Structural phase transition in quasi-one-dimensional conductors (BDTFP)2X(PhCl)0.5 (X = PF6 and AsF6) [BDTFP = 5,7-bis(1,3-dithiol-2-ylidene)-5,7-dihydrofuro[3,4-b]pyrazine; PhCl = chlorobenzene]

Abstract

We present the low-temperature crystal structures of (BDTFP)₂PF₆ and (BDTFP)₂AsF₆ below the phase transition temperatures (T_p). In the former compound, the c-axis is doubled, and the dimerized structure is therefore changed into a tetramerized structure. At the same time, a new charge-transfer transition appears at ca. 7000–8000 cm⁻¹. These two findings are consistent with the non-magnetic state below T_p. In the latter compound, on the other hand, the dimerized structure is maintained below T_p, and the AsF₆ ion rotates along with the ca. 10° rotation of BDTFP. This structural change is the origin of the first-order phase transition to a paramagnetic state.