Bromine-doping dependence of crystal structure and superconductivity in FeSe$_{1-x}$Br$_{x}$

Our experimental data indicate that the crystal structure formation in FeSe$_{1-x}$Br$_{x}$ is dependent on the amount of Br. For the samples with $x$ between 0.18 and 0.3, the single tetragonal $\beta $-FeSe phase with space group P4/nmm could be obtained by carrying out the low-temperature (400 $^{\mathrm{o}}$C) annealing after reaction at 680 $^{\mathrm{o}}$C. As to the sample with $x=$ 0.1, a heat firing at 680 $^{\mathrm{o}}$C is in favor of forming single hexagonal $\delta $-FeSe phase with space group P6$_{3}$/mmc. Powder x-ray diffraction and crystallographic data provide the suggestion that the $\beta $ tetragonal PbO-type phase is related to the superconducting state while the $\delta $ hexagonal NiAs-type phase has no effect on the superconductivity. Magnetization data confirm the bulk character of the superconducting state with a T$_{c}$ around 5 K. Larger superconducting volume fraction appears in the single-phase sample with $x$ near 0.2.