Charge ordering in low dimensional organic conductors: Structural aspects

Pouget, Jean‐Paul; Foury-Leylekian, P.; Alemany, Pere; Canadell, Enric

doi:10.1002/pssb.201100750

Cited by 22 publications

(34 citation statements)

References 27 publications

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“…The anions could also interact indirectly with the π -conduction holes via the polarization of the methyl groups and the σ -bond skeleton, as outlined in the last section and previously discussed in references. 13,51,52 The importance of this last effect has not yet been evaluated. Another important indirect effect is the lattice deformation caused by the libration of the anions and methyl groups and their low-temperature H-bond linkage.…”

Section: Discussionmentioning

confidence: 99%

“…The enhanced charge response for electric fields polarizing the methyl-group cavity when applied along c * can be explained using a mechanism previously proposed for (BEDT-TTF) 2 X salts. 51,52 A field-induced local shift of the anion toward a donor inside the methyl-group cavity will increase the H bonding with the donor's methyl groups, which will make the methyl group's hydrogen/deuterium more positive. This in turn will induce a negative σ charge shift leading to an excess of positive π charge in the core of the TMTSF.…”

Section: The Structural Change Around 55 Kmentioning

confidence: 99%

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Low-temperature structural effects in the (TMTSF)2PF6and AsF6Bechgaard salts

et al. 2013

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We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF) 2 PF 6 and (TMTSF) 2 AsF 6 (TMTSF: tetramethyl-tetraselenafulvalene). The 4 K neutron-scattering structure refinement of the fully deuterated (TMTSF) 2 PF 6 -D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF) 2 PF 6 -H12 salt previously determined at the same temperature. Surprisingly, it is found that deuteration corresponds to the application of a negative pressure of 5 × 10 2 MPa to the H12 salt. Accurate measurements of the Bragg intensity show anomalous thermal variations at low temperature both in the deuterated PF 6 and AsF 6 salts. Two different thermal behaviors have been distinguished. Small Bragg-angle measurements reflect the presence of low-frequency modes at characteristic energies θ E = 8.3 K and θ E = 6.7 K for the PF 6 -D12 and AsF 6 -D12 salts, respectively. These modes correspond to the low-temperature methyl group motion. Large Bragg-angle measurements evidence an unexpected structural change around 55 K, which probably corresponds to the linkage of the anions to the methyl groups via the formation of F. . .D-CD 2 bonds observed in the 4 K structural refinement. Finally we show that the thermal expansion coefficient of (TMTSF) 2 PF 6 is dominated by the librational motion of the PF 6 units. We quantitatively analyze the low-temperature variation of the lattice expansion via the contribution of Einstein oscillators, which allows us to determine for the first time the characteristic frequency of the PF 6 librations: θ E ≈ 50 K and θ E = 76 K for the PF 6 -D12 and PF 6 -H12 salts, respectively.

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Section: Discussionmentioning

confidence: 99%

Section: The Structural Change Around 55 Kmentioning

confidence: 99%

Low-temperature structural effects in the (TMTSF)2PF6and AsF6Bechgaard salts

et al. 2013

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“…The change of orientation of the anion in its cavity should break both the short F…Se contact distance and the weak F…H-CH 2 bonds (see Section 2.4). This will respectively modulate directly and indirectly (via the polarization of the H bonds and of the σ intra-molecular electrons [21]) the Hartree potential experienced by the π holes. These fluctuations of the Hartree potential should provide new scattering mechanisms of holes and electrons at the Fermi level, resulting in either an incoherent interstack charge transfer and/or intra-stack backward and forward scatterings.…”

Section: Figurementioning

confidence: 99%

“…It was only recently that the finding [24] of a lattice thermal expansion anomaly at T CO provides convincing evidences of a structural counterpart at the electronic ferroelectricity. Then evidence of a tiny structural modification was found from the detection of weak (<15%) variations at T CO of the intensity of several main Bragg reflections using neutron diffraction on (TMTTF) 2 PF 6 deuterated powders [21,105]. These quite small structural modifications explain why earlier attempts to solve the (TMTTF) 2 PF 6 and AsF 6 structures on hydrogenated crystals in the P1 space group from conventional neutron scattering data collection at 4K were unsuccessful [106].…”

