Structural modulations and phase transitions in beta-eucryptite: an in-situ TEM study

Beta-eucryptite as grown by the flux method has been investigated by in-sit u cold- and hot-stage transmission electron microscopy (TEM). Using electro n diffraction and dark-field TEM imaging, we found no evidence for the mero hedral twinning that accompanies structural collapse at the beta-to-alpha-q uartz transition, suggesting a true hexagonal symmetry for the aluminosilic ate framework of beta-eucryptite. Selected-area electron diffraction (SAED) patterns exhibited a variety of incommensurate structures along the three a axes with an average modulation period of about 6.5 a. These modulated st ructures arise from superperiodic stacking parallel to (100) of two structu ral units with different Li configurations. In-situ heating experiments sho w that the modulated structures undergo a reversible transition to the diso rdered modification at elevated temperatures. This transition arises from p ositional order-disorder of Li ions along the main structural channels. In addition, it appears that at constant low temperature, electron beam irradi ation alone can induce a sequence of transitions similar to those that are thermally induced from the ordered modification to modulated states.