Figure 4. Doping dependent δ values in (a) Yb³⁺-Er³⁺ doped CaWO₄, (b, c, d, e) Yb³⁺-Er³⁺-xLi⁺ (x = 0.1, 0.2, 0.3, and 0.5 mol%) doped CaWO₄, (f,g,h,i) Yb³⁺-Er³⁺-ySr²⁺ (y = 0.5, 1.0, 1.5 and 2.0 mol%) doped CaWO₄, and (j,k,l,m) Yb³⁺-Er³⁺-zMg²⁺ (z = 0.25, 0.5, 1.0, and 1.5 mol%) doped CaWO₄.