A comparable study of the optical spectra and EPR parameters for 3d 1 - and 3d 9 -doped MgNH 4 PO 4 ·6H 2 O.

VO 2+ (3d 1 ) and Cu 2+ (3d 9 ) are the two complementary states that usually show opposite distortions when they are doped in crystals. In this work, the optical absorption spectra (OAS), electron paramagnetic resonance (EPR) parameters, and local structure (LS) for VO 2+ (and Cu 2+ ) in MgNH 4 PO 4 ·6H 2 O (MPPH) are uniformly investigated on the basis of the high-order perturbation formulas for a 3d 1 (and 3d 9 ) ion in tetragonally compressed (and elongated) octahedra, respectively. In the calculated formulas, the required crystal-field parameters can be obtained from the superposition model and reasonably linked with the LS distortion for VO 2+ and Cu 2+ centers. Based on the calculations, the tetragonal compressed [VO(H 2 O) 5 ] 2+ cluster (and tetragonal elongated [Cu(H 2 O) 6 ] 2+ cluster) is found to suffer tetragonal compression ratio of 1.65% and tetragonal elongation ratio of 3.8% along C 4 -axis, respectively, due to the Jahn-Teller (JT) effect. The theoretical EPR parameters based on the above lattice distortions agree well with the experimental data, and the LS of the VO 2+ and Cu 2+ centers in MPPH is discussed.