Microwave dielectric study of spin-Peierls and charge ordering transitions in (TMTTF)$_{2}$PF$_{6}$ salts

Using a microwave cavity perturbation technique at 16.5 GHz, we report a temperature and magnetic field study of the complex dielectric function along the stacking $a$-axis for a (TMTTF)$_{2}$PF$_{6}$ single crystal and its deuterated analog (d$_{12})$. For both salts, the charge ordering transition (CO) is characterized by a decrease of the dielectric constant $\varepsilon _{a}$ centered at T$_{CO}$ (65 K and 85 K); concomitantly, the dielectric losses go through a maximum near T$_{CO}$ and decreases rapidly below. The spin-Peierls transition (SP) is rather signalled by a rapid increase of $\varepsilon _{a}$ below T$_{SP}$ (16.5 K and 13 K) accompanied by a small peak in the losses. For the deuterated salt, we have observed important relaxation effects below 40 K that complicate the analysis of the dielectric function in the SP ground state. The temperature dependence of the SP anomalies was analysed in magnetic field values up to 18 Tesla.