Theoretical investigations of the spin Hamiltonian parameters for the two trigonal Cr3+ centers in GASH

The spin Hamiltonian (SH) parameters (zero-field splitting D, anisotropic g factors g∥, g⊥, the hyperfine structure constants A∥, A⊥, the nuclear quadrupole interaction constant Q′ and the effective nuclear g value gn′) for the two trigonal 53Cr3+ centers I and II in guanidinium aluminum sulfate hexahydrate are theoretically investigated by using the perturbation formulas of the SH parameters for a 3d3 ion in trigonal symmetry. According to the studies, the impurity-ligand bonding angles βi in center I (or II) are found to be about 1° (or 0.9°) smaller than the corresponding metal-ligand bonding angles βiH of the host Al3+ sites, and then the trigonal distortions for the impurity centers are also smaller than those in the hosts. The calculated SH parameters based on the above local angles are in good agreement with the observed values of EPR and ENDOR measurements. Meanwhile, the negative D values for both centers are theoretically verified. Further, the quantitative expressions are established to associate the orbital coefficients a and b in the formula of A∥ (or A⊥) with the trigonal distortion of the studied impurity centers for the first time. The validity of the results is discussed.