Figure 5.

a) Room‐temperature upconversion emission spectra of KMg_(1–x–y)F₃:Yb³⁺ _y/Mn²⁺ _x (0.01 ≤ x ≤ 0.40; y = 0.005); all samples were excited with a 976 nm LD at a power density of 10 W cm⁻²; b) proposed two‐photon upconversion mechanism for KMgF₃:Yb³⁺,Mn²⁺; c) room temperature upconversion emission spectra of AB_(1–x–y)F₃:Yb³⁺ _y/Mn²⁺ _x (x = 0.20; y = 0.005; A = K, Rb, Cs; B = Mg, Zn, Cd); here, too, all samples were excited with a 976 nm LD at a power density 10 W cm⁻²; d) emission peak positions of VIS and NIR emissions from AB_(1–x–y)F₃:Yb³⁺ _y/Mn²⁺ _x (x = 0.20; y = 0.005) as a function of the lattice constant a for perovskite KMgF₃, KZnF₃, KCdF₃, RbCdF₃, and CsCdF₃; and e) Tanabe–Sugano energy diagram of a 3d⁵ system in an octahedral crystal field.