Spin-Hamiltonian parameters for the tetragonal GdM3+-Fi- centers in CaF2 and SrF2 crystals

► Spin-Hamiltonian parameters of GdM3+-Fi- center in CaF2 and SrF2 are calculated. ► The calculation is made by using the diagonalization (of energy matrix) method. ► This diagonalization method is based on one-electron crystal field mechanism. ► Defect structural data of GdM3+-Fi- centers obtained from ENDOR data are confirmed.