you are way too cryptic! Unless you specify what kind of reaction you do, it's kind of hard to guess! Remember carbonates decompose into oxides if you do not pay attention

You should be ready that high amounts of CO2 release due to solid state reaction. It produces many pores in the compound.

They decompose at very high temp. and their solubility is poor in many solvents

CaCO3 decompose at above 860-900C and BaCO3 decompose at above 1300C to their respective oxides. So, you hav to pay attention in heating

CaCO3 decomposes at a higher temperature than BaCO3 which should be taken into consideration

Please specify your compound. A single compound can have many different phase, which will be reflected in XRD. For example when your are preparing (Ba,Ca)TiO3, you will endup with 2-3 different phases even after heating till 1300C. There are methods to avoid it.