The solubilities of carbonates decrease down the group due to decrease in

As we move down the group, the lattice energies of carbonates remain approximately the same. However the hydration energies of the metal cation decreases from Be⁺⁺ to Ba⁺⁺, hence the solubilities of carbonates of the alkaline earth metal decrease down the group mainly due to decreasing hydration energies of the cations from Be⁺⁺ to Ba⁺⁺.