Thermodynamic studies at the charge-ordering and spin-Peierls transitions in (TMTTF)2X(TMTTF)2X

We report results of high-resolution thermal expansion measurements on the charge-ordering (TCO)(TCO) and spin-Peierls (TSP)(TSP) transition in (TMTTF)2X(TMTTF)2X for X=AsF6X=AsF6 and PF6PF6. Based on distinct lattice effects observed at TCOTCO in the c*c*-axis expansivity, a scheme is proposed which involves a charge modulation along the TMTTF stacks and its coupling to displacements of the anions. It is also the c*c*-axis expansivity which is most strongly affected by the transition at TSPTSP. The combination of expansivity and specific heat results for the X=AsF6X=AsF6 salt enables us to separate for both quantities the contribution associated with the spin-Peierls transition from the ordinary lattice effects and to provide a thermodynamic determination of the uniaxial-pressure dependence of TSPTSP.