Alkali chalkogenido metalates(II/III)

• Applicant: Professorin Dr.-Ing. Caroline Röhr
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 313431712

Alkali metal (A) salts of mixed-valent chalcogenido(Q) ferrate(II/III) anions (A=Na, K, Rb Cs; Q=S, Se, Te) are a structurally variable class of compounds, which are furthermore models for much complexer systems like e.g. redox enzymes or iron-based superconductors.Their crystal chemistry, which has been recentyl extended by many mixed-valent Fe-rich salts (mainly via using reductive synthetic strategies) is based on edge-sharing [FeQ4] tetrahedra. Particularly diverse are the tetrahedra connections and the anions/crystal structures along the section A[FeQ2]-A2[FeQ2]: At one hand, chains of edge-sharing tetrahedra in several conformations, which depend on the Fe(II):Fe(III) ratio and the A cations, are found. On the other hand [exclusively in mixed-valent ferrates] 4Fe4S analog tetrahedra clusters [Fe4Q8] occur as characteristic building blocks. Commonly, the ferrates(II/III) are mixed-valent compounds of Robin&Day class III, with magnetic properties determined by a strong antiferromagnetic coupling of the HS-Fe(2/3)+- ions. However, the magnetic moments [similar to those in pure ferrates(II) and (III)] are considerably decreased with respect to the expected 'spin-only' values, especially in the condensed polyanions. Additionally, the dimer in the new sodium salt Na7[Fe2S6] is one of the very rare examples, for which charge fluctuations result in a ferromagnetic coupling between the two Fe ions.Based on these structurally and magnectically diverse mixed-valent ferrate(II/III) salts, the aim of this project is to synthesize new alkali chalcogenido metallates(II/III) with two different cations M2+ (Mn, Co, Zn) and M3+ (Al, In), to determine their crystal structures and to measure their magnetic properties. For the newly obtained (and related) compounds, the chemical bonding is calculated using spin-polarized FP-LAPW DFT band structure methods.The investigation of these salts containing heterobimetallic polyanions, for which the ratio M(II):M(III) can be ajusted much easier than for the ferrates, does not only allow to mimic the ferrates(II/III): First results on mixed manganates/indates of this structure family show, that the crystal chemistry along the section A[MQ2]-A2[MQ2] is considerably extended by acombined edge/corner and a pure edges-sharing of [MQ4] tetrahedra. This structure diversity as well as the variation of the metal cations themselves (e.g. number of d electrons, magnetic dilution, ionic radii, M-Q bond character) results in a wide range of the chemical bonding and the physical properties. The investigation of these basic alkali salts with low(er)-dimensional metallate(II/III) anions and the comparison with the ferrates and the pure metallates(II) and (III), allows to improve the general understanding of the bonding and the magnetism also in much complexer mixed-valent systems, e.g. like those mentioned above.

• DFG Programme: Research Grants