicant: Privatdozent Dr. Mohammad Mangir Murshed
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 492161439

Since decades, the stereochemical activity of lone electron pair (LEP) has been a subject of deep insights. LEPs are known to play important roles in ferroelectricity, ionic/thermal conductivity, high-refractive index, non-linear optical property, and insulator/semiconductor-metal transitions. The LEP driven anharmonic potential in chemical bonding explains the large static dielectric constants, large Born-effective charge, large phononic bandgap and anharmonic thermal expansion. Recently we coined Wang-Liebau eccentricity (WLE) and Liebau density vector (LDV) to characterize the stereochemical activity of the LEP-element in a crystalline system. The absolute value of WLE measures the magnitude of the stereochemical activity of the LEP due to its experimentally determined environmental distortion. Herein we propose to optimize the stereochemical activity of the LEP-site mullite-type compounds, leading to crystallize new schafarzikite structures. The chemical composition of schafarzikite (FeSb2O4) can be written as ML2O4 where L refers to stereochemically active LEP cation. Using the WLE parameter, it could be shown that a mid-range value of WLE favors the formation of centrosymmetric schafarzikite structure. A comparison of WLE parameters of the L-cations between almost all known ML2O4 structures reveal that by changing the M- and/or the L-sites a rich chemistry of schafarzikites within the composition of M(III)L(II)L(III)O4 (M(III) = Cr, Mn, Fe; L(II) = Pb, Sn; L(III) = Sb, Bi) is possible. Due to two different L-cations with different WLE values, some compounds may crystallize in polar space groups with possible non-linear properties. Trials of syntheses followed by preliminary investigations showed clear evidence for the formation of M(III)L(II)L(III)O4 schafarzikites, indicating the success and the prospects of the project.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Thorsten Michael Gesing