Charge Ordering in α-(BEDT-TTF)₂I₃by Synchrotron X-ray Diffraction

Abstract

The spatial charge arrangement of a typical quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 is revealed by single crystal structure analysis using synchrotron radiation. The results show that the horizontal stripe type structure, which was suggested by mean field theory, is established. We also find the charge disproportion above the metal-insulator transition temperature and a significant change in transfer integrals caused by the phase transition. Our result elucidates the insulating phase of this material as a 2k F charge density localization.KEYWORDS: α-(BEDT-TTF)2I3, Charge Ordering, Quasi two dimensional organic conductor, Synchrotron Radiation, Crystal structure analysisAs an origin of a metal-insulator (M-I) transition, the charge ordering caused by the long rangeCoulomb interaction between electrons is a prominent phenomenon as well as the Peierls transition or the Mott Hubbard transition. Organic materials having low-dimensionality provide us a field of studying such interesting phenomena. 1 Among them, an organic conductor α-(BEDT-TTF) 2 I 3 , which undergoes a M-I transition at T M I = 135 K, 2 attracts considerable attention because of its possibility of zero-gap semiconductor under hydrostatic pressures. 3 The insulating phase is a nonmagnetic state with a spin gap, 4 and interpreted to be caused by a charge disproportionation theoretically 5, 6 and experimentally. 7,8 According to these studies, horizontal-charge stripes with valences of +1e and 0 are expected on BEDT-TTF molecules while no structural evidence has been reported. In this letter, we report a precise structure of α-(BEDT-TTF) 2 I 3 as a function of temperature based on synchrotron radiation diffraction measurements in order to provide evidence of the charge disproportionation.α-(BEDT-TTF) 2 I 3 , which has a two-dimensional (2D) conduction band, is a typical organic conductor made of I − 3 ions and BEDT-TTF 0.5+ on average. The crystal structure of this material is a sandwich structure of I 3 insulating layers and 2D-BEDT-TTF conduction layers having a quarterfilled hole band. The space group at room temperature is P1. Together with the metal-insulator transition, it shows paramagnetic-nonmagnetic transition. 4 According to several reports, 6, 9, 10 this transition is not a Peierls transition but a charge ordering caused by a strong Coulomb repulsion.

Theoretical study of the zero-gap organic conductor α-(BEDT-TTF)2I3

“…Then the charge disproportionation does not break any symmetry of crystal and does not have any phase transition. Recent theoretical results on the charge disproportionation [15] are consistent with the x-ray experiment [16] and NMR experiment [17]. The charge disproportionation exists in the ZGS theoretically [34].…”

“…The charge orderings appearing in both those two phases are the horizontal stripe, and do not exhibit any phase transitions between them. Recent theoretical estimation on charge on each site [15], based on the transfer energies obtained by the first-principles calculation [33], is consistent with both the x-ray experiment [16] and the NMR experiment [12], where the A and B sites are hole-rich and the A and C sites are electron-rich. The ZGS emerges with a further increase of P a .…”

“…The Raman studies suggest the charge ordering in the insulating phase below T MI and also charge disproportionation above T MI [13,14]. Recent theoretical results on the charge disproportionation [15] are consistent with the x-ray experiment and NMR experiment [16,17].…”

Magnetic ordering and charge dynamics in κ-(BEDT-TTF)2Cu[N(CN)2]Cl

“…For the prime example of α-(BEDT-TTF) 2 I 3 , twinned domains form due to symmetry breaking in the charge-ordered phase (2δ = 0.6 from NMR, Raman and IR measurements [40,41,20,35]), and the motion of domain-wall pairs gives rise to temperature-independent weak dielectric relaxation with ε ≈ 400 as well as a negative differential resistance in dc electric fields.…”

C13NMR study on the charge-ordering saltα′−(

“…8 Indeed, a CO state was confirmed in ␣-͑BEDT-TTF͒ 2 I 3 and -͑BEDT-TTF͒ 2 RbZn͑SCN͒ 4 and in similar salts. [9][10][11][12][13][14] Moreover, superconductivity was observed in ␣-͑BEDT-TTF͒ 2 I 3 under uniaxial stress and in -͑BEDT-TTF͒ 2 I 3 ͑Refs. 15 and 16͒ under ambient pressure.…”