Why is $Li_{2}CO_{3}$ decomposed at a lower temperature whereas $Na_{2}CO_{3}$ at higher temperature?

(N/A) As we move down the alkali metal group,the electropositive character increases,which leads to an increase in the thermal stability of alkali metal carbonates.
However,$Li_{2}CO_{3}$ is relatively unstable to heat because the $Li^+$ ion is very small in size and has a high polarizing power. It polarizes the large $CO_{3}^{2-}$ ion,leading to the formation of stable $Li_{2}O$ and $CO_{2}$ gas.
$Li_{2}CO_{3} \xrightarrow{\Delta} Li_{2}O + CO_{2}$
In contrast,$Na_{2}CO_{3}$ is more stable due to the larger size of the $Na^+$ ion,which has a lower polarizing power,thus requiring a much higher temperature for decomposition.