Vibrational Spectral Analysis of Natisite (Na₂TiSiO₅) and its Structure Evolution in Water and Sulfuric Acid Solutions

Natisite (Na₂TiSiO₅) is a layered sodium titanosilicate containing TiO₅ square pyramids. The structure evolution of natisite in water and acid solutions is the basis for its potential applications. With Na₂SiO₃ as the silicon source, natisite with the shape of the square sheet was selectively prepared from the hydrothermal method with 14.3 mol/L NaOH solution at 240 °C. Natisite has 20 Raman active modes and 22 infrared active modes from the first-principles calculations within density functional theory, and the calculated Raman and infrared spectra agree well with the experimental ones. The characteristic Raman peak at 844 cm^-1 is caused by the symmetric stretching of the apical Ti-O bond in the TiO₅ unit, assigning to A_1g and B_2g modes. Natisite remains relatively stable in water with a sodium leaching percentage of lower than 6%. When washing with sulfuric acid solutions, the interlayer spacing of natisite is reduced due to the extensive removal of sodium ions, and an intermediate composed of SiO₄ and newly formed TiO₆ units may be formed. Moreover, after washing with water and acid solutions, 95.5%, 63.4%, and 35.2% of Na, Si, and Ti in natisite can be leached in total, respectively, resulting in the structural disintegration of natisite.