Charge Ordering Transitions of the New Organic Conductors δm- and δo-(BEDT-TTF)2TaF6

Tadashi Kawamoto,Reiji Kumai,Takehiko Mori,Kohei Kurata

doi:10.3390/magnetochemistry3010014

Abstract

Structural, transport, and magnetic properties of new organic conductors composed of (BEDT-TTF) 2 TaF 6 , where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, have been investigated. Two δ -type polymorphs, monoclinic and orthorhombic phases are obtained by the electrocrystallization. Both phases show a semiconductor-insulator phase transition at 276 K and 300 K for the monoclinic and orthorhombic phases, respectively; the ground state of both salts is a nonmagnetic insulating state. The low-temperature X-ray diffraction measurements show two-fold superlattice reflections in the intercolumnar direction. The low-temperature crystal structures show a clear charge ordered state, which is demonstrated by the molecular shape and intramolecular bond lengths. The observed checkerboard charge ordered state is in agreement with the charge ordering in a dimer Mott insulator. If we distinguish between the monoclinic and orthorhombic phases, the transition temperature of the δ -type (BEDT-TTF) 2 M F 6 conductors ( M = P, As, Sb, and Ta) increases continuously with increasing the anion volume.

Highlights

Among molecular conductors, bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts with octahedral anions, so-called β-(BEDT-TTF)2PF6 type salts, have a unique arrangement of the donor molecules with a twisted stacking structure (Figure 1) [1,2,3]
Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts with octahedral anions, so-called β-(BEDT-TTF)2PF6 type salts, have a unique arrangement of the donor molecules with a twisted stacking structure (Figure 1) [1,2,3]. This molecular arrangement is categorized into the δ type; the donor column consists of alternate stacking of two interaction modes: the twisted stacking mode (a1 in Figure 1c) and the parallel stacking mode (a2) slipped along the molecular short axis [3]
The superlattice reflection disappears at 277 K and 302 K for the δm and δo phases, respectively. These results indicate that the temperatures at which the resistivity shows anomaly are the phase transition temperatures

Summary

Introduction

Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) salts with octahedral anions, so-called β-(BEDT-TTF)2PF6 type salts, have a unique arrangement of the donor molecules with a twisted stacking structure (Figure 1) [1,2,3]. This molecular arrangement is categorized into the δ type; the donor column consists of alternate stacking of two interaction modes: the twisted stacking mode (a1 in Figure 1c) and the parallel stacking mode (a2) slipped along the molecular short axis (ring-over-atom mode) [3]. The AsF6 salt has been known as a monoclinic compound, but later the orthorhombic crystal has been found [4,5,6,7]

Methods

Results

Discussion

Conclusion