Electronic properties and vibrational spectra of (NH₄)₂M″(SO₄)₂·6H₂O (M = Ni, Cu) Tutton's salt: DFT and experimental study

The complex crystals of the family of the Tutton's salt have been investigated through the numerous experimental and theoretical studies to understand their physical properties and their potential technological applications. In spite of the more than 60 years of research, there are very few studies about the electronic properties of Tutton's salt. In our present work, we have calculated the stability, electronic properties and the first theoretical study of band structure of the three different crystals of the Tutton's salt, ammonium nickel sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O), ammonium nickel-copper sulfate hexahydrate ((NH₄)₂Ni_0.5Cu_0.5(SO₄)₂·6H₂O) and ammonium copper sulfate hexahydrate ((NH₄)₂Ni(SO₄)₂·6H₂O) with the help of periodic ab-initio calculations based on density functional theory (DFT). In addition to this, the internal Raman and FTIR modes of the ionic fragments [Ni(H₂O)₆]²⁺, [Cu(H₂O)₆]²⁺ NH₄⁺ and SO₄^2- of the sample crystals were obtained by employing the ab initio and the orientation of the molecular vibrations of the ionic fragments have been presented in picturized form. Furthermore, the Raman and FTIR spectroscopy of the sample crystals were measured in the range 100-4000 cm^-1 and 400-4000 cm^-1 respectively, and the internal vibrational modes obtained from experimental measurement have been compared with those obtained from DFT calculations.