β-Sulfur 1: introduction

See α-sulfur for the general properties of sulfur. β-sulfur is not found in nature but it can be made from α-sulfur. If sulfur is melted and cooled very slowly then just the more stable S₈ rings will form. If it is left at around 100° to crystallise then β-sulfur will form as it is the more thermodynamically stable polymorph above 95°. Once the β-sulfur has crystallised at 100° it must then be cooled rapidly to room temperature so it doesn't have time to revert to the more stable α-sulfur. Once it is at room temperature the β-sulfur can last for several weeks before it has transformed into α-sulfur. This is an example of metastability - where the substance is in a thermodynamically unstable state but doesn't change due to there being insufficient energy to provide the activation energy for change. This amounts to saying there is a kinetic barrier, making the reaction exceedingly slow. Technically diamond is carbon in a metastable state (see graphite).

There is another metastable form of sulfur called plastic sulfur. This is formed if molten sulfur is cooled straight down to room temperature very quickly. It is amorphous (ie there is no regular repeating structure) rather than crystalline since there isn't time in the cooling process for the atoms to find their most stable arrangement in space. Glasses are also examples of amorphous substances.

A fragment of the bulk structure of β-sulfur is shown to the left, with a black wireframe indicating the monoclinic unit cell. In this type of crystal system the three types of cell length are different and of the three types of bond angle only two are 90°. This gives unit cells the kind of symmetry adopted by a pack of playing cards that has been nudged slightly in a direction parallel to one of its edges.

Go to page 2 to focus on the S₈ ring.

Other elements/compounds with a monoclinic unit cell:

AlCl₃, SiCl₄, LaPO₄, FeSO₄.7H₂O, XeF₄, KClO₃, cyclohexane (C₆H₁₂), cumingtonite ((Mg,Fe)₇Si₈O₂₂(OH)₂), gypsum (CaSO₄.2H₂O), malachite (CuCO₃.Cu(OH)₂), borax (Na₂B₄O₇.10H₂O), cryolite (Na₃AlF₆), talc (Mg₃Si₄O₁₀(OH)₂), orpiment (As₂S₃), realgar (AsS), nahcolite (NaHCO₃), monazite (LnPO₄, Ln=various rare earths), azurite (2CuCO₃.Cu(OH)₂), crocoite (PbCrO₄), argentite (Ag₂S), gibbsite (Al(OH)₃), hessite (Ag₂Te), kieserite (MgSO₄.H₂O), manganite (MnO(OH)), trona (Na₂CO₃.NaCO₃.2H₂O), vivianite (Fe₃(PO₄)₂.8H₂O), whewellite (CaC₂O₄.H₂O) and xonotlite (Ca₆Si₆O₁₇(OH)₂).

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