Structural and electronic changes of pentacene induced by potassium doping

Abstract

Potassium is introduced into the crystalline herringbone structure of pentacene searching for a compound showing metallic electronic transport properties and, hopefully, superconductivity at small enough temperatures. Several possible structures for stoichiometric KPentacene (1:1), K 2 Pentacene (2:1), and K 3 Pentacene (3:1) compounds are theoretically investigated. Detailed densities of states for all of them are presented. As a more prominent result, a new monoclinic structure has been stabilized for the potassium richer material that could correspond to the recently synthesized superconducting phase of K 3 Pentacene. Although energetically unfavorable, it is the only metallic candidate found to date.

Prediction of a metallic phase for Cs3Pentacene compound

“…Figure 4 gives a graphical view of the organic molecules rearrangement. This kind of relaxation was previously observed for K x Pentacene compounds 9 . Figure 1 shows the structure at 3:1 stoichiometry.…”

“…They constitute the cornerstone of our work. It should be pointed out at this point that the procedure we are using to predict crystalline phases was satisfactorily tested in our previous work on pentacene alloying 9 . We successfully reproduced there the experimental crystal structures of two naturally occurring polymorphs of pentacene, while regarding more specific interactions, the structure of the prototypical intercalation compound KC 8 was correctly obtained as well.…”

“…Even in the specific search looking for new structural patterns conducted by Naghavi and Tosatti using an evolutionary algorithm 33 , the prevalence of the herringbone arrangement was always patent. On the other hand, pentacene alloying has been systematically ignored un-til recently 8,9 . It is therefore very important to show the crucial differences that appear between picene and pentacene using a unified framework.…”

“…Figure 1 provides a picture of this structure for a 3:1 metal-pentacene compound. Recently, the possible existence of quite different phases has been considered both experimentally 8 and computationally 9 . This new phase shows a space group change from triclinic to monoclinic in both cases although important differences between the experimentally measured sample and the theoretical prediction exist.…”

“…In spite of these structural differences, both experiment and theory predict a metallic scenario at low temperatures that makes possible the occurrence of superconductivity. Unfortunately, the new phase of K 3 Pentacene is rather unfavorable from an energetic point of view 9 . Only a high temperature synthesis might yield significant amounts of this phase within a polycrystalline heterogeneous sample.…”