Why is `Li_2CO_3` decomposed at a lower temperature whereas `Na_2CO_3` at higher temperature?

Explanation

`Li^(+)` ion being smaller in size forms a more stable lattice with the smaller oxide ion `(O^(2-))` than the larger carbonate ion `(CO_3^(2-))` and consequently `Li_2CO_3` decomposes into `Li_2O` at a much lower temperature. The high polarising power of very small `Li^(+)` ion also facilitates the decomposition of `Li_2CO_3`. On the other hand, the larger `CO_3^(2-)` ion than with the smaller `O^(2-)` ion. Therefore, `Na_2CO_3` is quite stable and decomposes only at very high temperature.