Picoscale electron density analysis of organic crystals

The atomic arrangement within a crystal defines the overall features of the system, such as one-, two-, or three-dimensional systems. Thus, structural information provides a fundamental idea of the physical understanding of a material. In some cases, precise structural information makes it possible to perform theoretical calculations that explain the physical/electronic properties. The band structure of organic conductors is well explained by molecular orbital calculations based on the structure, and therefore x-ray structure analysis has been done for many materials [1,2]. Another characteristic phenomenon in organic systems is the periodic arrangement of valence electrons, i.e., charge density waves or charge ordering. Here, we take a-(BEDT-TTF)2I3 as an example. This compound shows a metal-insulator transition at 135 K, and the low-temperature insulator phase is a charge-ordered phase. Theoretical work [3] predicted that the valence arrangement of the Bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF) molecule can show various structures as a function of the ratio between the on-site, U, and inter-site, V, Coulomb interactions. The valence arrangement was clarified by an x-ray structure analysis [4], and the resulting structure agreed with the theoretical predictions with a reasonable U/V ratio.