X-ray observation of 2k_F and 4k_F charge orderings in (TMTTF)₂ReO₄ and (TMTTF)₂SCN associated with anion orderings

Electronic transfer integrals (t) and TMTTF molecular ionicities (ρ) in several Fabre salts were estimated using the atomic coordinates obtained by systematic X-ray structure analyses at room and low temperatures. Among these Fabre salts containing the octahedral anions, (TMTTF)₂PF₆ has the largest degree of 4k_F bond dimerization in t, (TMTTF)₂AsF₆ does the middle and (TMTTF)₂SbF₆ has the smallest. This tendency corresponds well to the degree of 2k_F spin-Peierls instability at low temperatures which has been estimated. However opposite tendency was observed between the magnitude of the 4k_F bond dimerization and the reported critical temperature of the charge disproportionation. The charge modulation patterns and the transfer integral modulation patterns of two salts containing non centrosymmetric anions, (TMTTF)₂ReO₄ and (TMTTF)₂SCN were compared.