THEORY OF THE ZERO-FIELD SPLITTING OF S-6(3D(5))-STATE IONS IN CUBIC-CRYSTALS

A study is made of the zero-field splitting (ZFS) of S-6(3d(5)) ions i n cubic crystals, based on an extended crystal-field (CF) model which assumes two constants xi(te) and xi(tt) in the description of the spin -orbit (SO) interaction. In addition to the recognized origin for the ZFS, namely, the combined effect of the CF and the SO couplings, a sec ond source is found to arise from the SO interaction alone through a d ifference between xi(te) and xi(tt) caused by covalency. To understand this second effect, we have investigated the SO coupling processes wh ich contribute to the ZFS, using the Macfarlane-Zdansky perturbation p rocedure. Processes in which the couplings are all between states of d ifferent configurations t(2)(m)e(5-m) are found to make a positive con tribution proportional to xi(te)(4). Other processes contribute negati vely through a term in xi(te)(2) xi(tt)(2). The ZFS is thus determined by the relative magnitudes of these two parts, and a small difference between xi(te) and xi(tt) will cause a great change in its value. App lication of this new theory is successfully made to Mn2+ ions in tetra hedral II-VI compounds and in fluoroperovskites.