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(1), g(2), the hyperfine structure constants A(parallel to), A(perpendicular to), the nuclear quadrupole interaction constant Q' and the effective nuclear g value g(n)') for the two trigonal Cr-53(3+) centers I and II in guanidinium aluminum sulfate hexahydrate are theoretically investigated by using the perturbation formulas of the SH parameters for a 3d(3) ion in trigonal symmetry. According to the studies, the impurity-ligand bonding angles beta(i) in center I (or II) are found to be about 1degrees (or 0.9degrees) smaller than the corresponding metal-ligand bonding angles beta(i)(H) 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 oil 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 h in the formula of A(parallel to) (or A(perpendicular to)) with the trigonal distortion of the studied impurity centers for the first time. The validity of the results is discussed. (C) 2004 Elsevier B.V. All rights reserved.