by Synchrotron X-ray Diffraction*

(TMTTF)₂PF₆ undergoes a multistep phase transition as the temperature decreases [1]. One of these transitions is called a “structure-less transition” [2,3], and these detailed structures were unknown for many years. With synchrotron X-ray diffraction, we observed a slight structural difference owing to effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis (CDFS) method [4]. Furthermore, we found that the ground state due to tetramerization, called the spin-Peierls phase, is a three-dimensional transition with interchain correlation.

[1] B. Kohler et al., Phys. Rev. B 84, 035124 (2011).
[2] C. Coulon et al., Phys. Rev. B 31, 3583 (1985).
[3] M. de Souza et al., Phys. Rev Lett. 101, 216403 (2008).
[4] S. Kitou et al., Phys. Rev Lett. 119, 065701 (2017).