From Ta2S5 Wires to Ta2O5 and Ta2O5–xSx

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 P 2/ 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.