THE STABILITY AND VIBRATIONAL-SPECTRA OF 3-RING CONTAINING ZEOLITIC SILICA POLYMORPHS

Zeolitic silica polymorphs containing three-membered rings are modeled using three different atomistic potential sets for lattice energy minimization calculations of their structure and using a general valence force field to calculate vibrational spectra. In some cases, large deviations from the experimental symmetry occur upon energy minimization. The lattice energy calculations show that structures with tetrahedra that participate in one or two three-rings can exist in the pure silica form. A hypothetical structure in which all tetrahedrally coordinated atoms are part of a three ring, and with a corresponding very low framework density, is shown to be significantly less stable. The calculated vibrational spectra are very sensitive to small-scale structural features like bond lengths and bond angles. Because three-ring structures generally have small Si-O-Si angles compared with other zeolites, relatively low-frequency asymmetric stretch modes are predicted. No specific spectral peak can be assigned to a three-ring mode. The differences in structural detail computed using the three potential sets for silica produce variations in predicted infrared spectra that are larger than found between different structural polymorphs. This indicates the need for further improvement of potential parameters applicable to structure relaxation.