From Ta(2)S(5) Wires to Ta(2)O(5) and Ta(2)O(5–x)S(x)

[Image: see text] Synthesis routes to forming novel materials are oftentimes complicated and indirect. For example, Ta(2)S(5) has only been found as an unwanted byproduct of certain chemical reactions, and its properties were unknown. However, here we demonstrate the growth of Ta(2)S(5) wires with steel-like tensile strength, which are also precursors for the first controlled synthesis of long, mesoscopic Ta(2)O(5) wires and superconducting Ta(2)O(5–x)S(x) wires. Single-crystal wires of tantalum pentasulfide, Ta(2)S(5), were first grown using vapor transport from polycrystalline XTa(2)S(5), sulfur, and TeCl(4) 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 μm(2). 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 Ta(2)S(5) 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 (Ta(2)O(5)) wires after calcinating them for 30 min in air at 900 °C. Finally, tantalum pentoxide wires were converted to tantalum oxysulfide (Ta(2)O(5–x)S(x)) wires after annealing them in CS(2) vapor for 30 min at 900 °C. The oxysulfide crystal structure was determined using X-ray diffraction to be that of β-Ta(2)O(5). Electrical and magnetic measurements reveal Ta(2)O(5–x)S(x) to be metallic and superconducting with T(c) = 3 K.