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

(hbox {Yb}³⁺/hbox {Er}³⁺), (hbox {Yb}³⁺/hbox {Tm}³⁺), or (hbox {Yb}³⁺/hbox {Tm}³⁺/hbox {Gd}³⁺) co-doped (hbox {KLu}₂hbox {F}₇) up-conversion (UC) materials were synthesized through a hydrothermal method or an additive-assisted hydrothermal method. The X-ray diffraction (XRD) results suggested that the materials crystallized in orthorhombic phase, yet, the potassium citrate (CitK) introduction affected immensely the crystalline purity of final material. The field emission scanning electron microscopy (FE-SEM) results suggested that the additive adding had effects on size and morphology of the material, which affected the UC emissions further. Green/red UC emissions of (hbox {Er}³⁺), UV/blue/IR UC emissions of (hbox {Tm}³⁺), and UV UC emissions of (hbox {Gd}³⁺) were observed in the orthorhombic phase of (hbox {KLu}₂hbox {F}₇) materials. The excitation power-dependent UC emissions illustrated that the UC emission intensity initially increased, then decreased with the increase in excitation power. At the same time, the variation rates of different transitions in (hbox {Er}³⁺) or (hbox {Tm}³⁺) are also different. In addition, the (hbox {Er}³⁺) or (hbox {Tm}³⁺) concentration-dependent UC emission results suggested that the optimal doping concentration of (hbox {Er}³⁺) is 2 mol% and (hbox {Tm}³⁺) is 0.5 mol% with the (hbox {Yb}³⁺) concentration fixed as 20 mol%. The experimental results suggest that the orthorhombic phase of (hbox {KLu}₂hbox {F}₇) should be a good host lattice for UC emitters.