Mas Nmr Spectroscopic Study of the Polytypes of the Titanosilicate Penkvilksite

Simulations of 29Si and 23Na MAS NMR spectra indicate the presence of differing proportions of two polytypes and a third phase in three samples of penkvilksite, Na4Ti2Si8O22•5H2O. The most abundant peaks are assigned to the 2O and 1M polytypes, and a third set of peaks are provisionally assigned to a proposed 2M polytype. The quadrupolar interaction parameters for 23Na, Cq and , were calculated using the fullcrystal linearized augmented plane-wave model (FC LAPW) for the 1M and 2O polytypes. The values obtained for Cq from the simulations of 23Na spectra and from the LAPW calculations for the 1M polytype are close, 3.3 and 3.5 MHz, respectively. However, for the 2O polytype, these values are 3.3 and 1.5 MHz, indicating that the Rietveld refinement of the structure for the 2O polytype is not as accurate as the single-crystal refinement of the 1M polytype, from which the 2O structure was optimized to obtain a value of Cq closer to the measured one. The ratios of these polytypes for the three samples of penkvilksite, obtained independently from simulations of 29Si and 23Na spectra, agree within 9% for the 1M and 2O polytypes.