  It is shown that the d8 electron system occurring in the Ni2+ ion can have
the non-magnetic ground state in the atomic scale formed by the Jahn-Teller
effect. The tetragonal/trigonal off-cubic lattice distortion for the octahedral
site causes the splitting of the triplet 3A2g into a singlet-doublet structure
with the non-magnetic singlet lying lower provided the intra-atomic spin-orbit
coupling is taken into account.
