Syntheses, Crystal and Electronic Structures, and Characterizations of Quaternary Antiferromagnetic Sulfides: Ba2MFeS5 (M = Sb, Bi)

Two new quaternary sulfides, Ba2SbFeS5 and Ba2BiFeS5, were synthesized by using a conventional high-temperature solid-state reaction method in closed silica tubes at 1123 K. The two compounds both crystallize in the orthorhombic space group Pnma with a = 12.128(6) Å, b = 8.852(4) Å, c = 8.917(4) Å, and Z = 4 for Ba2SbFeS5 and a = 12.121(5) Å, b = 8.913(4) Å, c = 8.837(4) Å, and Z = 4 for Ba2BiFeS5. The crystal structure unit can be viewed as an infinite one-dimensional edge-shared MS5 (M = Sb, Bi) tetragonal-pyramid chain with FeS4 tetrahedra alternately arranged on two sides of the MS5 polyhedral chain via edge-sharing (so the chain can also be written as 1[MFeS5]4−). Interestingly, the compounds have the structural type of a Ba3FeS5 high-pressure phase considering one Ba2+ is replaced by one Sb3+/Bi3+, with Fe4+ reduced to Fe3+ for in order to maintain the electroneutrality of the system. As a result, the isolated iron ions in Ba3FeS5 are bridged by intermediate MS polyhedra in Ba2MFeS5 (M = Sb, Bi) compounds and form the 1[MFeS5]4− chain structure. This atom substitution of Ba2+ by one Sb3+/Bi3+ leads to a magnetic transition from paramagnetic Ba3FeS5 to antiferromagnetic Ba2MFeS5, resulting from an electron-exchange interaction of the iron ions between inter- or intrachains. Magnetic property measurements indicate that the two compounds are both antiferromagnetic materials with Néel temperatures of 13 and 35 K for Ba2SbFeS5 and Ba2BiFeS5, respectively. First-principles electronic structure calculations based on density functional theory show that the two compounds are both indirect-band semiconductors with band gaps of 0.93 and 1.22 eV for Ba2SbFeS5 and Ba2BiFeS5, respectively.