In situ ²⁹ Si MAS NMR studies of structural phase transitions of tridymite

The structural phase transitions of monoclinic tridymite that occur at elevated temperatures, as well as the structure of the high-temperature modifications, have been studied by ²⁹Si MAS NMR. The sample, originating from the surface of a refractory silica brick, displays sharp discontinuities in the first moment (m₁), second moment (m₂), or both, of the ²⁹Si MAS NMR spectrum at the following phase transitions: 108 ± 1 °C for the monoclinic (MC) to orthorhombic (OP) transition, 165 ±5 °C for the transition from OP to a second orthorhombic phase (OS), and 206 ± 4 °C for the transition from OS to a third orthorhombic phase (OC). Mv for the OC phase becomes more negative with increasing temperature up to ∼323 °C, at which there is also a small discontinuity in m₂, indicating a transition from OC to a hexagonal phase (LHP). The spectrum of the MC phase consists of 12 narrow lines of equal area, consistent with the known crystal structure. The spectrum of the OP phase is broad and asymmetric, indicating a distribution of local environments at the Si sites. It can be simulated assuming an incommensurate structural modulation with a maximum amplitude of ∼5 x 10^-2Å, but it is also consistent with a recent domain-formation model, provided that most disorder occurs at the Si6 site. The spectrum of the OS phase is asymmetric, indicating a nonlinear incommensurate modulation with a maximum amplitude of ∼2 x 10^-2 Å. The spectra of higher temperature phases are consistent with the single Si site of the known crystal structures, but m₁ values imply significantly smaller mean Si-O-Si angles (±), e.g., (±) = 152.6° at 220 °C from NMR, compared with 168.0° from X-ray diffraction data. This difference is consistent with disorder of O atoms around Si-Si axes.