BaCO3<CaCO3<SrCO3<MgCO3
MgCO3<CaCO3<SrCO3<BaCO3
MgCO3<SrCO3<CaCO3<BaCO3
BaCO3<SrCO3<CaCO3<MgCO3
The thermal stability of alkaline earth metals carbonates increases down the group because down the group polarizing power of cation decreases. So thermal stability increases.Hence, thermal stability order :MgCO3<CaCO3<SrCO3<BaCO3