Origin of the Ground Kramers Doublets for Co2+(3d7) Ions with the Effective Spin 3/2 Versus the Fictitious "Spin' 1/2

Experimental spectroscopic and magnetic data for Co2+(3d(7)) ions in various systems are reviewed and critically examined. The focus is on Co2+ ions with the electronic spin S=3/2, properties of which may be interpreted using the spin Hamiltonian with the effective S=3/2 or the fictitious spin' S (S)=1/2. Possible distinct ground states of Co2+(3d(7)) ions arising from crystal field energy levels are discussed. Distinctions between the concepts of the effective spin S and the fictitious spin' S are outlined to clarify the terminological confusion encountered in literature. Sample cases of the ground state assignments and options for the spin' S=1/2 origin are considered for better understanding of the Co2+ ions local environment in various systems, including low symmetry cases. Present study is motivated by potential applications of Co2+(S=3/2) complexes exhibiting very large or moderate zero-field splitting as molecular nanomagnets.