COMMENT ON THE CRYSTALLINE FIELD IN LaS:Er AND LaSe:Er INTERMETALLIC COMPOUNDS.

Electron spin resonance (ESR) measurements yield GAMMA //7 crystalline field ground state for Er in both LaS and LaSe. This in contrast with previous measurements on the analogue intermetallic compounds LaSb:Er and LaBi:Er where GAMMA //8 ground state has been observed. This indicates an appreciable reduction in the crystalline field parameters ratio A//4r**4 direction/A//6 r**6 direction upon going from LaBi and LaSb to LaS and LaSe. Taking the change of the lattice constants into consideration, it was not possible to interpret the appreciable change in A//4r**4 direction/A//6r**6 direction in terms of the point charge model. It seems, however, that the change in A//4r**4 direction/A//6r**6 direction might be associated with the charge on the ligands, noting that both S and Se belong to the VI**t**h column in the periodic table, whereas Sb and Bi belong to the V**t**h.