Chlorine-doping effects on crystal structure and superconductivity in FeSe$_{1-x}$Cl$_{y}$

According to the assessed Fe-Se phase diagram,\footnote{``Binary Alloy Phase Diagrams'' 2$^{nd}$ edition, ASM International, Editor-in-Chief: Thaddeus B. Massalski, pp 1769-1770 (1992).} the FeSe compound could crystallize either in tetragonal Pb-O type structure with space group P4/nmm (called $\beta $ FeSe) or hexagonal Ni-As type structure with space group P6$_{3}$/mmc (called $\delta $ FeSe) depending on the materials preparation process. In general, different starting composition of FeSe$_{x}$ results in a mixture of superconducting phase with almost identical T$_{c}$ and different amounts of magnetic impurity, therefore, it was proposed that the tetragonal $\beta $-FeSe superconducting phase (T$_{c} \quad \sim $ 8 K) only exists in a very narrow Se-deficiency range.\footnote{Zhaofei Li, et. al., J. Phys. {\&} Chem. Solids, \textbf{71}, 495-498 (2010).} Our experimental data indicated that the single hexagonal $\delta $ phase sample could be obtained by carrying out the low-temperature (400 $^{\circ}$C) annealing for FeSe$_{x}$Cl$_{y}$ after reaction at 680 $^{\circ}$C. This result is contrary to what is observed in the FeSe system in which the tetragonal $\beta $ phase is the predominant stable one at the low annealing temperature 400 $^{\circ}$C. As compared with the $\beta $-FeSe superconductor reported in the literature, higher T$_{c}$ values ($>$ 8 K) and larger superconducting volume fraction could be achieved by suitable tuning and heat treatments in the FeSe$_{x}$Cl$_{y}$ system.