Origin of the Crystal Field inTi3+-Substituted Alums

An examination of the relationship between the crystal-field energy-level splittings and the crystal-structure details for several ${\mathrm{Ti}}^{3+}$-substituted alums is presented in an attempt to explain the peculiar paramagnetic-resonance behavior of these salts. With recently acquired data for K${\mathrm{A}\mathrm{l}\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·12${\mathrm{H}}_2$O and Na${\mathrm{A}\mathrm{l}\left(\mathrm{S}{\mathrm{O}}_4\right)}_2$·12${\mathrm{H}}_2$O added to the previously reported results for Rb, Tl, and Cs alums, a clear correlation is found to exist between the magnitude of the lower-symmetry crystal-field component and the respective aluminum-sulfur distances as determined by x-ray crystallography. After a careful study of the unit cell, it is concluded that the probable cause of the orthorhombic character of the crystal field is a displacement of the ${\left(\mathrm{T}\mathrm{i}\mathrm{·}6{\mathrm{H}}_2\mathrm{O}\right)}^{3+}$ ion from the trigonal symmetry axis. The reasons for the distortion are believed to be the positions of the surrounding ${\left(\mathrm{S}{\mathrm{O}}_4\right)}^{2-}$ groups and the radius of the ${\mathrm{Ti}}^{3+}$ ion, which is significantly larger than that of the host ${\mathrm{Al}}^{3+}$ ion.