Exothermic Supercooled Liquid—Liquid Transition in Amorphous Sulfur

Abstract

Amorphous sulfur (a-S) is prepared by rapidly compressing molten sulfur to high pressure. From differential scanning calorimeter measurements, a large exothermic peak has been observed around 396 K. Online wide-angled x-ray scattering spectra indicate that no crystallization occurs in the temperature range 295-453 K, suggesting that the exothermal process corresponds to an amorphous-to-amorphous transition. The transition from amorphous sulfur to liquid sulfur is further verified by the direct observation of sulfur melt at the temperature of the associated transition. This is the first time of reporting that a-S transforms to liquid sulfur directly, which has avoided a crystallization process. What is more, the transition is an exothermic and a volume expansion process.

Pressure and Time Dependences of the Supercooled Liquid-to-Liquid Transition in Sulfur

“…The XRD results from the prepared sample of sulfur indicate a fully amorphous structure, which is consistent with our earlier reports. [9] The DSC results at ambient pressure obtained at 10 K/min are also very similar to those reported in Refs. [7,9], which include broad exothermic and endothermic peaks.…”

“…[9] The DSC results at ambient pressure obtained at 10 K/min are also very similar to those reported in Refs. [7,9], which include broad exothermic and endothermic peaks. [7,9] It has been made clear that the former peak corresponds to a direct transition from supercooled liquid to liquid, [9] and the latter is the known liquid-liquid transition of sulfur (𝜆 transition).…”

“…The transition is a distinct exothermic process with a volume expansion. [9] This novel melting phenomenon is different from the glass transition or crystal melting behavior, and also varies from the direct transition to liquid of known oxides and organic glass. [5] Through Raman spectral analysis we have attributed the exothermic energy to a possible chain-ring transition of molecules in a-S when it was heated to the melting point.…”

“…[7,9], which include broad exothermic and endothermic peaks. [7,9] It has been made clear that the former peak corresponds to a direct transition from supercooled liquid to liquid, [9] and the latter is the known liquid-liquid transition of sulfur (𝜆 transition). [12,13] It is also confirmed that the supercooled-liquid-to-liquid transition of a-S is exothermic and expands the liquid volume.…”

Chain-ring transition during exothermic "melting" in amorphous sulfur

“…化 [2] . 非晶硫 的放热熔化与 无机化学 家 Tammann 理论预言的"放热熔化"不 同, 后者提出在一个普适的压力温度 相图中包含一个闭合的相平衡线, 预 言会发生一定压力下加热液体发生晶 化, 反之冷却导致晶体熔化, 即吸热 结晶、放热熔化, 该"放热熔化"是发生 在降温过程中 [3] .…”