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The phase transitions of the strongly correlated tetrahedral V 4 -cluster compound GeV 4 S 8 have been studied by low-temperature powder x-ray diffraction, magnetic susceptibility, and specific heat measurements. The crystal structure is cubic at room temperature ͑space group F43m͒ and transforms to orthorhombic ͑space group Imm2͒ at T S = 30 K. A Jahn-Teller distortion reduces the symmetry of the V 4 -cluster from 43m to mm2. The second transition at 18 K is the onset of antiferromagnetic ordering without symmetry change but with a certain increase in the distortion. The latter reflects a strong magnetoelastic coupling at T N . Specific heat anomalies at 30 and 18 K confirm the two phase transitions.

Superconductivity in doped BaFe2As2 is controlled by the charge of the (FeAs)δ− layers. Adjustment from electron to hole doping in Ba1−xKxFe1.86Co0.14As2 tailors the system from superconductivity to static magnetic order and back to superconductivity. When the charges compensate each other, the magnetic phase similar to BaFe2As2 is recovered. Structural parameters play minor roles in the superconductivity but are important for the highest possible critical temperatures.

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We present a detailed investigation of the electronic phase diagram of effectively charge compensated Ba 1−x K x (Fe 1−y Co y ) 2 As 2 with x/2 ≈ y. Our experimental study by means of x-ray diffraction, Mössbauer spectroscopy, muon spin relaxation and ac-susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations. For low substitution levels of x/2 ≈ y 0.13, the system displays an orthorhombically distorted and antiferromagnetically ordered ground state. The low-temperature structural and magnetic order parameters are successively reduced with increasing substitution level. We observe a linear relationship between the structural and the magnetic order parameter as a function of temperature and substitution level for x/2 ≈ y 0.13. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum T c of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels x/2 ≈ y 0.25, a tetragonal nonmagnetic ground state is observed. Our DFT calculations yield a significant reduction of the Fe 3d density of states at the Fermi energy and a strong suppression of the ordered magnetic moment in excellent agreement with experimental results. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density.

In situ spatially resolved neutron diffraction was used to investigate the processes occurring in the cathode of a sodium metal halide cell with mixed Fe/Ni chemistry during cell operation. The in situ diffraction data makes it possible to follow NaCl consumption, Na 6 FeCl 8 formation and consumption and Ni 1-x Fe x Cl 2 formation during charge as well as the reverse processes during discharge. Data collected at three different positions at different depths within the cathode permit mapping of the reaction progress in time and space. Reactions start near the β -alumina and proceed towards the cathode interior. Instead of one single reaction front moving through the cell during charge and discharge, separate reaction zones are found for Fe and Ni oxidation as well as for Na 6 FeCl 8 and NaCl usage as Cl − and Na + source during Ni oxidation. Thus there is one reaction zone per reaction, during charge as well as discharge. The broadness of the reaction zones varies with time, depth and the respective reaction. Our data also yield information about processes like the formation of Ni 1-x Fe x Cl 2 , a possible slight overcharge close to the β -alumina and finally allow to sketch a simplified mechanism of the processes that occur in the cell during charging.Sodium metal halide batteries, first invented in the 80's 1 are used today both for stationary and mobile applications. 2 Advantages of such molten salt Na/MCl 2 (M = Ni, Fe) batteries are a high specific energy density of ∼140 Wh/kg-four times higher than lead acid batteriesin combination with a high cyclability (more than 3000 cycles at 80% depth of discharge (DoD)) without need of maintenance. These features make sodium metal halide batteries well suited for stationary telecom applications, for example in areas with poor grid connection and frequent power outages, where also the fact that the batteries can be operated under elevated or extreme temperature conditions proves to be useful. Additionally, sodium metal halide cells can be charged and discharged rapidly and are safe in operation, which makes them especially suitable for mobile applications such as cars, vans and busses. Current applications also comprise use as energy backup system for railed vehicles. Finally, sodium metal halide batteries have the benefit of being 100% recyclable: The metal is used for alloys while salt and ceramic go into road beds. Figure 1 shows a simplified scheme of sodium metal halide cell operation. In the discharged state, the cathode of a sodium metal halide cell consists of a porous Ni or Fe metal electrode, sodium chloride and NaAlCl 4 , which is liquid above 154 • C and serves as electrolyte, ensuring good ion conductivity in the cathode. The cathode is separated from the anode compartment by a tube of ceramic β -alumina (BASE), that offers a high sodium conductivity at the optimal cell operation temperature of 270-350 • C. During charging, NaCl reacts with the metal (M = Ni or Fe) to form MCl 2 and Na + ions. The sodium ions travel through the β -alumina separator and a...