Structure, thermal and optical properties of hydrogen bond stabilized hybrid rare-earth selenates: [(CH3)3NH][Ln(H2O)4(SeO4)2], (Ln = Pr, Nd)

•The double selenates of trimethylammonium and Pr3+/Nd3+ ions were isolated.•All the trimethylammonium lanthanide selenates are isostructural and have triclinic (P-1) lattice.•Structure is stabilized by network of Cyclic and Acyclic bifurcated H-bonds and almost linearly arranged Ln3+ ions.•Solid-State Fluorescence, Reflectance, Vibrational properties, and stability of the compounds are presented.•The compounds decompose to their respective lanthanide oxide at high temperature.