# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 3 Results
### RT Photoluminescence and Photoluminescence Excitation  
Figure 4a shows the normalised RT PL, PLE, and DR spectra of non-intentionally doped and chromium-doped ZGO and ZGGO samples. By exciting the samples with 325 nm/3.81 eV, the main recombination process on ZGO:Cr and ZGGO:Cr is due to the intraionic Cr\({}^{3+}\) luminescence in the red/NIR spectral region. As mentioned in the introduction, emission lines corresponding to transitions from the first excited level, \({}^{2}E\), to the fundamental one, \({}^{4}A_{2}\), are observed. In a single configurational coordinate diagram, these electronic states exhibit a small offset, being responsible for the narrow emission lines, the R-lines (see also Figure 5). Cr\({}^{3+}\) in the ZGO and ZGGO lattices preferentially occupy the slightly trigonal distorted Ga\({}^{3+}\) octahedral sites [11; 18; 24]. By the interplay of the crystalline field and spin-orbit interaction, the \({}^{2}E\) electronic level is split into two electronic states \(\overline{E}\) and 2\(\overline{A}\) (Figure 4b), for higher and lower energies, respectively, with an energy separation of ca. 5 meV [11; 18; 24; 29]. Additionally, it should be also emphasised that the ground \({}^{4}A_{2}\) state is also split into two electronic states, with a zero-field splitting of ca. 0.1 meV [18; 24; 29]. Consequently, two ZPL are observed from the upper \(\overline{E}\) and 2\(\overline{A}\) states \begin{table}
\begin{tabular}{c c c c} \hline
**Mode** & **Theoretical Calculated** & **ZGO:Cr (cm\({}^{-1}\))** & **ZGGO:Cr (cm\({}^{-1}\))** \\
**Assignment** & **ZGO (cm\({}^{-1}\))**[30] & **[This Work]** & **[This Work]** \\ \hline \(T_{2g}\) & 186 & & 186 \\  & & 273 & 270 \\ \(E_{g}\) & 395 & 369 & 367 \\ \(T_{2g}\) & 488 & 476 & 471 \\ \(T_{2g}\) & 618 & 611 & 611 \\ \(A_{1g}\) & 717 & 717 & 717 \\ \hline \end{tabular}
\end{table} 
Table 1: Frequency of the vibrational modes measured experimentally in this work for ZGO:Cr and ZGGO:Cr. For comparison, the theoretical values for ZGO calculated in ref [30] are also included. to the fundamental one, corresponding to the R2- and R1-lines (Figure 4b) placed at higher and lower energies, respectively. In the case of the used excitation (325 nm/3.81 eV), the RT broad spectra show the peak position of the R-lines in the ZGO:Cr and ZGGO:Cr samples at 686.6 nm/1.806 eV and 688.1 nm/1.802 eV, respectively. The RT PL spectra also evidence a rich vibronic structure on the low/high energy side of the R-lines, with the former/latter corresponding to the vibrational coupled Stokes/anti-Stokes luminescence. Additionally, superimposed with the intraionic Cr\({}^{3+}\) luminescence in substitutional sites, N emission lines due to exchange-coupled ion pairs and/or Cr-related complex defects in the presence of nearby intrinsic/extrinsic defects are observed, in line with what was reported by other authors [7, 11, 24, 25]. As aforementioned, N-lines have been reported in ZGO:Cr [22, 23, 24, 25] in the low energy side of the R ones, namely at 688.9 nm/1.80 eV (N1), 693.5 nm/1.79 eV (N2), 700.3 nm/1.77 eV (N3), and 704 nm/1.76 eV (N4), and their intensity was found to be dependent on the chromium concentration and excitation energy [24, 25]. In the here-studied samples, the N-line with the highest intensity is the N2, and the defect from which this line originates has a major influence on the observation of red/NIR persistent luminescence, as will be further discussed. When monitored at the R-lines, the PLE spectra (Figure 4a) are dominated by asymmetric broad excitation bands due to the spinallowed \({}^{4}A_{2}\to{}^{4}T_{2},{}^{4}T_{1}\) transitions with maxima at 556 nm/2.23 eV and 404 nm/3.07 eV in both doped hosts. A higher energy transition (\({}^{4}A_{2}\to{}^{4}T_{1}({}^{4}P)\)) is also expected to occur superimposed with the materials' band edge and/or with a defect-related excitation band [7].