Why is `Li_(2)CO_(3)` decomposed at a lower temperature whereas `Na_(2)CO_(3)` at higher temperature?

Explanation

`Li^(+)` ion being small in size , cannot stabilize large `CO_3^(2-)` ion therefore , `Li_2CO_3` is unstable. On the other hand sodium is larger in size and can easily stabilize large `CO_3^(2-)` ions and form stable `Na_2CO_3`. This is because of lattice energy effects .