A red-luminescent long-afterglow phosphor, represented by Mg.sub.xZn.sub.yGe.sub.zO.sub.3: aMn.sup.2+, bEu.sup.3+, cR.sup.3+. The phosphor is a kind of oxide compound based white powder. The phosphor powder synthesis process is environmentally friendly, no sintering-assisted gas required and no harmful gas generated during or after sintering. The phosphor powder can be excited by UV and is chemically stable.

The present invention relates to compounds of formula I,

M.sup.IM.sup.II.sub.3 M.sup.III.sub.3M.sup.IV.sub.3N.sub.2O.sub.12:EuI

wherein, M.sup.I, M.sup.II, M.sup.III, and M.sup.IV have one of the meanings as given in claim 1, to a process of their preparation, the use of these compounds as conversion phosphors or in an emission-converting material, the use of these phosphors in electronic and electro optical devices, such as light emitting diodes (LEDs) and solar cells, and especially, to illumination units comprising at least one of these phosphors.