Influence of charge order on the ground states of TMTTF molecular salts

Abstract

Abstract. (T M T T F ) 2 AsF 6 and (T M T T F ) 2 SbF 6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using 13 C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (T M T T F ) 2 SbF 6 . An extension of the well-known temperature/pressure phase diagram is proposed.

Coexistence of charge order and antiferromagnetism in (TMTTF)2SbF6: NMR study

“…This value is almost same as that estimated from the infrared spectroscopy [8]. On the other hand, larger values for ∆ρ were estimated from NMR T 1 -1 measurements [3,7]. The discrepancy is well explained by considering the contribution of the commensurate AF fluctuation in Fig.…”

“…In addition, the almost same ∆ρ in the sP phase was reported in the optical investigation [6], whereas the previous NMR study suggested the disappearance of the charge disproportionation in the sP phase [4]. For (TMTTF) 2 Sb 6 with the AF ground state ∆ρ was estimated as 0.50e and 0.55e by the NMR T 1 -1 [3,7] and 0.29e by the infrared spectroscopy [8]. In each case the estimated ∆ρ by T 1 -1 is much larger than that by the optical measurement.…”

“…However, there is inconsistency between the estimated values of ∆ρ for each salt depending on the measuring method. For example, for the CO phase (TMTTF) 2 AsF 6 with the sP ground state ∆ρ was estimated as 0.50e and 0.33e from the ratio of T 1 -1 [3,4], while 0.21e and 0.16e were obtained by the infrared spectroscopy of two independent groups [5,6]. In addition, the almost same ∆ρ in the sP phase was reported in the optical investigation [6], whereas the previous NMR study suggested the disappearance of the charge disproportionation in the sP phase [4].…”

Comprehensive Optical Investigations of Charge Order in Organic Chain Compounds (TMTTF)2X

“…The position in the phase diagram can be tuned by external or chemical pressure. (TMTTF) 2 PF 6 (TMTTF) 2 SbF 6 First indications of a phase transition at T CO = 157 K in (TMTTF)) 2 SbF 6 were provided by thermopower and transport measurements [9], but it took another 15 years before one-and two-dimensional 13 C spin labeled NMR spectroscopy could prove charge disproportionation developing for T < T CO [10][11][12][13][14][15]. Dielectric permittivity measurements on several of these TMTTF salts finally yielded evidence for ferroelectric behavior of purely electronic origin [16][17][18][19][20][21][22].…”

13C NMR study of the chemical pressure effect in (TMTTF)2[(AsF6)x(SbF6)1-x] (x∼ 0.5)

“…The ratio of the charge occupancy at 101 K is 0.32 : 0.68 (the amplitude is ρ ∼ 0.18), assuming that the 1/T 1 is proportional to the square of the charge quantity on the TMTTF molecule. The ρ is also intermediate between the values of ρ ∼ 0.17 for AsF 6 salt [11] and ρ ∼ 0.25 for SbF 6 salt [13].…”