## 3 Results and Discussion
### Photoluminescence Analysis
The R-, R\({}^{\prime}\)-, and B-lines result from the spin-orbit admixture of the \({}^{4}\)T\({}_{1}\) and \({}^{4}\)T\({}_{2}\) electronic states with the \({}^{2}\)E, \({}^{2}\)T\({}_{1}\), and \({}^{2}\)T\({}_{2}\) levels [4,50]. According to the measured spectra, the highest emission intensity occurs for the blue and green photon excitation. As such, the data are consistent with the fact that at RT, after excitation to any of the Cr\({}^{3+}\) higher excited states, the system efficiently relaxes to the \({}^{2}\)T\({}_{1}\) and \({}^{2}\)E levels, from which emission to the ground \({}^{4}\)A\({}_{2}\) state occurs. Simultaneously, the used excitation also promotes the recombination of the chromium ion pairs (N-lines, as observed in the PL spectra of Figure 4b). Nevertheless, the presence of other Cr\({}^{3+}\)-complex defects emitting in the same spectral region cannot be discarded [45, 46, 47, 48, 49, 51]. Figure 4: _Cont._  
To acquire additional information on the spectral features of the Cr-doped \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) spheres, the temperature of the samples was lowered down to 14 K. Figure 5 shows a highly resolved PL spectrum obtained at 14 K with 3.81 eV photon excitation. The low-temperature spectrum exhibits additional spectral complexity, with the presence of several lines at lower energies than the R ones, which are absent at high temperatures. At 14 K, the R-lines are peaked at 1.793 eV (R\({}_{2}\)) and 1.789 eV (R\({}_{1}\)). Besides the appearance of new lines on the low energy side of the R ones, on the high energy side, the R\({}^{\prime}\)-lines are observed, meaning that the emission from the \({}^{2}\)T\({}_{1}\) level to the ground state occurs even at 14 K. Additionally, new doublet lines emerge at 14 K, with maxima at 1.809 eV and 1.805 eV, here assigned as R\({}_{2}\)\({}^{\prime\prime}\)- and R\({}_{1}\)\({}^{\prime\prime}\)-lines, respectively. Analogously to the R-lines, the two R\({}^{\prime\prime}\)-lines are 4 meV apart and are likely to be due to a \({}^{2}\)E \(\rightarrow\)\({}^{4}\)A\({}_{2}\) transition from a distinct trivalent charged chromium optical center, thus justifying a model corresponding to multi-non-equivalent Cr\({}^{3+}\) optical centers. Figure 4: RT (**a**) PLE and (**b**) PL spectra of the analyzed \(\alpha\)-Al\({}_{2}\)O\({}_{3}\):Cr sphere; **(c)** Tanabe–Sugano diagram for the single Cr\({}^{3+}\) ions (3d\({}^{3}\) electronic configuration) in octahedral symmetry (O\({}_{\rm h}\)). The dashed line corresponds to Dq/B = 2.8, as reported for isolated Cr\({}^{3+}\) in Al\({}^{3+}\) substitutional sites [3; 4; 5; 6]; **(d)** schematic representation of the split emitting levels under trigonal distortion for a single Cr\({}^{3+}\) and their nonradiative transfer to the N pairs, namely second- (N\({}_{1}\)) and fourth- (N\({}_{2}\)) nearest neighbors. The energy values correspond to the ones measured in the present samples. The levels’ energy positions are not at scale. The vertical full arrows denote the observed radiative transitions (R-, R-, and N-lines), while the dashed arrows represent energy transfer. Scheme based on reference [47]. Figure 6 depicts the PL spectra obtained at several temperatures under 3.81 eV photon excitation. As seen, between 14 K and RT, the overall luminescence intensity decreases with increasing temperatures. The PL intensity of the distinct Cr\({}^{3+}\) optical centers exhibits distinct sensitivities to the temperature rise. It was verified that the peak position of the R\({}^{\prime}\)-lines slightly shifts toward lower energies with increasing temperature. This deviation is accompanied by an increase in the PL intensity of the lines from 14 K to RT, suggesting the existence of a thermal population process of the \({}^{2}\)T\({}_{1}\) electronic state.