Investigation on the valence state stability and optical properties of Mg2GeO4:Cr

In this work, the valence states and optical properties of chromium-doped Mg2GeO4 are investigated via first-principles methods. The formation energies and thermal charge transition levels show that the site occupation and valence states of chromium are strongly related to the growth condition: Cr3+ dominates in the Mg sites under Mg-poor conditions, and Cr4+ dominates in the Ge sites under Ge-poor conditions. With the lithium co-doping, the Fermi-level is pinned below the midgap, and the formation energies of chromium-doped Mg sites become the lowest either Mg-poor or Ge-poor conditions, suggesting that the co-doping of lithium ions promotes the chromium to preferentially occupy Mg sites, in good agreement with experiments. Furthermore, the excitation and emission processes of chromium ions are investigated, and our results recommend that the near-infrared emission is related to the 4T2 to 4A2 transition of Cr3+ at the Mg sites and the infrared emission is related to the 3T2-3A2 transition of Cr4+ at the Ge site. Our results help to understand the stability of the valence states of chromium ions doped Mg2GeO4 and the effects of ions co-doping in regulating the chromium valence states, as well as the corresponding optical properties of different types of chromium ions.