Well, your sample offers smaller breakdown strength than your expectation. Porous nature of the sample, presence of moisture inside, diffusion of electrode material, small thickness of the film, organic impurities, conduction channels etc can be the reasons for small breakdown strength.

You should prepare thicker films, which are fully dry and very pure in nature. Then slowly increase the maximum voltage. Long before the breakdown occurs, you should be able to hear the sound of small sparks. If you hear them, don't increase the voltage further. Optimize your sample for the best ferroelectric properties.

Thank you for your reply Souvik. It is not for single sample or for composite structure with higher chances of leakage. All of my sample both ceramic and polymer are showing breakdown at lower voltage. However it is well reported in literature that the breakdown strength of material is far higher than the observed value.

Try first to measure I(V) and to determine the source of the conductivity. In my experience, ambient air moisture can significantly impact properties of polymer samples. It is a good idea to dry samples in vacuum first, and to perform experiments in evacuated or dry nitrogen filled chamber. Another thing is to restrict the current, which can flow through your sample, to prevent electric avalanche. Do you have a series resistor in your circuit?

Alexander Sorokin Thank you for your reply Sir. I am using Radiant instrument for ferroelectric loop measurement, so I don't need to design any circuit for that. I have tried to minimize the effect of moisture and leakage effect. It is getting better than before now. Still there is a lot of gap between between obtained values of breakdown field to the those available in literature.

What should be optimum size and thickness of electrode for measurement?

I have observed that increasing thickness of electrode also breaking it.

How thick is a polymer in your samples and what kind of electrodes do you use?