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72 Ralf Steudel · Bodo Eckert above the bcc structure was found to be energetically unfavorable in the case of sulfur. Accordingly, the differences between the structural sequences of sulfur and selenium were interpreted in terms of differences in the electronic core states, especially arising from the presence of a strongly attractive d-pseudopotential in sulfur due to the lack of d-states. On the other hand, the d-pseudopotential in selenium, which has d-states in the core region, is relatively weak. Therefore, in the energy calculations the electron-ion contri- bution was found to stabilize the sc structure for sulfur in comparison with the bcc structure. A drop of the temperature of the superconducting transition in sc sulfur down to 7 K (at 280 GPa) and 1 K (at 550 GPa) was predicted on the basis of changes in the phonon density of states, that is a distinct stiffening of the sc lattice in comparison with the b-Po and the bcc structures [192]. 3.4 Conclusion In conclusion, orthorhombic sulfur a-S8 seems to be stable up to at least 20–30 GPa. However, in this pressure range several additional phases (a-S, p-S, S6) were observed which are generated by photo-induced ring-opening and re-crystallization processes due to the application of laser radiation. At high-pressure high-temperature conditions (3–8 GPa, >600 K) a two-chain structure with a trigonal unit cell was found. In the pressure range of about 20–35 GPa sulfur exists in an amorphous state and, upon increasing pres- sure, transforms to an unknown (trigonal?) phase existing up to about 80– 90 GPa. Around 90 GPa, sulfur undergoes a transition to a bco phase and at about 160 GPa to the b-Po phase both structures being metallic with super- conducting properties below 20 K. A transition to the more close-packed bcc structure was estimated to oc- cur at pressures ranging from 500 to 700 GPa. However, theoretical calcula- tions suggest the existence of a sc phase stable between 280 and 500 GPa pri- or to the bcc structure. Accordingly, the structural phase sequence of sulfur is different from those of selenium and tellurium in following the order bco!b-Po!sc!bcc at pressures above ~80 GPa. Acknowledgements One of the authors (B.E.) appreciates the support of parts of the work (Photo-induced Amorphization of Sulfur at High Pressures and High-Pressure Low-Tem- perature Phase Diagram of Sulfur) by NATO research grants and by the European Commu- nity (LENS facility). This work has also been supported by the Deutsche Forschungsge- meinschaft and the Verband der Chemischen Industrie. References 1. Gmelin Handbuch der Anorganischen Chemie, 8. Aufl., Schwefel, Teil A, Lieferung 2, Springer, Berlin, 1974, and Lieferung 3, VCH, Weinheim, 1953. 2. M. Schmidt, Angew. Chem. 1973, 85, 474; Angew. Chem. Int. Ed. Engl. 1973, 12, 445. 3. B. Meyer, Chem. Rev. 1976, 76, 367. 4. R. Steudel, Top. Curr. Chem. 1982, 102, 149.PDF Image | Topics in Current Chemistry
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