On the Phase Diagram of FeSe$_{1-x}$As$_{x}$

Structural, magnetic and transport measurements have been performed on FeSe$_{1-x}$As$_{x}$ samples in the normal and superconducting state in an effort to obtain the phase diagram. It is found that As-substitution drastically suppress the superconducting state while it has little effect of structural transformation as the temperature decreases. Moreover, low As-concentration ($\sim$ 1{\%}As) has been found very effective in stabilizing the $\beta $-FeSe phase.