Mixed rare earth hydrogen selenite crystals, neodymium praseodymium hydrogen selenite (NdxPr1−x(HSeO3)(SeO3)⋅2H2O), Neodymium samarium hydrogen selenite (NdxSm1−x(HSeO3)(SeO3)⋅2H2O) and praseodymium samarium hydrogen selenite (PrxSm1−x(HSeO3)(SeO3)⋅2H2O) were prepared by gel diffusion technique. Simultaneous thermogravimetric and differential thermal analysis were carried
out on the grown crystals. Decomposition is observed to occurs in six steps, which gives the evidence of successive losses
of H2O and SeO2. The final product due to decomposition is a mixed rare earth oxides. FT-IR spectrum of the crystal samples heated at different
temperatures complemented to the TG-DTA results.