Synthesis and up-conversion emissions of $$\hbox {Yb}^{3+}{/}\hbox {Er}^{3+}$$ Yb 3 + / Er 3 + , $$\hbox {Yb}^{3+}{/}\hbox {Tm}^{3+}$$ Yb 3 + / Tm 3 + and $$\hbox {Yb}^{3+}{/}\hbox {Tm}^{3+}{/}\hbox {Gd}^{3+}$$ Yb 3 + / Tm 3 + / Gd 3 + co-doped $$\hbox {KLu}_{2}\hbox {F}_{7}$$ KLu 2 F 7