S₈ring content in liquid sulfur determined from x-ray diffraction

The S₈ring content in liquid sulfur has been determined from the 4.47 Å peak in the x-ray pair distribution function over the temperature range -55 °C -330 °C. This peak represents the characteristic 3rd and 4th nearest atom-atom distances across an S₈-ring. Three procedures were used to calculate the S₈-ring fraction; scaling to crystallineβ-S, a linear combination fit and a direct integration method. All methods gave similar values, indicating that approximately one third of the S₈-rings break up upon melting into a distribution of short chains with an average length of 5 ± 2 atoms. Our results contradict established models of low temperature sulfur that indicate below theλ-transition the liquid comprises almost entirely of S₈-rings. In addition, we find the S₈-ring fraction increases to 76 ± 10% at -30 °C in the supercooled liquid, and decreases to 23% ± 3% at 330 °C in the high temperature melt. First principle molecular dynamics calculations confirm the onset of dynamic ring distortion via broad bond angle distributions, associated with the interstitial region between the first and second peaks in the pair distribution function. Our results are consistent with the presence of a propensity of short chains in the low temperature liquid, that are required for the bond switching mechanism responsible for the polymerization process to occur at theλ-transition temperature,T_λ.