Polythiazyl

The compound was first reported as early as 1910 by F.P. Burt, who obtained it by heating tetrasulfur tetranitride in vacuum over silver wool.^[6]

The compound was the first compound with only non-metallic elements in which superconductivity could be demonstrated.^[7][8]

Polythiazyl is a metallic-golden and shiny, crystalline but fibrous material with anisotropic electrical conductivity.^[8] It has at least two crystal polymorphs, which are usually intermixed after chemical synthesis. About 90% is the monoclinic form I, while the remainder is an orthorhombic form II.^[9]: 10 The orthorhombic portion can be increased mechanically through e.g. grinding,^[10] "although full conversion is unlikely."^[9]: 10

Along the fibres (SN chains), polythiazyl is electrically conductive; perpendicular, an insulator. The one-dimensional conductivity arises from bonding conditions in the S-N chain, where each sulfur atom provides two π electrons and each nitrogen atom provides one π electron to form two-center 3π electron bonding units.^[8] At temperatures below 0.3 K, it becomes a electrical superconductor.^[8] The Peierls distortion is inhibited by weak inter-chain interactions, which resist the characteristic bond alternation.^{[9]: 12–13}

Polythiazyl is "completely insoluble";^[9]: 13 and mostly inert to oxygen and water.^[11][12] But it decomposes rapidly in alkaline solutions,^[9]: 13 and slowly in air to a grey powder after months of exposure.^[11][12] Halogens intercalate between the strands.^{[9]: 15–16}

At temperatures above 240 °C explosive decomposition can occur.^[13] The compound also explodes on impact.^[12] Explosion generally proceeds via decomposition to the elements.

Vapor sublimed from polythiazyl is actually a cracked form of the polymer, the six-membered radical ring (SN)₃. Condensation reverts it back to the chain form.^[14]

The oldest-known polythiazyl synthesis is the polymerization of the cyclic formal dimer disulfur dinitride (S₂N₂), which is in turn synthesized from the formal tetramer tetrasulfur tetranitride (S₄N₄),^[2] in the presence of hot silver wool.^[2][1][20] The reaction begins when silver abstracts sulfur from S₄N₄ to produce a Ag₂S catalyst; the resulting gaseous S₂N₂ is then isolated through sublimation onto a cold surface:

S₄N₄ + 8 Ag → 4 Ag₂S + 2 N₂

S₄N₄ (low-pressure gas at 250-300 °C; Ag₂S catalyst) → 2 S₂N₂ (gas) → 2 S₂N₂ (stable solid at 77 K)

When warmed to room temperature, the additional heat induces impurity-catalyzed polymerization:^[21]

S₂N₂ (0 °C) → (SN)_x

An alternative is the azide reduction of thiazyl chloride trimer,^[22] itself made from thiazyl fluoride:^[23]

(NSF)₃ + Cl₂ → (SNCl)₃ + ClF

(SNCl)₃ + NaN₃ → NaCl + N₂ + (SN)_∞

To eschew explosive reagents entirely, iron filings in nitromethane reduce the thiazyl chloride trimer to [(SN)₅]⁺[FeCl₄]⁻,^[24] which a platinum cathode reduces to polythiazyl.^[25]

Due to its electrical conductivity, polythiazyl is used in LEDs, transistors, battery cathodes, and solar cells.^[20]