An investigation of the properties of large crystals of the zeolites dodecasil-3C and ferrierite by high-temperature birefringence microscopy and X-ray diffraction

Optical birefringence has been measured as a function of temperature for two types of siliceous zeolite crystals that contain organic template molecules. The specimens were prepared using modified solvothermal synthesis to produce large (∼1 mm dimension) crystals. In the case of the clathrasil dodecasil-3C the material undergoes a first-order phase transition at ∼433 K that is reversible after heating to 873 K and cooling to room temperature. Comparison with powder X-ray diffraction data from a bulk sample shows that this is a ferroelastic tetragonal (I2d) to cubic (Fdm) transition, which is supported by the functional form of temperature variation of the birefringence. There is apparently no loss of the organic template involved in this transition. For the zeolite ferrierite, the plate-like crystals show a pronounced domain-like structure, which, although not due to twinning, shows a distinctive optical birefringence change on heating, suggesting that variable concentrations of organic template might be present in different domain-like regions. In this material there is no evidence for a change in crystal symmetry up to 873 K (Pnnm), despite apparent loss of some organic template from the material which, in turn, gives rise to strain birefringence at the edges parallel to the [010] direction.