# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 3 Results
### The 15 K PL: Site-Selective Excitation  
Most of the recently published works on the persistent luminescence of the chromium ion in the trivalent state in the ZGO and ZGGO lattices refer to the behaviour observed at RT [7; 8; 9; 10; 11; 22; 23; 24; 25]. However, the identification of optically active centres and their correlation with intrinsic/extrinsic defects requires the use of cryogenic temperatures and only a few works have been reported so far [9; 10; 11; 18; 22]. To clarify if, in addition to the two Cr\({}^{3+}\) environments identified at RT (by the R- and N2-lines), there are further Cr\({}^{3+}\)-related optical centres in the red/NIR spectral region, the samples were cooled down to 15 K. Figure 6 shows the low-temperature Cr\({}^{3+}\) PL spectra obtained upon excitation with different wavelengths for the studied ZGGO:Cr sample. The spectrum acquired for the ZGO:Cr sample under 325 nm excitation was also added to the figure for comparison. Besides the R- and N2-lines, the spectra exhibit a complex vibronic structure in the lower energy region that hampers individual classification of the vibronic progression due to spectral overlapping. However, it is also identified from Figure 6 that the spectral position and relative intensity of the lines are dependent on the used excitation wavelength. Figure 7 shows the highly resolved 15 K PL spectra in the spectral range of the R2- to N2-lines. By varying the wavelength excitation, a set of lines can be discerned that clearly show the presence of more than two optical centres in the samples under study. Firstly, and comparing the spectra of the ZGO:Cr and ZGGO:Cr samples obtained under the same excitation conditions (325 nm photons), it can be seen that the position of the R-lines in ZGGO:Cr is slightly shifted towards lower energies, as indicated in Table 2. This is a compelling indication that the environment/distortion of the Cr\({}^{3+}\) emitting ions in ZGGO is distinct from the one observed on ZGO. Additionally, for the case of the ZGGO:Cr samples, the relative intensity of the R-lines was found to be strongly sensitive to the used wavelength excitation conditions, indicating that there are preferential excitation mechanisms of the different Cr\({}^{3+}\)-emitting centres. In all cases, the energy separation of the R-lines is \(\sim\)5 meV, in agreement with previously reported values for the unfolding of the \({}^{2}E\) state due to the crystal field effect and spin-orbit interaction and the calculated activation energies for  
Figure 5: (**a**) RT spectra of ZGO:Cr and ZGGO:Cr obtained under different excitation wavelengths: onset of the band-to-band absorption (275 nm), defect absorption band (310 nm), and intraionic \({}^{4}A_{2}\to{}^{4}T_{1},{}^{4}T_{2}\) absorption bands (400 nm and 560 nm, respectively). Besides the R- and N2-lines, the spectra are dominated by Stokes and anti-Stokes phonon sidebands for longer and shorter wavelengths. The spectra were vertically shifted for clarity. (**b**) Photograph of the ZGGO:Cr pellet under white light (**top**) and 256 nm lamp (**bottom**) illumination. the lines thermalization, as described below [11; 20; 25]. The spectral comparison further suggests that the chromium-doped ZGGO samples also exhibit chromium optical centres with the same environment as those observed in ZGO. However, the preferential excitation mechanism of these centres is now accomplished by intraionic absorption rather than by high-energy photons, in line with the PLE measurements, as depicted in Figure 4a Besides the R-lines and vibronic progression, luminescence features peaked at the same spectral position of the N1- and N2-lines reported by [PERSON] et al. [22] were also observed. Figure 6: The 15 K PL spectra of ZGGO:Cr obtained upon excitation on the defect absorption band and intraionic \({}^{4}A_{2}\to{}^{4}T_{1},{}^{4}T_{2}\) transitions. The spectrum obtained for the ZGO:Cr under 325 nm excitation (onset of band-to-band absorption) was added for comparison purposes.