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Solid Sulfur Allotropes 71 Table 26 Lattice parameters of sulfur at room temperature and pressures above 20 GPa ob- tained by X-ray diffraction studies [200, 201, 217, 218] Structure Pressure (GPa) CNb Monoclinicc 5.3 Trigonal 38–40 Base-centered 89.4 orthorhombic (bco) Lattice parameters (pm) Za d Cmcm (D2h17) b-Po (rhombohedral)e 207 R3m(D3d5) a Z=Atoms per unit cell b CN=Coordination number Trigonal setting a=207.9 a=104.03 Hexagonal setting a=327.7 c=258.4 c/a=0.789 a=983 b=1033 c=2065 b=79.34 a=b~724 c~385 a=346.96(36) b=524.22(68) c=222.02(34) ? ? ? ? 4 4 1 6 3 6 145 a=330.93 b=496.98 c=214.46 c The monoclinic form at above 5.3 GPa reported by Luo et al. might be assessed as an incorrect assignment [219] d Metallic phase with superconducting temperatures of about 10 K (at ~90 GPa) raising to 17 K (at ~160 GPa, where the transition to the b-Po structure occurs) e Metallic phase with decreasing superconducting temperature upon increasing pressure (~15 K at 231 K) [179] proposed based on a monoclinic distortion of the rhombohedral unit. In this model, the rhombohedral structure was considered as a base-centered monoclinic structure of B2/m (C2h3) symmetry with Z=6 atoms per unit cell [191]. In the X-ray studies of Luo et al. the b-Po phase could be observed up to at least 212 GPa, the highest pressure achieved in the experiments [200]. From an analogy of the ratio of the lattice constants c/a in Se and Te (~0.72), at pressures where the body-centered cubic (bcc) structure is reached, a sim- ilar transition for sulfur was estimated to take place around 700 GPa [200]. Zakharov and Cohen predicted a pressure of about 550 GPa for the b-Po to bcc transition on the basis of pseudopotential ab-initio calculations [190]. In addition, a critical temperature of 15 K for the superconducting transition in the vicinity of the b-Po to bcc transition was estimated. This relatively high value was attributed to an enhancement of the electron-phonon cou- pling near the structural transition. Although the lattice parameters of bco and b-Po sulfur have fairly been reproduced by the calculations, the model failed in reproducing the experimentally observed bco to b-Po transition. At 145 GPa, the calculated total energy of the bco structure was found higher than that of the b-Po structure. On the other hand, Rudin et al. [192] found theoretically a progression b-Po!simple cubic (sc)!bcc instead of the previously expected b-Po!bcc transition for sulfur by analogy with selenium and tellurium. In addition, the face-centered cubic (fcc) structure of Se (and Te) stable at pressuresPDF Image | Topics in Current Chemistry
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