The invention provides a lighting unit comprising a light source, configured to generate light source light and a particulate luminescent material, configured to convert at least part of the light source light into luminescent material light, wherein the light source comprises a light emitting diode (LED), wherein the particulate luminescent material comprises particles comprising cores, said cores comprising a phosphor comprising M′xM2-2xAX6 doped with tetravalent manganese, wherein M′ comprises an alkaline earth cation, M comprises an alkaline cation, and x is in the range of 0-1, wherein A comprises a tetravalent cation, at least comprising silicon, wherein X comprises a monovalent anion, at least comprising fluorine, and wherein the particles further comprise a metal phosphate based coating, wherein the metal of the metal phosphate based coating is selected from the group consisting of Ti, Si and Al.