The crystallite size measured using Debye Scherrer method shows 50nm in polycrystalline ferroelectric ceramics. Is it possible to increase the crystallite size to micrometer range. If so, what will be the possible method?
In principle, thermal annealing should be the answer, though to go from 50 nm to micrometers will probably require a long time. You'll have to research conditions on your own, focusing on literature pertaining to your compound and related ferroelectrics. Caveat: If the compound has a phase transition you'll need to stay below its temperature, which in some cases makes effective thermal annealing impractical.