A process for preparing a Mn+4 doped phosphor of formula I

Ax[MFy]:Mn+4  I



includes combining a first solution comprising a source of A and a second solution comprising H2MF6 in the presence of a source of Mn, to form the Mn+4 doped phosphor;
wherein

A is Li, Na, K, Rb, Cs, or a combination thereof;
M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Y, La, Nb, Ta, Bi, Gd, or a combination thereof;
x is the absolute value of the charge of the [MFy] ion;
y is 5, 6 or 7; and


wherein a value of a Hammett acidity function of the first solution is at least −0.9.




Particles produced by the process may have a particle size distribution with a D50 particle size of less than 10 μm.