Atomic thermal parameters and thermodynamic functions for chrysoberyl (BeAl2O4) from vibrational spectra and transfer of empirical force fields

Using empirical atomic charges and valence force fields derived from a best fit to the Raman-IR spectra, and applying a rigid-ion Born-von Karman model, atomic thermal parameters have been calculated for chrysoberyl (BeAl2O4). The agreement with the experimental values (redetermined here for this purpose) is good; and excellent agreement with the experimental data is obtained for the estimates of some thermodynamic functions such as entropy and the molar heat. As for other oxides and silicates, the zero-point contribution is particularly important with respect to both the vibrational energy and to the mean-square amplitude of motion (about 83 and 70% of the value at room temperature, respectively). In order to test transferability of the force fields, the vibrational frequencies have also been calculated using empirical valence force-field parameters derived from a best fit to the Raman-IR spectra of corundum (Al2O3) and bromellite (BeO), without fitting the chrysoberyl spectra. The agreement with the experimental values is good, especially for the low frequencies. However, there is significant disagreement for the higher frequencies, which apparently require larger stretching-force constants than for both corundum and bromellite: These constants also appear to be more dependent upon bond distance (especially for the Be-O bonds). This behaviour may reflect some inadequacy of the rigid-ion model used here for these calculations. However, the agreement with the experimental atomic thermal parameters and thermodynamic functions is good, because the lowest energy vibrational levels (corresponding to the softest modes, including the acoustic branches near the origin of the Brillouin zone) are the most important for these applications. (orig.)