Bromine-doping dependence of crystal structure and superconductivity in FeSe1−xBrx

Our experimental data indicate that the crystal structure formation in FeSe1−xBrx is dependent on the amount of Br. For the samples with x between 0.18 and 0.3, the single tetragonal β-FeSe phase with space group P4/nmm could be obtained by carrying out the low-temperature (400 °C) annealing after reaction at 680 °C. As to the sample with x=0.1, a heat firing at 680 °C is in favor of forming single hexagonal δ-FeSe phase with space group P63/mmc. The compound FeSe0.9Br0.1 with single δ hexagonal NiAs-type phase shows neither magnetic order nor superconductivity above 1.8 K according to dc magnetization measurements while the bulk character of the superconducting state with a Tc around 5 K for the single β tetragonal PbO-type phase in FeSe1−xBrx is confirmed. Larger superconducting volume fraction appears in the single-phase sample with x near 0.2.