Ab initio quantum mechanical study of akdalaite (5 Al2O3 · H2O): structure and vibrational spectrum

The structure and the vibrational spectrum of akdalaite (5Al 2 O 3 ·H 2 O, also known as tohdite) have been investigated at the periodic ab-initio quantum-mechanical level by using a high quality Gaussian type basis set and the hybrid B3LYP Hamiltonian with the CRYSTAL06 code. Three space groups proposed in the literature, namely P6 3 mc and its two P31c and Cmc2 1 subgroups, have been considered, obtaining essentially the same energy (the largest total energy difference is 0.2 kJ/mol per cell) and geometry. The harmonic frequencies at the τ point have been computed. Isotopic substitution and graphical representation permit a complete classification of normal modes in terms of simple models (octahedra and tetrahedra modes, hydrogen stretching and bending). The Al-O octahedra and tetrahedra modes appear below 880 cm -1, Al-OH bending modes are located in the range 870-900 cm -1, and OH stretching modes are at 3330-3400 cm -1.