# Optical Studies in Red/NIR Persistent Luminescent Cr-Doped Zinc Gallogermanate (ZGGO:Cr)
## 1 Introduction  
Persistent luminescent phosphors have attracted extensive research interest due to the feasibility of their use on glow-in-the-dark applications such as anti-counterfeiting, optical information storage, night vision surveillance, photocatalysis, and optical bioimaging [1, 2, 3, 4, 5, 6, 7]. Since the first work of [PERSON] et al. [8], it is well known that Cr-doped ZnGa\({}_{2}\)O\({}_{4}\) (ZGO:Cr) [8, 9, 10, 11] and ZGGO:Cr [12, 13, 14] spinel structures give rise to Cr\({}^{3+}\) red/near-infrared (NIR) long-lasting afterglow phosphors, with a luminescence signal that can remain up to several hours after ceasing the excitation source [7, 8, 9, 10, 11, 12, 13, 14]. In the ZGGO with lower molar fractions, \(0<x<0.5\), the Ge ions typically replace Ga sites in the lattice (see Figure 1) but only the single ZGO crystalline phase is expected to be present [13, 14], with a cubic lattice constant of ca. 8.33 A (from JCPDS data file card 38-1240). Indeed, in this case, these compounds exhibit a normal AB\({}_{2}\)O\({}_{4}\) cubic spinel belonging to the \(\mathcal{O}_{h}^{7}\)(_Fd_\(\overline{3}\)_m_) space group symmetry, with eight formula units in the unit cell [9, 10]. Zn\({}^{2+}\) ions are surrounded by a tetrahedral environment of oxygen ions, or A-sites, being placed in \(T_{d}\) point group sites and Ga\({}^{3+}\) ions in a six-fold distorted octahedral coordination (\(D_{3d}\)), or B-sites [9, 10]. On the other hand, for \(x\geq 0.5\), the hexagonal Zn\({}_{2}\)GeO\({}_{4}\) phase also starts to be present and the two phases co-exist [13, 14]. ZGO and ZGGO are ultra-wide bandgap semiconductors, with RT bandgap energies estimated to be around 5.0 eV [15; 16; 17]. These materials are suitable host lattices for the incorporation of several dopants' energy levels, as is the case of the ones resulting from the incorporation of trivalent chromium ions. Typically, Cr\({}^{3+}\) will replace Ga\({}^{3+}\) sites exhibiting a preference to occupy the distorted octahedral ones [10; 18]. In particular, it should be emphasised that the Cr\({}^{3+}\) ionic radius of 0.62 A matches well to the one of the Ga\({}^{3+}\) in octahedral sites (0.62 A) [19]. With a 3d\({}^{3}\) electronic configuration, the \({}^{4}\)F Cr\({}^{3+}\) ground level of the free ion is known to split into the \({}^{4}A_{2}\left(t_{2}^{3}\right)\), \({}^{4}T_{2}\left(t_{2}^{2}e\right)\), and \({}^{4}T_{1}\left(t_{2}^{2}e\right)\) electronic states when in an octahedral crystal field [10; 18]. The levels of the ion in the cubic field are further split by the interplay of the trigonal field strength and the spin-orbit interaction [10; 18]. According to Tanabe and Sugano diagrams [20; 21], under intermediate and high crystalline field strengths, the lowest excited level corresponds to the \({}^{2}E\left(t_{2}^{3}\right)\) electronic state originated from the splitting of the \({}^{2}G\) free ion level. Therefore, and as a result of the electron-phonon coupling of the different states, Cr\({}^{3+}\) in octahedral sites exhibits broad spin allowed \({}^{4}A_{2}\left({}^{4}F\right)\rightarrow{}^{4}T_{2},{}^{4}T_{1}\left({}^{4}F\right)\) absorption bands, as well as narrow zero-phonon lines (ZPL), the so-called R-lines, due to the parity and spin-forbidden \({}^{4}A_{2}\left({}^{4}F\right)\rightarrow{}^{2}E\left({}^{2}G\right)\) transition [18; 22]. Besides the intraionic R-lines emission of isolated Cr\({}^{3+}\) that occurs nearby 688 nm (-1.8 eV), complex defects involving Cr\({}^{3+}\) and neighbouring defects (e.g., Cr\({}^{3+}\) pairs and Cr-intrinsic defects complexes) are known to emit in the same red/NIR spectral region, resulting in the so-called N-lines [7; 8; 9; 10; 11; 12; 13; 14]. According to [PERSON] et al. [22], the most prominent N-lines are placed at 688.9 nm/1.80 eV, 693.5 nm/1.79 eV, 700.3 nm/1.77 eV, and 704 nm/1.76 eV, corresponding to the N1-, N2-, N3-, and N4-lines, respectively. The assignment of the defects that give rise to these N-lines has been a matter of discussion in literature [10; 22; 23; 24; 25].