文摘
The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [1b]2 has been investigated over the range 0鈥?1 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se鈥揝e---S 蟽-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped 蟺-stack arrays running along the x-direction of the monoclinic (space group P21/c) unit cell. Pressurization to 11 GPa causes the unit cell dimensions a and c to undergo a slow but uniform compression, while the b-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped 蟺-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [1a]2, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4鈥? GPa, characterized by a sharp contraction in a and c, and a correspondingly large increase in b. At the molecular level, the transition is associated with a buckling of the 蟽-bonded dimer to a more conventional 蟺-bonded arrangement. Geometry optimized DFT band structure calculations on [1b]2 replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction Eact, which indicates that a metallic state requires pressures > 10 GPa.