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Two optical bands and four EPR (or spin-Hamiltonian) parameters (g factors g , g ⊥ and hyperne structure constants A , A ⊥ ) of the tetragonally-compressed Nb 4+ octahedral center in glasses are calculated from two methods, the complete diagonalization (of energy matrix) method and the perturbation theory method. Both methods are founded on the two-spinorbit-parameter model where both the contribution to the spectral data from the spinorbit parameter of central d n ion and that of ligand ion via covalence eect are considered. The calculated results from both the complete diagonalization (of energy matrix) method and perturbation theory method show reasonable agreement with the experimental values. The signs and possible misprint for the observed hyperne structure constants A and A ⊥ are suggested, and the local structure of the tetragonal Nb 4+ center in glasses due to the JahnTeller eect is gained from the calculations. The results are discussed.

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