Color stable phosphors for LED lamps and methods for preparing them

A method for preparing a color stable phosphor, said method comprising contacting a phosphor of formula I in particulate form with a coating precursor at an acidic or basic pH in a solvent comprising water;

• ((Sr1-zMz)1-(x+w)AwCex)3(Al1-ySiy)O4+y+3(x−w)F1-y-3(x−w) I
• wherein
• the coating precursor is selected from tetraalkylothosilicates, fumed silica, alkali metal silicates, phosphates of formula Rn(PO4)m, and combinations thereof;
• A is Li, Na, K, or Rb, or a combination thereof;
• M is Ca, Ba, Mg, Zn, or a combination thereof;
• 0<x≦0.10, 0≦y≦0.5, 0≦z≦0.5, 0≦w≦x;
• R is hydrogen, NH4, Li, Na, K, Rb, or a combination thereof;
• with the proviso that when the coating precursor is fumed silica, pH is less than or equal to about 4.

An LED lamp comprising:

• a light source configured to emit radiation with a peak intensity at a wavelength between about 250 nm and about 550 nm;
• and
• a phosphor composition configured to be radiationally coupled to the light source, and comprising particles of a phosphor of formula I, said particles having a coating composition disposed on surfaces thereof;
• ((Sr1-zMz)1-(x+w)AwCex)3(Al1-ySiy)O4+y+3(x−w)F1-y-3(x−w) I
• wherein
• the coating composition comprises aluminum oxide, magnesium oxide, calcium oxide, barium oxide, strontium oxide, zinc oxide, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, barium hydroxide, strontium hydroxide, zinc hydroxide, aluminum phosphate, magnesium phosphate, calcium phosphate, strontium phosphate, barium phosphate, silica-containing material derived from a precursor selected from tetraalkylothosilicates, fumed silica, alkali metal silicates, or a combination thereof;
• and
• A is NH4, Li, Na, K, or Rb, or a combination thereof;
• M is Ca, Ba, Mg, Zn, or a combination thereof;
• and
• 0<x≦0.10, 0≦y≦0.5, 0≦z≦0.5, 0≦w≦x.