The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at Tc = 38 K in (Ba1−xKx)Fe2As2 with x ≈ 0.4. The parent compound BaFe2As2 as well as KFe2As2 both crystallize in the tetragonal ThCr2Si2-type structure, which consists of (FeAs) δ− iron arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a spin density wave (SDW) anomaly at 140 K. By substituting Ba 2+ for K + ions we have introduced holes in the (FeAs) − layers, which suppress the SDW anomaly and induce superconductivity. This scenario is very similar to the recently discovered arsenide-oxide superconductors. The Tc of 38 K in (Ba0.6K0.4)Fe2As2 is the highest critical temperature in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr2Si2-type structure. Our results suggest, that superconductivity in these systems evolves essentially from the (FeAs) δ− layers and may occur in other related compounds.

The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (I4/mmm) to orthorhombic (F mmm). 57 Fe Mössbauer spectroscopy experiments show a single signal at room temperature and full hyperfine field splitting below the phase transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can serve as a parent compound for oxygen-free iron arsenide superconductors.

Doping improves performance: Iron arsenides (Ba1−xKx)Fe2As2 with the ThCr2Si2‐type structure exhibit superconductivity at 3–38 K depending on the potassium doping level. Superconductivity occurs before the structural distortion of the parent compound BaFe2As2 (x=0) is completely suppressed by doping (see phase diagram; • critical temperature, ○ phase‐transition temperature). Doping decreases the bond angles in the iron arsenide layers, suggesting a strong coupling of structural and electronic degrees of freedom.

The development of medium-bandgap solar cell absorber materials is of interest for the design of devices such as tandem solar cells and building-integrated photovoltaics. The recently developed perovskite solar cells can be suitable candidates for these applications. At present, wide bandgap alkylammonium lead bromide perovskite absorbers require a high-temperature sintered mesoporous TiO2 photoanode in order to function efficiently, which makes them unsuitable for some of the above applications. Here, we present for the first time highly efficient wide bandgap planar heterojunction solar cells based on the structurally related formamidinium lead bromide. We show that this material exhibits much longer diffusion lengths of the photoexcited species than its methylammonium counterpart. This results in planar heterojunction solar cells exhibiting power conversion efficiencies approaching 7%. Hence, formamidinium lead bromide is a strong candidate as a wide bandgap absorber in perovskite solar cells.

GaV 4 S 8 was prepared by direct heating of the elements at 1123 K. The crystal structures were refined from single-crystal data at room temperature (RT-GaV 4 S 8 , GaMo 4 S 8 -type, F4 h3m, a ) 9.661(1) Å, Z ) 4) and by the Rietveld method at 20 K (LT-GaV 4 S 8 , R3m, a ) 6.834(1) Å, R rh ) 59.66(2)°, Z ) 1). Magnetic measurements show weak paramagnetism with a temperature-dependent magnetic moment between 1.5µ B (50 K) and 1.9µ B (300 K) compatible with one unpaired electron per V 4 cluster. Ferromagnetic ordering was detected below T C ) 10 K. DC electrical conductivity measurements between 30 and 320 K reveal semiconducting behavior. GaV 4 S 8 undergoes a structural phase transition at 38 K from cubic to rhombohedral symmetry, as known for GaMo 4 S 8 , but with an opposite change of the rhombohedral angle R rh . This behavior is explained by self-consistent band structure calculations and simple cluster orbital schemes for GaV 4 S 8 in comparison with GaMo 4 S 8 . The results prove that the directions of the structural deformations are determined by orbital interactions and depend on the electron count in the tetrahedral metal cluster units.