Investigations of MX-1 tridymite by 29Si MAS NMR ? Modulated structures and structural phase transitions

Abstract

MX-1 tridymite is one of the room-temperature polymorphs of SiO2 tridymite and has an underlying monoclinic structure (Cc) with incommensurate modulations along a* and c* (Hoffmann et al. 1983; Lons and Hoffmann 1987). With increasing temperature up to 500° C, MX-1 is reported to experience at least five structural phase transitions. However, its structures and the relationships to other tridymite polymorphs are unclear. We present here a 29Si MAS NMR study of the room-temperature incommensurate structure of MX-1 and its structural phase transitions up to 540° C. Our results suggest that at room temperature, all the Si sites in MX-1 tridymite are in positions with similiar ∠Si-O-Si of ∼150° and are consistent with the presence of two incommensurate modulations proposed by Hoffmann et al. (1983). Simulations of the spectra yield modulation amplitudes of 1.33 and 0.87 ppm, corresponding to 0.009 and 0.006 A for Si-Si. The maximum atomic displacements along a and b due to the modulations appear to be ∼0.01–0.02 A. The structural phase transitions of MX-1 are significantly different from those of MC tridymite below 220° C. Our high temperature results confirm that MX-1 tridymite transforms to the H5 phase at about 65° C. The most important transition occurs near 110° C, where the H5 phase transforms to a phase yielding a single, narrow NMR peak, indicating the disappearance of the superstructure and possibly the onset of the dynamic averaging. The NMR lineshapes of H5 are consistent with the metrically orthorhombic unit cell and commensurate superstructure of 2a, 2b and 10c proposed by Graetsch and Florke (1991). The phase present above 110° C is probably similar to the OC phase, but has a mean ∠Si-O-Si of ∼152.0° at 113° C, 152.9° at 185° C and 154.1° at 500° C. The transitions at ∼160 and 220° C for MX-1 are subtle and probably due to impurity MC. Analysis of the modulations in the OS phase of MC tridymite indicates that their amplitudes are of the order of 0.02 A, significantly less than the value 0.3 A proposed by Nukui et al. (1979).