Vibrational Spectra of β″-Type BEDT-TTF Salts: Relationship between Conducting Property, Time-Averaged Site Charge and Inter-Molecular Distance

Abstract

Abstract:The relationship between the conducting behavior and the degree of charge fluctuation in the β″-type BEDT-TTF salts is reviewed from the standpoints of vibrational spectroscopy and crystal structure. A group of β″-type ET salts demonstrates the best model compounds for achieving the above relationship because the two-dimensional structure is simple and great diversity in conducting behavior is realized under ambient pressure. After describing the requirement for the model compound, the methodology for analyzing the results of the vibrational spectra is presented. Vibrational spectroscopy provides the time-averaged molecular charge, the charge distribution in the twodimensional layer, and the inter-molecular interactions, etc. The experimental results applied to 2/3-filled and 3/4-filled β″-type ET salts are reported. These experimental results suggest that the conducting property, the difference in the time-averaged molecular charges between the ionic and neutral-like sites, the alternation in the inter-molecular distances and the energy levels in the charge distributions are relevant to one another. The difference in the time-averaged molecular charges, ∆ρ, is a useful criterion for indicating conducting behavior. All superconductors presented in this review are characterized as small but finite ∆ρ.

Charge Fluctuations and Ethylene‐Group‐Ordering Transition in β′′‐(BEDT‐TTF)4[(H3O)Fe(C2O4)3]⋅Y Molecular Charge‐Transfer Salts

“…A similar infrared response, as to the anisotropy and the reflectance level, was recorded earlier for other metallic and superconducting β′′‐(BEDT‐TTF) 4 [A⋅M(C 2 O 4 ) 3 ]⋅Y materials 16. 48, 49 Our results can also be compared with the more anisotropic response of the β′′‐(BEDT‐TTF) 2 SF 5 CH 2 CF 2 SO 3 superconductor ( T c =5.2 K) 15. 47 The reflectance spectra of β′′‐2, β′′‐3 and α‐κ polarized in the low‐conducting interplane ∥c direction are flat with a rather low electronic background, as expected (Figure 1 (b)–(d)).…”

“…16 Of interest here are the β′′‐(BEDT‐TTF) 4 [(H 3 O)Fe(C 2 O 4 ) 3 ]⋅Y organic conductors. Recently, vibrational properties related to the CO fluctuating state have been reviewed by Yamamoto et al16, 48 for some β′′‐type salts including two organic metals, β′′‐(BEDT‐TTF) 4 [(H 3 O)Ga(C 2 O 4 ) 3 ]Pyridine and β′′‐(BEDT‐TTF) 4 [(H 3 O)Ga(C 2 O 4 ) 3 ]CH 2 Cl 2 , and superconducting β′′‐(BEDT‐TTF) 4 [(H 3 O)Ga(C 2 O 4 ) 3 ]PhNO 2 . It was shown that the conducting behavior in this class of materials is related to the time‐averaged molecular charges Δ ρ , defined as the charge difference between charge‐rich and charge‐poor donor molecules.…”

“…]·Y materials. [16,48,49] Our results can also be compared with the more anisotropic response of the b''-(BEDT-TTF) 2 SF 5 CH 2 CF 2 SO 3 superconductor (T c = 5.2 K). [15,47] The reflectance spectra of b''-2, b''-3 and a-k polarized in the low-conducting interplane k c direction are flat with a rather low electronic background, as expected ( Figure 1 (b)-(d)).…”

“…These modes display a linear relationship of frequency on charge and due to the large frequency shift and small time resolution they can be used for investigations of the time-averaged charge. [12,16] Of interest here are the b''-(BEDT-TTF) 4 [16,48] for some b''-type salts including two organic metals, b''-(BEDT-TTF) 4 Table 1. Drude-Lorentz model parameters extracted from analysis of the 300 K polarized reflectance spectra of b''-(BEDT-TTF) 4…”

Thermodynamics of a Liquid-like Spin State in Molecule-based Magnets with Geometric Frustrations

“…Even in the β-and κ-type salts which are considered to have rigidly dimerized structure, the spin correlations coupled with charge fluctuations should be taken into account. 39 The Et 2 Me 2 Sb[Pd(dmit) 2 ] 2 with similar molecular arrangement with the present spin-liquid system shows charge-ordered ground states of which charge distributions are different between compounds.…”