Core-shell materials with red-emitting phosphors

A process for preparing a coated phosphor, the process comprising contacting a suspension of a phosphor material in particulate form in a HF solution, with a source of Mn4+, A+, and M, to form a coated phosphor having a shell comprising a first phosphor of formula I,

• Ax[MFy]:Mn4+ I
• directly disposed on a core comprising a second phosphor;
• and
• converting the coated phosphor to a color stable coated phosphor by contacting the already formed coated phosphor:
• with a fluorine-containing oxidizing agent in gaseous form at an elevated temperature ranging from about 350 degrees Celsius to about 700 degrees Celsius,
• and with a compound of formula II, Ax[MFy] in an aqueous hydrofluoric acid, wherein the second phosphor is a material other than a compound of formula I or formula II;
• and
• Ax[MFy] II
• wherein:
• contact with the compound of formula II in the aqueous hydrofluoric acid is one of:
• 1, before, or 2, both before and after, the coated phosphor is contacted with the fluorine-containing oxidizing agent in gaseous form at the elevated temperature ranging from about 350 degrees Celsius to about 700 degrees Celsius,
• A is, independently at each occurrence, Li, Na, K, Rb, Cs, or a combination thereof;
• M is, independently at each occurrence, Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Hf, Y, La, Nb, Ta, Bi, Gd, or a combination thereof;
• x is the absolute value of the charge of the [MFy] ion;
• and
• y is 5, 6 or 7.