Microwave dielectric study of spin-Peierls and charge-ordering transitions in (TMTTF)2PF6 salts

We report a study of the 16.5 GHz dielectric function of hydrogenated and deuterated organic salts (TMTTF)(2)PF6. The temperature behavior of the dielectric function is consistent with short-range polar order whose relaxation time decreases rapidly below the charge-ordering temperature. If this transition has more a relaxor character in the hydrogenated salt, charge ordering is strengthened in the deuterated one where the transition temperature has increased by more than thirty percent. We give the first account of anomalies in the dielectric function related to the spin-Peierls ground state revealing some interaction between both phases in their domain of coexistence in temperature. The variation of the spin-Peierls ordering temperature obtained under magnetic field completes the structure of the phase diagram at low field and are analyzed in the framework of the mean-field prediction.