Dissymmetrization of crystal structures of alum with anomalous birefringence

Dissymmetrization of alpha -alum crystals of two isomorphous series, (NH4,K )Al(SO4)(2).12H(2)O and K(Al,Cr)(SO4)(2).12H(2)O, with different values of anomalous birefringence has been studied by X-ray diffraction methods. Diss ymmetrization is revealed mainly from the comparison of the intensities of crystallographically equivalent reflections in the space group Pa (3) over bar characteristic of alum. The degree of dissymmetrization depends on the position in which the isomorphous replacements occurs, as well as on the gr owth sector of the crystal, and shows a good correlation with birefringence . (C) 2000 MAIK "Nauka/Interperiodica".