Variation in the nature of the neutral-ionic transition in $\mathrm{DMTTF}-{\mathrm{QCl}}_4$ under pressure probed by NQR and NMR

We have investigated the microscopic charge/lattice/spin states in the neutral-ionic transition material $\mathrm{DMTTF}-{\mathrm{QCl}}_4$ under pressure by ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ and ${}^1\mathrm{H}-\mathrm{NMR}$ measurements to reveal the mechanism of the phase transition and the nature of the pressure-temperature phase diagram. At ambient pressure, the ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ frequency indicates low ionicity and shows neither discontinuous jump nor precursory decrease around the transition, suggesting that the spin degrees of freedom are not involved in a dimerization transition. These experimental features are consistent with the theoretically proposed picture that the electronic energy gain is acquired by modulations in transfer integrals (so-called Peierls mechanism) with the lattice dimerization. As pressure is applied, the ionicity is increased and the system is driven into an ionic state with the degree of charge transfer exceeding 0.5 above $\sim 9\mathrm{kbar}$ at room temperature, where the ${}^1\mathrm{H}-\mathrm{NMR}$ captures paramagnetic spin fluctuations indicative of the emergence of a nondimeric ionic phase. Furthermore, ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ and x-ray-diffraction measurements found a distinct antiferroelectric state at high pressures above 11 kbar. Based on these results, we propose a pressure-temperature phase diagram.

Figure 1

(a) Molecular structures of DMTTF and ${\mathrm{QBr}}_n{\mathrm{Cl}}_{4-n}$ (shown for the case of $n=0$). (b) Arrangement of DMTTF and ${\mathrm{QBr}}_n{\mathrm{Cl}}_{4-n}$ molecules in 1D chains in $\mathrm{DMTTF}-{\mathrm{QBr}}_n{\mathrm{Cl}}_{4-n}$ (for $n=0$). (c) Molecular arrangements in the plane parallel (upper) and perpendicular (lower) to the 1D chains in the neutral phase of $\mathrm{DMTTF}-{\mathrm{QBr}}_n{\mathrm{Cl}}_{4-n}$ (for $n=0$) [10, 11] and $\mathrm{TTF}-{\mathrm{QCl}}_4$ [8].

Figure 2

(a) ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ spectra at 6.9 kbar. Temperature dependence of NQR line frequency (b), NQR line splitting width (c), and center-of-gravity frequency of the NQR line, ${\nu }_{\mathrm{Q}}$, which is the NQR frequency (the averaged frequency) of the single line (the split lines) above (below) $T_{\mathrm{c}}$ (d) under various pressures. The vertical axis in (b) is shifted for each pressure as indicated by the reference value on the right axis for clarity.

Figure 3

Temperature dependence of the degree of charge transfer, $\rho$, in $\mathrm{DMTTF}-{\mathrm{QCl}}_4$ (a) and $\mathrm{TTF}-{\mathrm{QCl}}_4$ [18] (b), evaluated from ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ frequency, for several pressures. (c) Pressure evolutions of $\rho$ at 250 K for $\mathrm{DMTTF}-{\mathrm{QCl}}_4$ and $\mathrm{TTF}-{\mathrm{QCl}}_4$. The arrow represents the dimerization transition pressure.

Figure 4

Temperature dependence of ${}^{35}\mathrm{Cl}-\mathrm{NQR}$ spin-lattice relaxation rate, ${}^{35}T_1^{-1}$, under various pressures.

Figure 5

(a) Temperature dependence of ${}^1\mathrm{H}-\mathrm{NMR}$ spin-lattice relaxation rate, ${}^{1}T_1^{-1}$, under various pressures. Fits of the BPP model to ${}^{1}T_1^{-1}$ at ambient pressure (b) and 5.4 kbar (c). (d) Contribution of electron spin fluctuations to the relaxation rate, ${{}^{1}T_1^{-1}}_{\mathrm{spin}}$, at 8.9 kbar. ${{}^{1}T_1^{-1}}_{\mathrm{spin}}$ is obtained by subtracting the averaged ${}^{1}T_1^{-1}$ values for ambient pressure and 5.4 kbar from the ${}^{1}T_1^{-1}$ values at 8.9 kbar. The inset of (d) is the Arrhenius plot of ${{}^{1}T_1^{-1}}_{\mathrm{spin}}$. The blue lines are the fitting curves by the single exponential functions in $230<T<275$ K and $180<T<230$ K, which give the activation energies of 0.14 and 0.09 eV, respectively.

Figure 6

Pressure dependence of NQR frequency at room temperature (upper) and 276 K (lower). The blue-, green-, and orange-colored regions correspond to the neutral, dimeric ionic (Dimer I), and new (Dimer II) phases indicated in Fig. 9.

Figure 7

X-ray-diffraction patterns at room temperature. (a) Diffraction patterns in the $a^{*}-c^{*}$ plane at ambient pressure (left) and 15 kbar (right). (b) Diffraction patterns in the plane including the $b^{*}$ axis at ambient pressure (left) and 15 kbar (right).

Figure 8

Schematic crystal structures viewed along the $c$ axis (corresponding to the 1D axis) in the neutral, ionic (Dimer I), and high-pressure (Dimer II) phases. Green-colored circles represent the uniform stacking chains and red- and blue-colored circles represent the dimerized chains with the electric dipole moments oppositely directed with each other. Black parallelograms represent the unit cells.

Figure 9

(a) Pressure-temperature phase diagram of $\mathrm{DMTTF}-{\mathrm{QCl}}_4$. Red closed circles represent the $T_{\mathrm{c}}$ of dimerization transition determined by the present NQR measurements. Green closed and open squares indicate the previous results obtained by the dielectric and resistivity measurements, respectively [9]. Black triangles represent the transition points to Dimer II phase obtained from the results in Fig. 6. (b) Schematic global phase diagram of NI transition systems. $T$, $S$, and $V$ represent the temperature, the electron-lattice interaction, and the Madelung energy gain, respectively. The left (right) direction of the horizontal axis means the increasing role of $S \left(V\right)$.