Polar Transition-Metal Chalcogenide: Structure and Properties of the New Pseudo-Hollandite Ba_0.5Cr₅Se₈

Abstract

Single crystals of the new ternary selenide Ba0.5Cr5Se8 were synthesized using a self-flux method in fused silica tubes. This new phase was first considered to crystallize in the usual monoclinic symmetry for pseudo-hollandite compounds. However, single-crystal X-ray diffraction study showed unambiguously that Ba0.5Cr5Se8 crystallizes in the P1̅ triclinic space group with cell parameters a = 9.5084(4) Å, b = 7.1788(3) Å, and c = 8.9296(4) Å and α = 89.9979(16)°, β = 104.3958(22)°, γ = 100.8869(17)°, and Z = 2. Bulk samples were prepared by solid-state reaction and sintered using a spark-plasma sintering device. A combination of powder X-ray diffraction and transmission electron microscopy was used to perform structural analysis on spark plasma sintering prepared samples. Ba0.5Cr5Se8 orders antiferromagnetically with TN = 58 K and shows a semiconducting behavior with ρ300K = 0.35 Ω·cm and S300K = 230 μV·K–1. A maximum of the Seebeck coefficient of 315 μV·K–1 occurs at 635 K with ρ635K = 0.14 Ω·cm, while the thermal conductivity remains low and constant at about 0.8 W·m–1·K–1 from room temperature up to 873 K, leading to a maximum ZT of 0.12 around 800 K. A remarkably large increase of thermal conductivity is observed in the antiferromagnetic state.