Juergen Nuss

Homoatomic stella quadrangula [Tl8]6- in Cs18Tl8O6, interplay of spin-orbit coupling, and Jahn-Teller distortion.

Cs(18)Tl(8)O(6) was synthesized reacting the binary compounds CsTl and Cs(2)O. According to single crystal X-ray analysis, the title compound crystallizes as a novel structure type in the cubic space group I23 and is diamagnetic. The electronic structure of the extended solid and of excised Cs(6)Tl(8) clusters has been examined by relativistic density functional calculations including spin-orbit coupling. Cs(18)Tl(8)O(6) comprises a clusteranion [Tl(8)](6-) in the shape of a tetrahedral star. An isoelectronic cluster was found previously in Cs(8)Tl(8)O, however, with the shape of a parallelepiped. Both clusteranions can be derived from a homocubane unit by displacive distortions. It has been shown by quantum mechanical analyses that the closed-shell electronic structure of the parallelepiped is the result of a Jahn-Teller distortion, while in contrast the tetrahedral star in Cs(18)Tl(8)O(6) would still exhibit an open-shell degenerate HOMO within a scalar relativistic approximation. Only if spin-orbit coupling is considered, a closed-shell electronic system is obtained in accordance with the diamagnetic behavior of Cs(18)Tl(8)O(6).

Optical gap in herringbone and π-stacked crystals of [1]benzothieno[3,2- b ]benzothiophene and its brominated derivative

The optical gap of the organic semiconductor [1]benzothieno[3,2-b]benzothiophene and its 2,7-dibrominated analogue is measured in solution and in the crystalline state by means of UV-vis and emission spectroscopy. Bromination leads to a change in molecular packing from herringbone to π-stacked, resulting in a marked shift in the absorption and emission spectra which is found to be in accordance with TDDFT calculations.

Synthesis, crystal structure and magnetic properties of the new one-dimensional manganate Cs3Mn2O4.

Cs(3)Mn(2)O(4), a new member of the small family of ternary manganese (II/III) mixed-valent compounds, has been synthesized via the azide/nitrate route and studied using powder and single crystal X-ray diffraction, magnetic susceptibility measurements and density functional theory (DFT). Its crystal structure (P2(1)/c, Z = 8, a = 1276.33(1) pm, b = 1082.31(2) pm, c = 1280.29(2) pm, β = 118.390(2)°) is based on one-dimensional MnO(2)(1.5-) chains built up from edge-sharing MnO(4) tetrahedra. The title compound is the first example of an intrinsically doped transition metalate of the series A(x)MnO(2), (A = alkali metal) where a complete 1:1 charge ordering of Mn(2+) and Mn(3+) is observed along the chains (-Mn(2+)-Mn(3+)-Mn(2+)-Mn(3+)-). From the magnetic point of view it basically consists of ferrimagnetic MnO(2) chains, where the Mn(2+) and Mn(3+) ions are strongly antiferromagnetically coupled up to high temperatures. Very interestingly, their long-range three-dimensional ordering below the Néel temperature (T(N)) ~12 K give rise to conspicuous field dependent magnetic ordering phenomena, for which we propose a consistent picture based on the change from antiferromagnetic to ferromagnetic coupling between the chains. Electronic structure calculations confirm the antiferromagnetic ordering as the ground state for Cs(3)Mn(2)O(4) and ferrimagnetic ordering as its nearly degenerate state.

CsCoO(2) featuring a novel polyoxocobaltate(III) anion based on a two-dimensional architecture of interconnected tetrahedra.

CsCoO2 has been prepared along the azide/nitrate route as a reddish black microcrystalline powder at 833 K. According to single crystal X-ray analysis, the title compound crystallizes as a structure type novel for oxides (Cmca,Z = 8, a = 5.9841(1) Å, b = 12.2458(2) Å, c = 8.2650(1) Å). The prominent features of the structure are pairs of edge-linked CoO4 tetrahedra to form Co2O6 dimers, which are condensed by vertex sharing of the four remaining unshared oxygen atoms to form a two-dimensional architecture of a puckered polyoxyanion spreading along (010). Upon cooling, CsCoO2 undergoes a virtually second order phase transition at ∼100 K leading to a systematic dovetail twin (C2/c). The magnetic susceptibilities show the dominance of antiferromagnetic interactions with a remarkably high Néel temperature of 430 K indicating a very strong antiferromagnetic superexchange between the Co(3+) ions. The spin-exchange for CsCoO2 can be addressed as a set of strongly antiferromagnetically coupled quasi-one-dimensional chains, which are weakly coupled to neighboring chains by ferromagnetic interaction.

