Highly dispersed anti-Stokes phosphors based on KGd$_2$F$_7$:Yb,Er single-phase solid solutions

The possibility of doping the KGd$_2$F$_7$ matrix with ytterbium and erbium ions by introducing yttrium ions with a concentration of 25 mol.% was confirmed and the conditions were determined for the synthesis of anti-Stokes phosphors based on single-phase KGd$_2$F$_7$:Yb,Er solid solutions. The dependences were revealed of the sizes of coherent scattering regions, crystal lattice parameters, and energy yield of luminescence on the temperature and duration of heat treatment. Heat treatment conditions were determined to ensure the achievement of intense anti-Stokes luminescence. As a result, effective phosphors KGd$_2$F$_7$:Yb (20.0 mol.%),Er (4.0 mol.%) with an energy yield of up-conversion luminescence of 3.80% were developed. Disordering of the crystal structure (transition from cubic to tetragonal modification) at a temperature of 600$^\circ$C was recorded, corresponding to the rule of Ostwald steps.