High‐pressure studies of polymerization in sulfur

Abstract

By x‐ray and optical studies of thermally quenched products and by differential thermal analysis under pressure the effect of high pressure on the melting and polymerization of sulfur has been investigated to 31 kb and 500°C. At least four different liquid fields have been identified. DTA experiments indicate that pressure shifts the 159°C polymerization transition first toward higher temperatures and then toward lower temperatures until it finally coincides with the melting point at a pressure of about 0.7 kb. The depolymerization temperature was found by the same technique to increase with increasing pressure up to 0.4 kb. A liquid P—T boundary that may constitute the higher‐pressure extension to the depolymerization transition has been traced up to 7.5 kb and 480°C. A very sharp, practically temperature‐independent reaction in the liquid state has been located at about 9 kb extending from the liquidus to at least the limits of the apparatus at about 450°C. Evidence has been found for a possible second‐order phase transformation extending from about 10 kb at 400°C to the liquidus at approximately 360°C.

Chain breakage in liquid sulfur at high pressures and high temperatures

“…However, the depolymerization boundary was determined at low pressures, and has not been explored in the pressure range we investigated. Further investigations in a wide pressure and temperature range are necessary to clarify the relationship between the depolymerization of liquid sulfur [12], and our observed high-temperature chain breakage.…”

“…The λ-transition temperature decreases with increasing pressure and intersects the melting curve at 0.13 GPa [20]. The depolymerization temperature of the λ transition increases with increasing pressure with a slope of ß475 K/GPa [12]. If we linearly extrapolate the depolymerization boundary to higher pressures (red dash-dot line in Fig.…”

“…The red dash-dot line is the depolymerization boundary of the λ transition replotted after Ref. [12] and extrapolated to high pressure. The melting curve is taken from Ref.…”

“…Sulfur, next to phosphorus in the periodic table, also displays LLPT [12,13]. At ambient pressure and high temperatures, a so-called λ transition in liquid sulfur, linked to the breakdown of molecular S 8 rings and polymerization into long chains, has been reported, accompanied by viscosity, density, and heat capacity anomalies at 432 K [14][15][16][17][18][19].…”

“…In the low-pressure range of 0-2 GPa, five liquid phases were reported by differential thermal analysis and quenched samples analysis [12]. At higher pressures of 4-12 GPa, Brazhkin et al [13] proposed two phase boundaries with negative slopes for liquid sulfur from thermobaric analysis and resistance measurements.…”