A STUDY OF VIBRATIONAL-MODES IN NA-ALUMINA BY MOLECULAR-DYNAMICS SIMULATION( BETA)

The vibrational properties of crystalline Na+ beta-alumina (Na1.22Al11 O17.11) have been studied using the molecular dynamics simulation tech nique. The vibrational density of states was calculated from the veloc ity autocorrelation function, and the infrared spectrum from the dipol e-dipole autocorrelation function. Knowledge of the vibrations in diff erent crystallographic directions for the different atomic species fac ilitates the assignment of spectral peaks. The sodium in-plane vibrati ons are 59, 88 and 112 cm-1, and the out-of-plane vibrations are at 14 6 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 vib rations at 140 cm-1. The density of states is used to calculate thermo dynamic properties: heat capacity, entropy and internal and free energ y. The values obtained at 300 K are C(upsilon) = 410 J K-1 mol-1, S(up silon) = 300 J K-1 mol-1, U = 370 kJ mol-1 and F = 280 kJ mol 1. The h eat capacity and entropy values are in good agreement with experiment, and thus strongly support the empirical force field used in the simul ation.