Synthesis, crystal structure and chemical stability of the superconductor FeSe_{1-x}

We report on a comparative study of the crystal structure and the magnetic properties of FeSe1-x (x= 0.00 - 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 =700 C and a "high-temperature' method using solid pieces of Fe and Se at T= 1070 C. The effect of a starting (nominal) stoichiometry on the phase purity of the obtained samples, the superconducting transition temperature Tc, as well as the chemical instability of FeSe1-x at ambient conditions were investigated. It was found that in the Fe-Se system a stable phase exhibiting superconductivity at Tc~8K exists in a narrow range of selenium concentration (FeSe0.974(2)).