the invention provides a lighting unit comprising a light source , configured to generate light source light and a 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 and wherein the luminescent material comprises a phosphor comprising m 2 ax 6 doped with tetravalent manganese , wherein m comprises monovalent cations , at least comprising potassium and rubidium , wherein a comprises a tetravalent cation , at least comprising silicon , wherein x comprises a monovalent anion , at least comprising fluorine , and wherein m 2 ax 6 has the hexagonal phase .