ABSTRACT
The products of the solid-state reactions between potassium metal and tetracene (K:Tetracene, 1:1, 1.5:1, and 2:1) are fully structurally characterized. Synchrotron X-ray powder diffraction shows that only K2Tetracene forms under the reaction conditions studied, with unreacted tetracene always present for x < 2. Diffraction and 13C MAS NMR show that K2Tetracene has a crystal structure that is analogous to that of K2Pentacene, but with the cations ordered on two sites because of the influence of the length of the hydrocarbon on possible cation positions. K2Tetracene is a nonmagnetic insulator, thus further questioning the nature of reported superconductivity in this class of materials.
ABSTRACT
The Verwey transition in Fe3O4, a complex structural phase transition concomitant with a jump in electrical conductivity by two orders of magnitude, has been a benchmark for charge ordering (CO) phenomena in mixed-valence transition metal materials. CO is of central importance, because it frequently competes with functional properties such as superconductivity or metallic ferromagnetism. However, the CO state in Fe3O4 turned out to be complex, and the mechanism of the Verwey transition remains controversial. We demonstrate an archetypical Verwey-type transition in an open p-shell anionic mixed-valence compound using complementary diffraction and spectroscopic techniques. In Cs4O6, a phase change from a cubic structure with a single crystallographic site for the molecular O2x- building units to a tetragonal structure with ordered superoxide O2- and peroxide O22- entities is accompanied by a drastic drop in electronic conductivity and molecular charge fluctuation rates. The simple CO pattern of molecular units and the lack of magnetic order suggest Cs4O6 as a model system for disentangling the complex interplay of charge, lattice, orbital, and spin degrees of freedom in Verwey-type CO processes.
