Superconductivity in the iron selenide K x Fe 2 Se 2 ( 0 ≤ x ≤ 1.0 )

We report the superconductivity at above 30 K in a FeSe-layer compound ${\text{K}}_{0.8}{\text{Fe}}_{2}{\text{Se}}_{2}$ (nominal composition) achieved by metal K intercalating in between FeSe layers. It is isostructural to ${\text{BaFe}}_{2}{\text{As}}_{2}$ and possesses the highest ${T}_{c}$ for FeSe-layer materials so far under ambient pressure. Hall effect indicates the carriers are dominated by electron in this superconductor. We confirm that the observed superconductivity at above 30 K is due to this FeSe-based 122 phase. Our results demonstrate that FeSe-layer materials are really remarkable superconductors via structure and carrier modulation.