Synthesis, crystal structure, and chemical stability of the superconductor FeSe1-x

We report on a comparative study of the crystal structure and the magnetic properties of FeSe1-x (x=0.0-0.15) superconducting samples by neutron powder-diffraction and magnetization measurements. The samples were synthesized by two different methods: a low-temperature one using powders as a starting material at T similar or equal to 700 degrees C and a high-temperature method using solid pieces of Fe and Se at T similar or equal to 1075 degrees C. The effect of a starting (nominal) stoichiometry on the phase purity of the obtained samples, the superconducting transition temperature T-c, as well as the chemical stability of FeSe1-x at ambient conditions were investigated. It was found that in the Fe-Se system, a stable phase exhibiting superconductivity at T-c similar or equal to 8 K exists in a narrow range of selenium concentration (FeSe0.974 +/- 0.005).