Raman spectroscopy evidence of domain walls in the organic electronic ferroelectrics ${\left(\mathrm{TMTTF}\right)}_{2}X\left(X=\mathrm{Sb}{\mathrm{F}}_6,\mathrm{As}{\mathrm{F}}_6,\mathrm{P}{\mathrm{F}}_6\right)$

Charge ordering in the quasi-one-dimensional organic conductors ${\left(\mathrm{TMTTF}\right)}_{2}X\left(X=\mathrm{Sb}{\mathrm{F}}_6,\mathrm{As}{\mathrm{F}}_6,\mathrm{P}{\mathrm{F}}_6\right)$ was studied by using Raman spectroscopy. In the charge-ordered phase three vibrational features related to the ring breathing mode ${\nu }_{10}\left(a_g\right)$ of neutral ($\mathrm{TMTT}{\mathrm{F}}^0$) and ionized ($\mathrm{TMTT}{\mathrm{F}}^{+0.5}$ and $\mathrm{TMTT}{\mathrm{F}}^{+1}$) are observed at about 503, 507, and $526\mathrm{c}{\mathrm{m}}^{-1}$, respectively. The bands of donor molecules with charge $+0.5e$ are assigned to ferroelectric domains while the bands of neutral and fully ionized molecules to domain walls. The shape of the band at about $526\mathrm{c}{\mathrm{m}}^{-1}$, attributed to the charged domain walls (molecules with charge $+1e$), reveals important differences between salts, i.e., indicates the presence of relaxor ferroelectricity which is well seen in ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$.

Figure 1

Raman spectra of ${\left(\mathrm{TMTTF}\right)}_2\mathrm{As}{\mathrm{F}}_6$ salt obtained for the electrical vector of the exciting beam polarized perpendicular to the stacking axis for the red excitation $\lambda =632.8\mathrm{nm}$.

Figure 2

Raman spectra of ${\left(\mathrm{TMTTF}\right)}_2\mathrm{P}{\mathrm{F}}_6$ salt obtained for the electrical vector of the exciting beam polarized perpendicular to the stacking axis for red excitation $\lambda =632.8\mathrm{nm}$.

Figure 3

Raman spectra of ${\left(\mathrm{TMTTF}\right)}_2\mathrm{Sb}{\mathrm{F}}_6$ salt obtained for the electrical vector of the exciting beam polarized perpendicular to the stacking axis for red excitation $\lambda =632.8\mathrm{nm}$.

Figure 4

Temperature dependence of the Raman bands attributed to the ring breathing mode ${\nu }_{10}\left(a_g\right)$ in the salts ${\left(\mathrm{TMTTF}\right)}_{2}X$ ($X=\mathrm{Sb}{\mathrm{F}}_6,\mathrm{As}{\mathrm{F}}_6,\mathrm{P}{\mathrm{F}}_6$).