Quaternary antiferromagnetic Ba₂BiFeS₅ with isolated FeS₄ tetrahedra

We report the detailed physical properties of quaternary compound Ba₂BiFeS₅ with the key structural ingredient of isolated FeS₄ tetrahedra. Magnetization and heat capacity measurements clearly indicate that Ba₂BiFeS₅ has a paramagnetic to antiferromagnetic transition at about 30 K. The calculated magnetic entropy above ordering temperature is much smaller than theoretical value for high-spin Fe³⁺ ion with S=5/2, implying the possible short-range antiferromagnetic fluctuation in Ba₂BiFeS₅.