Successive Dimensional Transition in ${(\mathrm{TMTTF})}_2{\mathrm{PF}}_6$ Revealed by Synchrotron X-ray Diffraction

A quasi-one-dimensional organic charge-transfer salt ${(\mathrm{TMTTF})}_2{\mathrm{PF}}_6$ undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a “structureless transition,” and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer ${\delta }_{\mathrm{CO}}=0.20e$. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

Figure 1

(a) and (b) All electron density distribution and valence electron density distributions of the TMTTF molecule 1 (hole poor), respectively. (c) Atomic scattering factor of carbon [27]. The blue, orange, and black lines indicate the contributions of the core, valence, and total electrons, respectively. (d) CO patterns of hole-rich and hole-poor (yellow and green) molecules, which indicate a 2D Wigner crystal state. The region surrounded by a dotted line shows a TMTTF molecule dimer.

Figure 2

(a) X-ray diffraction data at 7 K; the all-even and all-odd indices indicate fundamental and superlattice reflections, respectively. (b) Temperature dependence of the integrated intensity of the superlattice reflection ($-9$, $-1$, $-21$). (c) and (d) peak profiles of the fundamental and superlattice reflections, which are indicated on the same scale as the background.

Figure 3

Schematic configuration along the molecular stacking direction for ${(\mathrm{TMTTF})}_2{\mathrm{PF}}_6$: (a) dimer Mott, (b) CO, and (c) spin Peierls phases. Large (small) circles denote hole-rich (hole-poor) TMTTF molecules. The black arrows on the circles show the molecular displacement corresponding to tetramerization.