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

The structure of a 2-chain helical form of sulphur with 9 atoms per unit-ce ll has been determined from powder synchrotron x-ray diffraction data obtai ned at 3 GPa and 400 degreesC, using a combination of global optimization, simulated annealing and Rietveld refinement techniques. Final refinement of the structure in trigonal space group P3(2)21 (no. 154) results in a = 7.0 897(2) Angstrom, c = 4.30238(9) Angstrom and V = 187.282(10) Angstrom (3). 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 = 0.6105(12) and the S2 3b with x = 0. 8755(8) Angstrom. Bond length and angle analysis shows S1-S1 = 2.070(4) Ang strom and S2-S2 = 2.096(7) Angstrom with an helical S1-S1-S1 divalent angle of 102.7(2)degrees and 101.7(3)degrees for S2-S2-S2. The 2 helices formed, contrary to other polymeric sulphur phases (principally, psi -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 qu enchable, is observed to back transform to a 4.04 Angstrom phase at pressur es less than 0.5 GPa. It is likely that this phase represents the true form of the phase XII of Vezzoli and Walsh.