The ternary iron arsenide ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at $T_c=38\mathrm{K}$ in $({\mathrm{Ba}}_{1-x}{\mathrm{K}}_x){\mathrm{Fe}}_2{\mathrm{As}}_2$ with $x\approx 0.4$. The parent compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ crystallizes in the tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure, which consists of $(\mathrm{FeAs}{)}^{\delta -}$ iron arsenide layers separated by ${\mathrm{Ba}}^{2+}$ ions. ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting ${\mathrm{Ba}}^{2+}$ for ${\mathrm{K}}^{+}$ ions we have introduced holes in the $(\mathrm{FeAs}{)}^{-}$ layers, which suppress the anomaly and induce superconductivity. The $T_c$ of 38 K in $({\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}){\mathrm{Fe}}_2{\mathrm{As}}_2$ is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure.

The ternary iron arsenide ${\text{BaFe}}_2{\text{As}}_2$, with the tetragonal ${\text{ThCr}}_2{\text{Si}}_2$-type structure, exhibits a spin-density-wave (SDW) anomaly at 140 K, very similar to LaFeAsO, which is the parent compound of the iron arsenide superconductors. ${\text{BaFe}}_2{\text{As}}_2$ 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 $\left(I4/mmm\right)$ to orthorhombic $\left(Fmmm\right)$. ${}^{57}\text{F}\text{e}$ 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 ${\text{BaFe}}_2{\text{As}}_2$ can serve as a parent compound for oxygen-free iron arsenide superconductors.

Innovative materials for phosphor converted white light-emitting diodes are in high demand owing to the huge potential of the light-emitting diode technology to reduce energy consumption worldwide. As the primary blue diode is already highly optimized, the conversion phosphors are of crucial importance for any further improvements. We report on the discovery of the high performance red phosphor Sr[Li₂Al₂O₂N₂]:Eu²⁺meeting all requirements for a phosphor’s optical properties. It combines the optimal spectral position for a red phosphor, as defined in the 2016 Research & Development-plan of the United States government, with an exceptionally small spectral full width at half maximum and excellent thermal stability. A white mid-power phosphor-converted light-emitting diode prototype utilising Sr[Li₂Al₂O₂N₂]:Eu²⁺shows an increase of 16% in luminous efficacy compared to currently available commercial high colour-rendering phosphor-converted light-emitting diodes, while retaining excellent high colour rendition. This phosphor enables a big leap in energy efficiency of white emitting phosphor-converted light-emitting-diodes.

It is widely believed that, in contrast to its electron-doped counterparts, the hole-doped compound ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ exhibits a mesoscopic phase separation of magnetism and superconductivity in the underdoped region of the phase diagram. Here, we report a combined high-resolution x-ray powder diffraction and volume-sensitive muon spin rotation study of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ showing that this paradigm does not hold true in the underdoped region of the phase diagram ($0\le x\le 0.25$). Instead we find a microscopic coexistence of the two forms of order. A competition of magnetism and superconductivity is evident from a significant reduction of the magnetic moment and a concomitant decrease of the magnetoelastically coupled orthorhombic lattice distortion below the superconducting phase transition.