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

Abstract

Abstract 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 P3221 (no. 154) results in a = 7.0897(2)Å, c = 4.30238(9)Å and V = 187.282(10)Å3. There are two unique S sites per unit cell, the general S1 6c position with x = 0.7727(9), y = 0.3067(11) and z =.6105(12) and the S2 3b 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 32-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.