Previously reported ABF3-type ferrimagnets with the hexagonal BaTiO3 structure all contain Ni++ as the predominate magnetic ion. This paper reports the discovery of new ferrimagnetic compositions in the RbMgF3Single Bond signRbCoF3 system where Co ++ is the only magnetic ion. Phase studies on this system show that at high temperatures the hexagonal phase is stable to about 70% RbCoF 3. Single crystals were grown over this compositional range and their magnetic and optical properties studied. Compositions containing in excess of about 35% RbCoF3 are ferrimagnetic and their magnetic moments (as high as 22 emu/g) and Curie temperatures (up to 36°K) vary with Co ++ concentration. The magnetization data are consistent with a model of a 1/3-2/3 ferrimagnetic spin arrangement. The B-B interaction, which is the resultant of the 90°Co++Single Bond signFSingle Bond signCo ++ and the direct Co++Single Bond signCo+ interactions, appears to be strongly positive. Optical absorption data were used to determine the magnitude of the crystal field parameter Dq (870-880 cm-1) and the spin orbit interaction constant λ (190 cm-1). These data further indicate the strength of the crystalline field acting on the Co++ ion in the hexagonal structure, where 2/3 of the CoF6 octahedra share faces, is similar to that in "cubic" KCoF3 and RbCoF3 where they share corners. © 1969 The American Institute of Physics.