Trivalent Nd, Dy, Ho, Er, Tm, Sm and Eu usually act as electron trapping centers in wide band gap compounds, whereas trivalent Ce, Tb and Pr act as hole trapping centers. When a deep electron trap is combined with a shallow hole trap, then during the thermoluminescence glow the hole is released, generating recombination luminescence at the electron trap. However, in the case of a shallow electron trap, the electron will be released to recombine at the hole trapping center. With the knowledge of the location of the lanthanide levels within the band gap, one may engineer the depth of the electron trap, the depth of the hole trap, and where the recombination will take place. This has all been tested and verified for the lanthanides in GdAlO₃ (Luo et al., J. Phys. Chem. C., 2016, 120, 5916). In this work, Cr³⁺ is combined with various trivalent lanthanides in GdAlO₃. By combining thermoluminescence with optical spectroscopy data, a consistent interpretation of all the data is obtained. Cr³⁺ can act both as a deep electron trap and a deep hole trap, which is different to all of the lanthanides. From the results we can deduce the location of the Cr²⁺ and Cr³⁺ levels within the band gap and with respect to the vacuum level. Besides thermoluminescence recombination via the conduction band, evidence is found for athermal (tunneling) recombination. The results for GdAlO₃ are compared with the results for LaAlO₃. It was found that they are related systems but LaAlO₃ has a lower lying conduction band and a higher lying valence band, which reduces the trap depths of the lanthanides and Cr in a predictive fashion.

In this work it is shown that CaZnOS:Eu²⁺ has no Eu²⁺ emission even at low temperature. The observed and earlier reported red emission originates from a CaS:Eu²⁺ impurity phase. By means of washing the as-prepared samples with diluted nitride acid, we were able to remove the CaS impurity phase and study the Eu2+ emission in the pure CaZnOS phase. A clear relation was found between the red emission intensity, the CaS XRD line intensities and the nitric acid solution washing time, with zero intensity after prolonged washing. A so-called VRBE (vacuum referred binding energy)-diagram was constructed showing the energy of the 4fⁿ and 4f^n-15d¹ states of the divalent and trivalent rare earth ions as dopants in CaZnOS with respect to the vacuum energy. This diagram shows that the 5d-levels of Eu²⁺ are located in the conduction band, which explains the absence of 5d→4f emission. By comparing the VRBE diagram with diagrams of other related compounds like CaO, CaS, ZnO and ZnS it becomes clear that the Eu²⁺ luminescence quenching is caused by a low lying conduction band, typical for Zn-based compounds.

Photoluminescence excitation (PLE) and emission spectra (PL) of undoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ as well as Eu³⁺- and Ce³⁺-doped samples have been investigated. The PL spectra show that Eu³⁺ enters into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce³⁺ gives two broad excitation bands in the range of 200−450 nm. First-principle calculations for Ce³⁺ on both dodecahedral and octahedral sites provide sets of 5d excited level energies that are consistent with the experimental results. Then the vacuum referred binding energy diagrams for (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ have been constructed with the lanthanide dopant energy levels by utilizing spectroscopic data. The Ce³⁺ 5d excited states are calculated by first-principles calculations. Thermoluminescence (TL) glow curves of (Ce³⁺, Sm³⁺)-codoped (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂ samples show a good agreement with the prediction of lanthanide trapping depths derived from the energy level diagram. Finally, the energy level diagram is used to explain the low thermal quenching temperature of Ce³⁺ and the absence of afterglow in (Sr, Ca)₃(Y, Lu)₂Ge₃O₁₂.