Why `Li_(2)CO_(3)` decomposes on heating but not other alkali metal carbonates?

Explanation

Since Li is very small in size, it has high polarising power which polarises `CO_(3)^(2-)` in `CO_(2)` and `O^(2-)` and also due to large size difference of `Li^(+)` and `CO_(3)^(2-)`, the lattice is very much unstable and decomposes on heating.