Electron paramagnetic resonance of chromium(III) ion in α, β and γ alums

The sign of the zero-field splitting (ZFS) of the ground state of Cr3+ ion in the α alums CsCr(SeO4)2.12H2O and CsAl(SeO4)2.12H2O has been determined by axial stress electron paramagnetic resonance. The angular overlap model is used to explain the signs and magnitudes of the ZFS in alums. For β alums the ZFS is mainly determined by the trigonal compression of the Cr(OH2)63+ ion with a minor contribution coming from the outer coordination sphere. The same obtains for α alums but, in addition, there is a larger contribution of opposite sign that arises from the misaligned valence orbitals and dipoles associated with the pyramidal geometry of the OH2 ligand.