In porous perovskites, the presence of pores can cause a decrease in the dielectric constant due to the reduction in the effective dielectric constant of the material. The effective dielectric constant of a material is determined by the dielectric constant of the material itself, as well as the relative amounts of the material and any voids or pores within it.

Porous perovskites have a microstructure that reveals pores, which can reduce the amount of perovskite material present in the sample and increase the amount of air or other voids. As a result, the effective dielectric constant of the material is reduced due to the lower dielectric constant of air compared to that of the perovskite material.

The decrease in the effective dielectric constant can also be attributed to the fact that the electric field lines are distorted near the pore surfaces, leading to a reduction in the polarization and therefore the dielectric constant. The presence of pores can also affect the mobility of charges within the material, which can further reduce the dielectric constant.

In summary, the decrease in the dielectric constant of porous perovskites can be attributed to the reduction in the effective dielectric constant of the material due to the presence of pores. The lower dielectric constant of air or other voids, the distortion of electric field lines near the pore surfaces, and the effects of the pores on charge mobility can all contribute to the reduction in the dielectric constant.

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When a material contains pores or voids, it effectively reduces the overall volume of the solid material that contributes to the dielectric response. The presence of air or other non-dielectric voids within the material creates regions with a lower permittivity compared to the bulk material. As a result, the effective permittivity of the porous material decreases.