On the possibility of magnetic Weyl fermions in non-symmorphic compound PtFeSb

Abstract Weyl fermions are expected to exhibit exotic physical properties such as the chiral anomaly, large negative magnetoresistance or Fermi arcs. Recently a new platform to realize these fermions has been introduced based on the appearance of a three-fold band crossing at high symmetry points of certain space groups. These band crossings are composed of two linearly dispersed bands that are topologically protected by a Chern number, and a flat band with no topological charge. In this paper, we present a new way of inducing two kinds of Weyl fermions, based on two- and three-fold band crossings, in the non-symmorphic magnetic material PtFeSb. By means of density functional theory calculations and group theory analysis, we show that magnetic order can split a six-fold degeneracy enforced by non-symmoprhic symmetry to create three- or two-fold degenerate Weyl nodes. We also report on the synthesis of a related phase potentially containing two-fold degenerate magnetic Weyl points and extend our group theory analysis to that phase. This is the first study showing that magnetic ordering has the potential to generate new three-fold degenerate Weyl nodes, advancing the understanding of magnetic interactions in topological materials.

Sr5Os3O13: a mixed valence osmium(V,VI) layered perovskite variant exhibiting temperature dependent charge distribution

New Sr5Os3O13, as synthesized from binary constituents, exhibits several uncommon features. Its crystal structure is dominated by quasi-2D poly-oxoanions that correspond to unprecedented cutouts of the perovskite type of structure, where corner sharing (OsO6) octahedra aggregate to form terraced slabs. The Os5+/Os6+ mixed valence oxide displays a particular charge ordering scheme. One osmium atom (Os1) per formula unit is in the valence state of 5+ in the whole temperature range studied, while the two remaining sites (Os2A and Os2B) show full charge disorder at high temperatures, resulting in an average charge of 5.5+. The latter, however, apparently undergo a process of continuous charge ordering at cooling. Full charge order appears to be established concomitantly with a phase transition to an antiferromagnetically ordered state at T(Néel) = 170 K. This kind of temperature dependent continuous charge ordering is reflected by structural changes with temperature as well as by changes in paramagnetic response above T(Néel). Disentangling the intimate interplay between magnetic and charge ordering degrees of freedom will require applying sophisticated spectroscopy and (neutron) diffraction techniques.

Novel Polyanionic Topologies in Ag2VAsO6 and Ag6V2As2O13

Two new silver vanadate arsenates, Ag2VAsO6 and Ag6V2As2O13, have been prepared applying high oxygen pressure syntheses in stainless-steel autoclaves. Ag2VAsO6 crystallizes in space group P1 with unit cell parameters a=639.1(1), b=646.1(1), c=706.6(1) pm, α =116.105(3), ß =91.759(4), γ = 90:067(4)°, and Z =2 (R1 =0.058, 3935 independent reflections). The structure consists of AsO4 tetrahedra and VO6 octahedra which are linked to form two-dimensional 2¥[VAsO6]2- polyanions, separated by silver cations. Ag6V2As2O13 (C2=c, a=1895.9(2), b=536.40(6), c=1308:5(2) pm, ß =113:578(2)°, Z =4; R1 =0.030, 2571 independent reflections) displays as primary building units AsO4 tetrahedra and VO5 trigonal bipyramids which are condensed by sharing edges to one-dimensional 1¥[V2As2O13]6- ladder-like strands, set apart by the silver cations. These heteropolyanions are without precedent Graphical Abstract Novel Polyanionic Topologies in Ag2VAsO6 and Ag6V2As2O13