The combined inelastic neutron scattering and solid state DFT study of hydrogen atoms dynamics in a highly ordered kaolinite

The dynamics of the hydrogen atoms in the highly ordered kaolinite was studied by vibrational spectroscopy based on inelastic neutron scattering method with the focus on the spectral region of 100–1,250 cm⁻¹. The experimental spectrum was interpreted by means of the solid state density functional theory calculations covering both normal mode analysis and molecular dynamics going beyond the harmonic approximation. The Al–O–H bending modes were found to be spread over the large interval of 100–1,100 cm⁻¹, with the dominant contributions located between 800 and 1,100 cm^-1. The shapes of the individual hydrogen spectra depend on the strengths of the individual interlayer O–H···O hydrogen bonds involving the inner surface hydroxyl groups. The modes assigned to the in-plane movements of the respective hydrogen atoms are well-defined and always appear on the top of the intervals of energy transfer. In contrast, the modes generated by the out-of-plane movements are spread over large intervals of energies spanning down to the region of external (lattice) modes.