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The structure of a 2-chain helical form of sulphur with 9 atoms per unit-cell has been determined from powder synchrotron x-ray diffraction data obtained at 3 GPa and 400ºC, using a combination of global optimization, simulated annealing and Rietveld refinement techniques. Final refinement of the structure in trigonal space group  P 3 2 21 (no. 154) results in  a  = 7.0897(2)Å,  c  = 4.30238(9)Å and  V  = 187.282(10)Å 3 . There are two unique S sites per unit cell, the general S1 6 c  position with  x  = 0.7727(9),  y  = 0.3067(11) and  z  =.6105(12) and the S2 3 b  with  x  = 0.8755(8) Å. Bond length and angle analysis shows S1-S1 = 2.070(4)Å and S2-S2 = 2.096(7)Å with an helical S1-S1-S1 divalent angle of 102.7(2)° and 101.7(3)° for S2-S2-S2. The 2 helices formed, contrary to other polymeric sulphur phases (principally,  Ψ -S [1]), are non-chiral and evidently have repeats along the (001) translation; that is one turn includes each of the 3 equivalent atoms about the 3 2 -screw axis (S1 helix) and vertex (S2) of the cell. This phase, although temperature quenchable, is observed to back transform to a 4.04Å phase at pressures less than 0.5 GPa. It is likely that this phase represents the true form of the phase XII of Vezzoli and Walsh.

In situ structure solution of helical sulphur at 3GPa and 400ºC