# Synthesis and Features of Luminescent Bromo- and Iodohectorite Nanolay Materials
## 3 Results
### Luminescence Properties
Since the octahedral coordination is the preferred geometry for titanium compounds, the initial impurities have been suggested to migrate to the octahedral layers substituting isomorphically either Mg\({}^{2+}\) or Li\({}^{+}\). The low reduction potential between Ti\({}^{\rm IV}\) and Ti\({}^{3+}\) allows the easy interconversion of these species; however, the trivalent form of titanium should be the majority species because it better matches the size and charge of the octahedral units [23; 24]. It can thus be assumed that a titanium-related center is responsible for the blue emission in Br-Hec and I-Hec. According to previous literature, this blue emission can be attributed to TiO\({}_{6}\) entities as reported for Benitoite (BaTiSi\({}_{3}\)O\({}_{9}\)) [43] or to Ti\({}^{3+}\)-V\({}_{\rm O}\) (oxygen vacancy) pairs [23; 24; 47; 48]. Moreover, the doping of the X-Hec materials with Ti\({}^{3+}\) (x\({}_{\rm Ti}\) = 0.1 mol %) increases the intensity of the main emission (Figure 6), without significant changes to its band position confirming titanium as the luminescent center. In addition to the blue emission band, there are also luminescence signals at ca. 720 and 830 nm. As is known from the Ti:sapphire laser [22], Ti\({}^{3+}\) can also emit in the red and NIR due to the \({}^{2}\)E\(\rightarrow^{2}\)T\({}_{2}\) transition. This is present as a wide band also in natural Benitoite [55]. For Br-Hec and I-Hec these red and NIR emissions are observed as bands or lines depending on the excitation wavelength (Figures 7, 4 and 5). The line-like shape suggests the presence of other emitters than Ti\({}^{3+}\). Thus, impurity 3d\({}^{3}\) ions Cr\({}^{3+}\) and Mn\({}^{4+}\) which show the characteristic emission R lines (\({}^{2}\)E\({}^{2}\)T\({}_{2}\)\(\rightarrow^{4}\)A\({}_{2}\) transition) in the red-NIR range seem to be probable candidates. These ions also show emission bands in the same spectral range at appropriate crystal field strength [22]. The contents of Mn and Cr in the present materials were determined with ICP-MS to be around 4 (Mn) and 0.5 ppm (Cr). Therefore, both Cr\({}^{3+}\) and Mn\({}^{4+}\) can be considered as possible emitters along with Ti\({}^{3+}\), as was previously reported for natural benitoite [55]. The main blue-green emission band has a red shift trend: the center of this blue emission has been identified at 460, 465, 470 and 490 nm for fluorohectorite [23], chlorohectorite [24], bromohectorite and iodohectorite, respectively. This observed emission red shift can be explained using the molecular orbital theory: the halides are \(\pi\)-donor ligands (\(\pi\)-donor strengths: F\({}^{-}\) < Cl\({}^{-}\) < Br\({}^{-}\) < I\({}^{-}\)), which have full \(\pi\) orbitals lower in energy than the 3d\({}^{1}\) orbitals of Ti\({}^{3+}\). When they form molecular orbitals with the t\({}_{\rm 2g}\) orbitals of Ti\({}^{3+}\), the hitherto nonbonding Ti\({}^{3+}\) t\({}_{\rm 2g}\) orbitals split into bonding and antibonding MOs. The energy is increased, bringing them closer in energy to the antibonding e\({}_{\rm g}\) orbitals [56]. The energy difference between the antibonding t\({}_{\rm 2g}\) and e\({}_{\rm g}\) MOs may be considered as the ligand field splitting responsible for the transition in visible. The higher is the \(\pi\)-donor strength, the lower is the energy difference between t\({}_{\rm 2g}\) and e\({}_{\rm g}\) generating the noticed red shift. Similar observations have been reported, for example, in perovskites showing the red shift when bromine was replaced by iodine in the inorganic octahedral PbX\({}_{6}\) (X = Br or I) [57]. The excitation spectra of the bromohectorite and iodohectorite (Figure 8) confirm the role of Ti\({}^{3+}\) as an activator center in these materials. The broad bands observed at ca.