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62 Ralf Steudel · Bodo Eckert Table 24 Critical points in the high-pressure high-temperature phase diagram of sulfur Phase boundary a-b-gas a b-liquid-gas a-b-liquid IV-VI-liquidb VI-VIII-liquidb VIII-XII-liquidc VIII-XI-XIIc g-L-L0d d-L0-L00 Pressure (Pa) 0.50 2.4 0.131·109 0.85(1) 109 0.9(1)·109 1.2(5) 109 1.8(2)·109 2.06·109 2.6·109 7.8(4) 109 12(5) 109 Temperature (K) 368.7 393 424 508(5) 528(2) 563(5) 617(2) 648 508 960(30) 1100(30) Reference [74] [197] [132, 194] [197] [132, 194] [132, 194] [132, 194] [196] a The a-phase and the b-phase are ften called SI and SII, respectively, which should be avoided since the use of Roman numbers is not consistent in the literature b Data derived from in-situ visual inspection c Phase XII can be assigned to (solid) polymeric sulfur on the basis of X-ray diffraction and Raman spectroscopic results. The critical points are results of temperature-pressure quenching experiments. Therefore, the data may only reflect the range of p-T conditions from which S XII has been obtained. The nature of the other forms (IV, VI, VIII, XI) is unknown d L and L0 denote non-metallic liquid phases. The phase L00 is believed to be a metallic phase of the melt. The symbols g and d denote solid phases; however, neither the struc- tures of these solids nor their relationships to known phases have been mentioned by the authors 3.3 High-Pressure Structures 3.3.1 General Generally, the group 16 elements of the periodic table show a tendency to- wards close-packing and increasing coordination number (CN) with in- creasing atomic number. The progression is: diatomic molecule (O2, CN=1)!insulating molecular solid (S8, CN=2)!semiconductor (Se and Te chain, CN=2+2)!metal (Po, CN=6). Under increasing high pressure these elements exhibit the same tendency [198]. From similarities of the melting curves of S, Se, and Te and from an analysis of the high-pressure structures of Se and Te it was to be expected that sulfur would undergo analogous phase transitions including metallization [195, 199–201]. A transition of sul- fur to conducting and superconducting states has been reported several times. The transition pressures were found in the range 17.5 to 50 GPa (by measurements of electrical resistivity and extrapolation from optical absorp- tion data), but at last it was stated that sulfur is “not very metallic” or “still a semiconductor” up to 50 GPa [202, 203]. More recent high-pressure X-rayPDF Image | Topics in Current Chemistry
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