Raman spectroscopy evidence of domain walls in the organic electronic ferroelectrics (TMTTF)2X(X=SbF6,AsF6,PF6)

Charge ordering in the quasi-one-dimensional organic conductors ${(\mathrm{TMTTF})}₂X\phantom{\rule{0.16em}{0ex}}(X=\mathrm{Sb}{\mathrm{F}}₆,\mathrm{As}{\mathrm{F}}₆,\mathrm{P}{\mathrm{F}}₆)$ was studied by using Raman spectroscopy. In the charge-ordered phase three vibrational features related to the ring breathing mode ${\ensuremath{\nu}}₁₀({a}_g)$ 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 $526\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, 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 $526\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$, 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 ${(\mathrm{TMTTF})}₂\mathrm{P}{\mathrm{F}}₆$.