# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 3 Results  
### Structural, Morphological, and Chemical Analysis  
Figure 2a displays typical X-ray diffractograms and Raman spectra of Cr-doped ZGO and ZGGO pellets. As identified, only the ZGO crystalline phase was observed and no other crystalline phases were present, apart from residual silica contamination likely arising from the mixing procedure of the precursors using the agate ball mill. The lattice constants of the doped spinels were estimated as 8.29 A and 8.32 A for the ZGO and ZGGO hosts, respectively, which are in line with the values reported in the literature [3; 8; 9; 10; 11] and close to the reference value of 8.33 A from ICDD #04-005-9208 card, which is included in the Figure for comparison purposes. The crystalline structure of the pellets was further assessed by Raman spectroscopy, as shown in Figure 2b Every pellet was measured in four different points to infer the samples' homogeneity. As the samples were seen to be fairly homogeneous, a representative spectrum of each is presented in Figure 2b. Since ZGGO is isostructural with ZGO, as seen in the XRD results, similar vibrational modes are expected for both compounds. Consequently, the signature of the vibrational modes was based on those of the ZGO [30]. Considering the reduced cell, ZGO has two formula units, corresponding to 14 atoms. Therefore, a total of 42 vibrational modes are generated. The total number of vibrational modes at the first Brillouin zone centre described by the irreducible representation of \(O_{h}\) point group is [30; 31]: \(\Gamma=A_{1g}(R)\ +E_{g}(R)\ +T_{1g}+3T_{2g}(R)\ +2A_{2u}+2E_{u}+5T_{1u}( IR)\ +2T_{2u}\), where the ones labelled with \(R\) and \(IR\) correspond to the modes that are active in Raman and infrared, respectively. The remaining \(T_{1g}\) mode is an acoustic phonon and all the other ones are silent modes, thus not contributing to the Raman nor the IR spectra. Therefore, at \(\overrightarrow{k}=\overrightarrow{0}\), five Raman active modes are expected: \(A_{1g},E_{g}\), and \(3T_{2g}\), with \(E\) and \(T\) modes doubly and triply degenerated, respectively [30; 31]. In spinel oxides, the highest frequency  
Figure 2: (a) X-ray diffractograms of ZGGO:Cr, ZGO:Cr, and ZGO (ICDD file #04-005-9208) with the assigned (hkl) diffracting planes of the cubic spinel structure. The * correspond to diffraction maxima due to SiO\({}_{2}\) contamination. (b) Raman spectra of the ZGGO:Cr and ZGO:Cr pellet samples. vibrational mode, assigned as \(A_{1g}\), is characteristic of stretching vibrations of O atoms inside the octahedral unit GaO\({}_{6}\)[30; 32]. The lowest energy vibrations (assigned as \(T_{2g}\)) are mainly due to Zn ions (e.g., ZnO\({}_{4}\) tetrahedrons), while the modes with intermediate frequencies (\(E_{g}\), \(T_{2g}\)) are related with Zn and Ga ions, with a higher contribution from Ga ions [30]. Table 1 lists the frequency of the vibrational modes measured experimentally in this work and compares them with the theoretically calculated values reported in the literature for ZGO [30]. By comparing the frequencies of the measured modes for the ZGO:Cr and ZGGO:Cr lattices with the theoretically predicted ones, some differences can be identified: (i) the peak positions of the vibrational modes of ZGGO:Cr are shifted to lower frequencies regarding those of ZGO:Cr; and (ii) an additional mode at ca. 270 cm\({}^{-1}\) was observed for both structures, which was not predicted by the space group analysis of the ideal spinel structure in [30]. Additional modes may appear in the spectra due to local lattice disorder effects, including those related to the inverse spinel (inversion of the cations, antisites), or local modes. For instance, for the case of ZGO, a slight inversion character of 3 % has been reported [10; 13; 25]. Therefore, the vibration at 270 cm\({}^{-1}\) may be assigned to a local lattice disorder.