Theoretical investigations of the microscopic spin Hamiltonianparameters including the spin–spin and spin–other-orbit interactions forNi2+(3d8) ions in trigonal crystal fields

Abstract

The microscopic origin of the spin Hamiltonian (SH) parameters forNi2+(3d8) ions in a trigonaltype I symmetry (C3v,D3d,D3) crystal field (CF) is studied. In addition to the spin–orbit (SO) interaction,we consider also the spin–spin (SS) and spin–other-orbit (SOO) interactions.The relative importance of the four (SO, SS, SOO, and combined SO–SS–SOO)contributions to the SH parameters is investigated using the CFA/MSH package andthe complete diagonalization method (CDM). The SO mechanism is dominantfor all CF parameter (CFP) ranges studied, except where the contributionsDSO to the zero-field splitting(ZFS) parameter D change sign. For the trigonal CFP, due to the other three mechanisms exceedsDSO. Although|DSOO| is quite small,the combined |DSO−SOO| is appreciable. The SO-based perturbation theory (PT) works generally well for theg-factors: and , while it fails for D in the vicinity of vc and for large and v>0. The high percentagediscrepancy ratio δD = 2020% forvc indicates unreliabilityof DSO (in PT). Applicationsto Ni2+ ions at trigonalsymmetry sites in LiNbO3, α-LiIO3, andAl2O3, areprovided. The theoretical SH parameters are in good agreement with the experimental data. The low symmetry(C3) effects inducedby the angle φ are tentatively studied, but appear to be quite small.