From Ta₂S₅ Wires to Ta₂O₅ and Ta₂O_5- S

Synthesis routes to forming novel materials are oftentimes complicated and indirect. For example, Ta₂S₅ has only been found as an unwanted byproduct of certain chemical reactions, and its properties were unknown. However, here we demonstrate the growth of Ta₂S₅ wires with steel-like tensile strength, which are also precursors for the first controlled synthesis of long, mesoscopic Ta₂O₅ wires and superconducting Ta₂O_5- S wires. Single-crystal wires of tantalum pentasulfide, Ta₂S₅, were first grown using vapor transport from polycrystalline XTa₂S₅, sulfur, and TeCl₄ in fused-quartz tubes, where X = Ba or Sr. Crystals form as long wires with lengths on the order of a few centimeters and varying cross sections as small as 25 μm². They were found to have steel-like tensile strength, and their crystal structure was determined using X-ray diffraction to be monoclinic with space group P2/m and with lattice parameters a = 9.91(7) Å, b = 3.82(5) Å, and c = 20.92(2) Å. Electrical resistivity measurements reveal Ta₂S₅ to be a narrow band gap semiconductor with E _g = 110 meV, while a Debye temperature Θ_D = 97.0(5) K is observed in specific heat. Tantalum pentasulfide wires were then converted to insulating tantalum pentoxide (Ta₂O₅) wires after calcinating them for 30 min in air at 900 °C. Finally, tantalum pentoxide wires were converted to tantalum oxysulfide (Ta₂O_5- S ) wires after annealing them in CS₂ vapor for 30 min at 900 °C. The oxysulfide crystal structure was determined using X-ray diffraction to be that of β-Ta₂O₅. Electrical and magnetic measurements reveal Ta₂O_5- S to be metallic and superconducting with T _c = 3 K.