Static and dynamic structure of deca-dodecasil 3R
Paper in proceedings, 2005
The structure of calcined deca-dodecasil 3R (DD3R), with formula Si120O240, a very well-suited material for the synthesis of inorganic/organic composites structured on a nanometer level, was investigated in details. So far, a highly complicated twinning has hampered its structure description on a desirable level of precision and accuracy. This twinning has been resolved and a new structure determination was completed. Structure refinement done in the R-3 space group revealed a rather large Ueq of a bridging oxygen (O), shorter than expected bridging Si - O bonds and the straight Si-O-Si bond angle dictated by the atom positions at a threefold axis. A structure model based on statically displaced bridging O-atom on one hand improved accuracy of Ueq s and of the Si-O bonds of interest, but provided unacceptable O-O contacts. To explain this dilemma ab initio molecular dynamics calculations were done in order to inspect possible configurations. The finite-temperature simulations of the dynamical properties were done at 300 K and a canonical ensemble with a Nosé thermostat procedure was used. The Verlet velocity algorithm with (i) a time step ∆t = 1fs (fast sampling) and simulation time of 4 ps and (ii) ∆t = 4fs (slow sampling) and simulation time of 20 ps were used. The x,y coordinates of Si-O-Si atoms corresponded to the time-averaged structure with all three atoms are on a threefold axis. It was thus evident, that structure analysis itself cannot provide a completely true picture of DD-3R structure. To reveal possible dynamic disorder in the structure and to analyze the spatial distribution of the atoms during a longer time interval, the 20ps MD simulation was used. To un-reveal basic features hidden in such a time series wavelet denoising and non-decimating wavelet transform were applied. Synchronous changes of both Si-O distances pointed to synchronous movement of the whole building units (tetrahedra). To calculate the frequency of such a Si-O-Si bending vibration total and partial vibrational density of states were calculated from atom velocities. The interpretation of calculated spectra could be based on INS spectra of various silica polymorphs collected by several authors. It can be concluded that a good agreement of the low frequency features in the INS spectra with our results from MD indicate, that the nature of disorder in DD-3R is dynamic rather than static.
X-ray structure analysis
molecular dynamics calculation