Complex changes in structural parameters hidden in the universal phase diagram of the quasi-one-dimensional organic conductors (TMTTF)2X ( X=NbF6, AsF6, PF6 , and Br)

Abstract

Structural parameters in quasi-one-dimensional organic conductors ${(\mathrm{TMTTF})}_{2}X$ ($X=\mathrm{Nb}{\mathrm{F}}_{6}, \mathrm{As}{\mathrm{F}}_{6}, \mathrm{P}{\mathrm{F}}_{6}$, and Br) are investigated by synchrotron x-ray diffraction. The temperature dependences of the crystal structures differ significantly between octahedral and monatomic anion systems, corresponding to the presence or absence of a charge-ordering transition. Changes in temperature and chemical pressure control the dimensionality of the lattice and the degree of dimerization. Furthermore, the antiferromagnetic and spin-Peierls ground states in this system depend on the spin frustration due to transfer integral paths between the one-dimensional chain molecules.