CRYSTALLIZATION OF BETA''-LINH4SO4 AND (NH4)(2)SO4 IN GELS - GROWTH-MORPHOLOGY AND EPITAXY PHENOMENA

The crystallization sequences in the system (NH4)(2)SO4-Li2SO4-H2O at 50 degrees C have been studied for solutions with an initial [(NH4)(2) SO4]/[Li2SO4] molar ratio higher than 1. The crystal growth experiment s were carried out under isothermal conditions in a gel medium, where supersaturation was achieved by ''drowning out''. The phases obtained are beta ''-LiNH4SO4 (LAS) and (NH4)(2)SO4 (AS) The similarities exhib ited by the structures of both compounds favours the development of or ientational overgrowth phenomena. The main epitaxial relationship foun d is the parallelism between [001](LAS) and [10(1) over bar](AS). The most common contact planes are (010)(LAS) and (101)(AS). Although less frequent, other epitaxial relationships have also been observed: (010 )(LAS) parallel to (010)(AS) and [100](LAS) parallel to [100](AS). The development of the overgrowth is interpreted on the grounds of the st ructural considerations and the characteristics of the crystallization medium.