Metal-insulator phase transition in the $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4$[2,6-anthracene- bis(sulfonate)]$·4{\mathrm{H}}_2\mathrm{O}$ studied by infrared spectroscopy

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4$[2,6-anthracene-bis(sulfonate)]$·4{\mathrm{H}}_2\mathrm{O}$ with $\frac{1}{4}$-filled band has been measured to investigate the metal-insulator phase transition at $T_{\mathrm{MI}}=85\mathrm{K}$ linked to charge ordering (CO). Vibrational bands related to $\mathrm{C}=\mathrm{C}$ stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen $\mathrm{OH}\cdots \mathrm{O}$ bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-$\mathrm{H}\cdots \mathrm{O}$ bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of ∼24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase transition is a consequence of both Coulomb interactions between charge carriers and donor-acceptor interactions.

Figure 1

Details of the structure of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ with (a) one BEDT-TTF layer with the two crystallographically independent molecules (noted A and B), (b) the overlap modes of neighboring donors, and (c) an anionic layer with hydrogen bonds (red dotted lines) between sulfonate groups of the ABS dianion and water molecules (O4, O5) [10].

Figure 2

(a) Polarized reflectance and (b) optical conductivity of the $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ salt in the $40–13000\mathrm{c}{\mathrm{m}}^{–1}$ frequency range at room temperature.

Figure 3

Reflectance and optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ as a function of temperature: in the left panels (a) and (b), spectra are polarized perpendicular to the stacks, and in right panels (c) and (d), parallel to the stacks.

Figure 4

Temperature dependence of the optical conductivity of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ $<1500\mathrm{c}{\mathrm{m}}^{–1}$. In both polarizations, development of a charge ordering (CO) energy gap is well seen. Arrows indicate the values of the energy gap (in $\mathrm{c}{\mathrm{m}}^{–1}$) at the temperature 8 K. Note the logarithmic frequency scale.

Figure 5

(a) Optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ in the frequency range of the ${\nu }_{27}$ mode for selected temperatures. (b) Positions of the ${\nu }_{27}$ components as a function of temperature.

Figure 6

(a) $c$-polarized and (b) $b$-polarized optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ in the frequency range of $\mathrm{C}=\mathrm{C}$ stretching vibrations at selected temperatures; (c) temperature dependence of the normalized integral intensity of the ${\nu }_3$ band measured for polarization perpendicular to the BEDT-TTF stacks and (d) the doublet of ${\nu }_5$ bands for both polarizations; (e) temperature dependence of the position of the ${\nu }_{5A}$ band for both polarizations.

Figure 7

Temperature dependence of (a) $b$-polarized and (b) $c$-polarized optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ in the frequency range of the ${\nu }_{60}$  mode. (c) Temperature dependence of the normalized integral intensity of the ${\nu }_{60}$ mode (marked as ${\nu }_{\mathrm{EMV}}$).

Figure 8

Temperature dependence of (a) $c$-polarized and (b) $b$-polarized optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ in the frequency range of stretching vibrations of $\mathrm{S}=\mathrm{O}$ and hydrogen bonds, respectively. (c) Temperature dependence of ${\nu }_{\mathrm{SO}3}$ components and (d) ${\nu }_{\alpha }$ and ${\nu }_{\beta }$ bands in the optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ in polarization $E//b$.

Figure 9

Fit of the charge-sensitive ${\nu }_{27}$ mode in the optical conductivity spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ at (a) 200 K and (b) 8 K in two CO regimes using Kubo model. (c) Velocity of charge fluctuation, (d) static, and (e) dynamic charge disproportion as a function of temperature estimated from infrared (IR) spectra using Kubo model.

Figure 10

Temperature dependence of the spectral weight of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ for polarization (a) perpendicular and (b) parallel to the stack for selected temperatures. (c) Temperature dependence of the center of the spectra weight for both polarizations in the $40–6000\mathrm{c}{\mathrm{m}}^{–1}$ frequency range.

Figure 11

Lorentz function fits of electronic bands in the spectra of $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$ at $T=8\mathrm{K}$ for polarization (a) parallel and (b) perpendicular to the BEDT-TTF stacks.

Figure 12

Zigzag stacks of BEDT-TTF in $\delta \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_4\left(\mathrm{ABS}\right)·4{\mathrm{H}}_2\mathrm{O}$. The charge-rich B (blue) and charge-poor A (red) molecules are arranged in stripes along the $c$ axis below $T_{\mathrm{MI}}=85\mathrm{K}$.