Deuteration Effects on the Transport Properties of (TMTTF)2X Salts

Toshikazu Nakamura,Martin Dressel,Andrea Rohwer

doi:10.3390/cryst10121085

Toshikazu Nakamura, Martin Dressel + Show 1 more

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https://doi.org/10.3390/cryst10121085

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Abstract

The electronic properties in the quasi-one-dimensional Fabre salts are strongly affected by electronic correlations along the molecular stacks, but also by the interactions with the anions located in a cage that is formed by the methyl end groups. We systematically compare the charge transport in deuterated and protonated (TMTTF)2X salts with the anions X = Br, PF6, SbF6, and ClO4, ranging from Mott and Efros–Shklovskii variable-range hopping to activated band transport with a temperature dependent energy gap. The strong dependence of charge localization and ordering on the anion size and deuteration confirms the subtle structural involvement of the anions in the charge transport along the TMTTF stack.

Highlights

Fascinated by the wealth of exciting physical phenomena summarized in the temperature-pressure phase landscape plotted in Figure 1, the Fabre salts (TMTTF)2 X—here TMTTF denotes tetramethyltetrathiafulvalene and X is a monovalent anion—have been intensely investigated since their discovery in the 1970s [1,2,3]. (TMTTF)2SbF6 (TMTTF)2PF6 (TMTTF)2Br Temperature (K) charge order (CO) SP AFM PressureAFM stands for antiferromagnetism and SP for spin-Peierls state.Crystals 2020, 10, 1085; doi:10.3390/cryst10121085 www.mdpi.com/journal/crystals
This finding agrees with previous studies [7], where variable-range hopping transport was found to dominate below T = 60 K in octahedral (TMTTF)2 Xsalts. (D12 -TMTTF)2 ClO4 could be fitted by an Arrhenius model with the same slope above and below its anion transition, indicating band transport with a constant activation energy across the phase transition, while the behavior observed in the protonated sister compound is best fitted by
TCO or TAO and T → 0 K, respectively. ∆ is the total gap energy as calculated according to Equation (4). This comparative study between deuterated and protonated Fabre salts aimed to advance our understanding how the methyl end-groups effect the electronic properties in quasi-one-dimensional charge transfer salts

Summary

Introduction

Fascinated by the wealth of exciting physical phenomena summarized in the temperature-pressure phase landscape plotted in Figure 1, the Fabre salts (TMTTF) X—here TMTTF denotes tetramethyltetrathiafulvalene and X is a monovalent anion—have been intensely investigated since their discovery in the 1970s [1,2,3]. For salts with smaller anions, e.g., X = PF6 and AsF6 , a weaker charge order is observed at temperatures between 100 and 50 K [4] These compounds exhibit a spin-Peierls ground state below 20 K. This structural phase transition normally occurs at temperatures TAO well below the charge-ordering temperature [7,8,9]. In order to study these interactions—and the effect of confinement of the anion within its methyl end-group cage—we compare protonated with deuterated Fabre salts: (TMTTF) X, for anions X = Br, PF6 , SbF6 , and ClO4.

Experimental Details

Results and Discussion

Charge Transport Mechanisms

Energy Gap

Summary