Effect of Codopant Ions on the IR Photoluminescence of Cu2+ Impurity Centers in Corundum (α-Al2O3)

We have studied broadband (1000–1600 nm) IR photoluminescence of polycrystalline corundum (α-Al₂O₃) samples containing copper impurity ions and additionally doped with ions of elements in the oxidation state 4+: Si⁴⁺, Ge⁴⁺, Ti⁴⁺, Zr⁴⁺, Hf ⁴⁺, and Sn⁴⁺. The results demonstrate that the IR photoluminescence intensity in corundum singly doped with copper is rather low. Additional doping of corundum with some tetravalent cations sharply increases the luminescence intensity. The largest increase in Cu²⁺ IR photoluminescence intensity is obtained in the case of additional doping with Ti⁴⁺, Ge⁴⁺, or Sn⁴⁺ cations. It seems likely that these ions ensure charge compensation when incorporated into the corundum lattice together with Cu²⁺ ions, so that the substitution process can be represented as 2Al³⁺ → Cu²⁺ + M⁴⁺ (M⁴⁺ = Ti⁴⁺, Ge⁴⁺, Sn⁴⁺,…). In this way, the additional doping of corundum with tetravalent ions raises Cu²⁺ solubility in α-Al₂O₃, leading to photoluminescence enhancement in the material.