A study of vibrational modes in Na(+)beta-Alumina by molecular-dynamics simulation

• The vibrational properties of crystalline Na+ beta-alumina (Na1.22Al11O17.11) have been studied using the molecular dynamics simulation technique. The vibrational density of states was calculated from the velocity autocorrelation function, and the infrared spectrum from the dipole-dipole autocorrelation function. Knowledge of the vibrations in different crystallographic directions for the different atomic species facilitates the assignment of spectral peaks. The sodium in-plane vibrations are 59, 88 and 112 cm-1, and the out-of-plane vibrations are at 146 cm-1. The stoichiometric compound is also studied, and in this case the sodium in-plane vibrations are at 80 cm-1 and the out-of-plane vibrations at 140 cm-1. The density of states is used to calculate thermodynamic properties: heat capacity, entropy and internal and free energy. The values obtained at 300 K are C(upsilon) = 410 J K-1 mol-1, S(upsilon) = 300 J K-1 mol-1, U = 370 kJ mol-1 and F = 280 kJ mol 1. The heat capacity and entropy values are in good agreement with experiment, and thus strongly support the empirical force field used in the simulation

Nyckelord

vibrational modes

NATURAL SCIENCES

NATURVETENSKAP

Publikations- och innehållstyp

ref (ämneskategori)

art (ämneskategori)