Composite anionic lattices

The first new materials in this project were obtained using two different synthesis routes. First, by reacting alkaline-earth metal oxides with elemental transition metals and heavier chalcogens, i.e. sulfur, selenium, and tellurium, pure powders were achieved though solid-state reactions. Second, to synthesize single crystals, large enough for structural characterization, two different flux growth methods were used: A self-flux growth method and a flux growth in alkali-metal halide melts.

As all constituents are either moisture sensitive or might oxidize in air, all synthesis steps were performed under inert conditions, either in a glove box with controlled argon atmosphere or in evacuated, closed silica glass vessels.

Figure 1. A perspective view of the BaFe2S2O structure with extended Fe sublattice to display the ladders. The local Fe coordination is displayed to the right. — Figure 1. A perspective view of the BaFe₂S₂O structure with extended Fe sublattice to display the ladders. The local Fe coordination is displayed to the right.

Figure 1. A perspective view of the BaFe₂S₂O structure with extended Fe sublattice to display the ladders. The local Fe coordination is displayed to the right.

Pure powders and small single crystals of three novel compounds with the general formula AEFe₂Ch₂O (AE = Ba, Sr, Ch = S, Se) were synthesized and characterized.[1,2] These compounds contain ladders of magnetic ions (high-spin Fe²⁺ d⁶), a structural features prone for low dimensional magnetism (Figure 1). The anions in these compounds order in layers perpendicular to the orthorhombic b-axis. The iron ions are found in tetrahedra consisting of three Ch and one O, which is a rare situation for an Fe²⁺ ion. Remarkably strong low dimensional magnetic interactions are observed in all three isostructural compounds and intrinsic geometric frustration prevents the system to order long ranged above about 250 K.

Figure 2. (left) The V-S-O lattice in Ba3V2S4O3, emphasizing the charge disproportionation order. (right) The polar VSO3 tetrahedron. — Figure 2. (left) The V-S-O lattice in Ba₃V₂S₄O₃, emphasizing the charge disproportionation order. (right) The polar VSO₃ tetrahedron.

Figure 2. (left) The V-S-O lattice in Ba₃V₂S₄O₃, emphasizing the charge disproportionation order. (right) The polar VSO₃ tetrahedron.

The crystal structure of Ba₃V₂S₄O₃ was revisited and its physical properties were investigated for the first time.[3] From simple calculations, vanadium could be assumed +4 but our new spectroscopic data clearly show that this compound possess an intrinsic charge disproportionation, which is coupled to the anionic lattice. Thus, the compound can be written Ba₃[VS⁵⁺O₃] V³⁺S_6/2], emphasizing that the crystal structure contains individual VSO₃ tetrahedra and one dimensional columns of face sharing VS₆ trigonal antiprisms (Figure 2). Obviously, the softer S ion stabilizes the lower oxidations state V³⁺ and the harder O ion is found next to the V⁵⁺ ion. Hence, it was concluded that this compound has a clear charge order and that the macroscopic electric conductivity could be described with an activation behavior (semiconducting). This was in contrast to previously reported band structure calculations, where metallic behavior was suggested. Moreover, the spatially separated V³⁺ chains (S = 1) are candidates for Haldane like properties.

Figure 3. A perspective view of (left) the BaCoSO crystal structure and (right) the Co-S-O lattice.

Figure 3. A perspective view of (left) the BaCoSO crystal structure and (right) the Co-S-O lattice.

An anionic super-lattice was known in the sulfide oxide BaZnSO. It proved possible to replace the nonmagnetic Zn²⁺ by Co²⁺, thus obtaining a new magnetic system containing S-O anionic ordering.[4] The divalent transition metals are heteroleptically tetrahedrally coordinated by two S and two O ions. These tetrahedra are corner-sharing and form layers separated by the relatively large Ba²⁺ ion. Hence, the alternative formula is Ba CoS_2/2O_2/2]. The crystal structure reveals a further feature important for the magnetic coupling strengths. There are two super-exchange paths between the Co ions: either via an O or via a S. Moreover, all Co-O-Co bridges extent in the same crystallographic direction and orthogonal to those are the Co-S-Co paths. This means that the layer can exhibit anisotropy and quasi 1-D physical properties might occur. Magnetic measurements show a typical low dimensional magnetic behavior even up to 750 K and a long range spin order at T_N = 222 K into an antiferromagnetic state. The anisotropy of the magnetic interactions in the layer was verified by theoretical calculations, but most prominent was the geometrical frustration.

Figure 4. (left) The layered crystal structure of CsV2S2O and (right) the trans-octahedral VS4O2 coordination. — Figure 4. (left) The layered crystal structure of CsV₂S₂O and (right) the *trans*-octahedral VS₄O₂ coordination.

Figure 4. (left) The layered crystal structure of CsV₂S₂O and (right) the *trans*-octahedral VS₄O₂ coordination.

Up to now, most of the compounds found in this project are insulating. However, vanadium in the new sulfide oxide CsV₂S₂O has a mean formal valence +2.5 and exhibits bad-metal properties along with Pauli-paramagnetic behavior. [5] The metallic layers in this compound are separated by the highly polarizable Cs⁺ ion (Figure 4) and it will be interesting to modify by changing the charge carriers concentration in this new material.

The various compounds found so far in this project exhibit anionic ordering and novel coordination types for transition metal ions in different oxidation states. The chemistry of "bichalcogenides", i.e. oxygen together with one heavier chalcogen, is very promising for discovering new solid-state crystalline materials with challenging physical properties.

References:

[1] M. Valldor, P. Adler, Yu. Prots, U. Burkhardt, L. H. Tjeng, Eur. J. Inorg. Chem. 2014, 36, 6150-6155. http://dx.doi.org/10.1002/ejic.201402805

[2] S. Huh, Yu. Prots, P. Adler,L. H. Tjeng, M. Valldor, Eur. J. Inorg. Chem. 2015, 2015, 2982-2988. http://dx.doi.org/10.1002/ejic.201500385

[3] E. J. Hopkins, Yu. Prots, U. Burkhardt, Yves Watier, Z. Hu, C.-Y. Kuo, J.-C. Chiang, T.-W. Pi, A. Tanaka, L. H. Tjeng, M. Valldor, Chem. Eur. J. 2015, 21, 7938-7943. http://dx.doi.org/10.1002/chem.201406511

[4] M. Valldor, U. Rößler, Yu. Prots, C. Y. Kuo, J.-C. Chiang,Z. Hu,T.-W. Pi, R. Kniep, L. H. Tjeng, Chem. Eur. J. 2015, 21, 10821-10828. http://dx.doi.org/10.1002/chem.201501024

[5] M. Valldor, P. Merz, Yu. Prots, W. Schnelle, accepted for publication in Eur. J. Inorg. Chem. 2015.