**7. Dielectric properties**

Doped Ln2CuO4 lanthanide cuprates are also characterized by their high dielectric properties due to the high value of the dielectric constant ε′ (ε′ > 104) and the low dielectric loss [59, 60]. The La2CuO4 oxide has a dielectric constant ε′ in the range

103–104 [61]. The dielectric constant of La1.95 Sr0.05CuO4±δ is about 2 × 105 much larger than that of La2CuO4 [11]. The increase in this constant was explained by the increase in the concentration of holes in the material [62]. For the cuprate Eu2CuO4, the dielectric constant was about 103–104 in the frequency range between 1 kHz and 1 MHz for the temperature range from 173 K to 423 K [63].

Indeed, at the microscopic scale, the dielectric permittivity of the material is linked to the electrical polarizability of the molecules or atoms constituting the material. Dielectric materials are polarized in an applied electric field [64]. A certain amount of time is required to orient the dielectric dipoles depending on the direction of the applied electric field. This period is called "relaxation time," which can be attributed to an inhomogeneous microstructure consisting of cuprate grains, separated by insulating grain boundaries.

The dielectric losses encountered in most materials, in particular noticed for cuprate, originate directly from polarization due to orientation because its range of variation can be located between 102 and 105 Hz [65]. This behavior can be explained in terms that at low frequency, there is a phase shift between the polarization vector and the electric field [64]. But at sufficiently high frequencies, the duration of the electric field is reduced compared to the relaxation time of the permanent dipoles. Thus, the orientation of the latter is no longer influenced by the electric field and remains random. We also observe that the dielectric loss decreases when the temperature increases because the conduction in the dielectric ceases and is believed to be a negligible phenomenon.
