A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

• Published in: Crystal growth & design Vol. 12; no. 9; pp. 4676 - 4684
• Main Authors: Lekin, Kristina, Leitch, Alicea A, Tse, John S, Bao, Xuezhao, Secco, Richard A, Desgreniers, Serge, Ohishi, Yasuo, Oakley, Richard T
• Format: Journal Article
• Language: English
• Published: Washington,DC American Chemical Society 05.09.2012
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Cross-disciplinary physics: materials science; rheology; Electron states; Exact sciences and technology; Materials science; Methods of electronic structure calculations; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Valence bands; Pressure effects; Space groups; Molecular structure; Fluorine; Diamonds; Molecular crystals; Energy gap; Unit cell; Activation energy; Synchrotron radiation; Crystal structure; Conduction bands; Band structure; Electronic properties; Phase transformations; Theoretical study; Monoclinic lattices; Pressure dependence; Electronic structure; Powder pattern; Experimental design; Thermal activation; Density functional method; Dimers; Arrays
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg3009255

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 0–11 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–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-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 π-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 4–5 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 σ-bonded dimer to a more conventional π-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 E act, which indicates that a metallic state requires pressures > 10 GPa.