Raman study of high-pressure phase transitions in dehydrated analcime

The high-pressure behaviour (up to 30 kbar) of dehydrated analcime, Na[AlSi 2O6], has been studied in detail using polarized microscopy and Raman micro -spectroscopy. Samples were compressed using a diamond anvil cell in a quas i-hydrostatic medium (glycerol or water-ice). Two transitions at 3.7 and 11 kbar were observed. At the first transition, phase II is observed under cross polarized light a s a contrasting black zone, moving from the edge to the centre of the sampl e. At this transition the strong Raman doublet at similar to 480 and 500 cm (-1) transforms discontinuously to a singlet, which is similar to that of q uasi-cubic natural analcime. This transition, with an increase of effective symmetry, seems unusual on increasing pressure. The analcime Framework may be thought of as an array of four-, six-, or eig ht-membered interconnected rings. Phase transitions in the analcime group a re considered in the literature in terms of deformations of the six-membere d rings. However, the present work shows that these phase transitions shoul d be associated with deformations of the four-membered rings las the minimu m size secondary building units by rotation of rigid TO4 units. The stronge st Raman band frequency is correlated with the mean T-O-T angle inside the four-membered ring: a rate of 4.5 cm(-1)/degree was found for analcimes and leucites. Using this correlation, one can estimate from Raman data the pos sible deformation of four-membered rings at the phase transitions.