Superconducting Fe-Based Compounds (A1-xSrx)Fe2As2 with A=K and Cs with Transition Temperatures up to 37K

New high-T_c Fe-based superconducting compounds, AFe₂As₂ with A=K, Cs, K/Sr, and Cs/Sr, were synthesized. The T_c of KFe₂As₂ and CsFe₂As₂ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50% 60% Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent p-type analogs of the n-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (Fe₂As₂) layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental A layers provides new avenues to superconductivity at higher T_c.