Section: Basic Features: the Historical Scenariomentioning

confidence: 99%

Structural Aspects of the Bechgaard and Fabre Salts: An Update

Pouget

2012

Crystals

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107

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Abstract:We review structural aspects of the Bechgaard and Fabre salts in relationship with their electronic, magnetic and superconducting properties. We emphasize the role of bond and charge modulations of the quarter filled organic stack in the various instabilities and ground states exhibited by these salts. A special consideration is also devoted to the influence of anions and methyl groups in these processes. In particular we point out the importance of the anions in achieving the inter-stack coupling by either direct or indirect (via the polarization of the methyl group cavities) interactions with the donors. In this framework we discuss the role of anions and methyl group disorders in the inhibition of the divergence of the high temperature bond order wave instability of the Bechgaard salts. We analyze the modulation in the magnetic ground states by considering explicitly the coupling of the magnetization with structural degrees of freedom. We consider the role of the anions and methyl groups in stabilizing the charge ordering pattern in the Fabre salts. We also discuss the spin-Peierls transition of the Fabre salts in relation with the charge ordering transition and the adiabaticity of the phonon field. We review the anion ordering transitions by considering more particularly the influence of the ordering process on the electronic structure and on the ground states which results. In this framework we show that the texture of the anion ordered structure has direct consequences on the superconducting properties of (TMTSF) 2 ClO 4 . Finally we conclude on the essential implication of the structural degrees of freedom on the generic phase diagram of the Bechgaard and Fabre salts. OPEN ACCESSCrystals 2012, 2 467

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“…It was also predicted that spin fluctuation mediates the SC pairing in a CO system [35]. Recently, it was pointed out that interaction with anions plays an essential role in the CO phase transition [36,37]. These subjects will be further investigated in future.…”

Section: Introductionmentioning

confidence: 99%

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

Yakushi

2012

Crystals

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This paper reviews charge ordering in the organic conductors, β″-(BEDT-TTF) (TCNQ), θ-(BEDT-TTF) 2 X, and α-(BEDT-TTF) 2 X. Here, BEDT-TTF and TCNQ represent bis(ethylenedithio)tetrathiafulvalene and 7,7,8,8-tetracyanoquinodimethane, respectively. These compounds, all of which have a quarter-filled band, were evaluated using infrared and Raman spectroscopy in addition to optical conductivity measurements. It was found that β″-(BEDT-TTF)(TCNQ) changes continuously from a uniform metal to a chargeordered metal with increasing temperature. Although charge disproportionation was clearly observed, long-range charge order is not realized. Among six θ-type salts, four compounds with a narrow band show the metal-insulator transition. However, they maintain a large amplitude of charge order (Δρ~0.6) in both metallic and insulating phases. In the X = CsZn(SCN) 4 salt with intermediate bandwidth, the amplitude of charge order is very small (Δρ < 0.07) over the whole temperature range. However, fluctuation of charge order is indicated in the Raman spectrum and optical conductivity. No indication of the fluctuation of charge order is found in the wide band X = I 3 salt. In α-(BEDT-TTF) 2 I 3 the amplitude of charge order changes discontinuously from small amplitude at high temperature to large amplitude (Δρ max~0 .6) at low temperature. The long-range chargeordered state shows ferroelectric polarization with fast optical response. The fluctuation of multiple stripes occurs in the high-temperature metallic phase. Among α-(BEDT-TTF) 2 MHg(SCN) 4 (X = NH 4 , K, Rb, Tl), the fluctuation of charge order is indicated only in the X = NH 4 salt. α′-(BEDT-TTF) 2 IBr 2 shows successive phase transitions to the ferroelectric state keeping a large amplitude of charge order (Δρ max~0 .8) over the whole temperature range. It was found that the amplitude and fluctuation of charge order in these compounds is enhanced as the kinetic energy (bandwidth) decreases. OPEN ACCESSCrystals 2012, 2 1292

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Charge ordering in low dimensional organic conductors: Structural aspects

Cited by 22 publications

References 27 publications

Low-temperature structural effects in the (TMTSF)2PF6and AsF6Bechgaard salts

Low-temperature structural effects in the (TMTSF)2PF6and AsF6Bechgaard salts

Structural Aspects of the Bechgaard and Fabre Salts: An Update

Infrared and Raman Studies of Charge Ordering in Organic Conductors, BEDT-TTF Salts with Quarter-Filled Bands

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