## 1 Introduction  
Optical materials with a spinel-type structure have attracted researcher's interest in the past ten years due to the possibility of using Cr\({}^{3+}\)-doped samples for an interesting application of bioimaging. Since the first report on an application of a zinc gallate spinel for bioimaging by [PERSON] et.al. [1], several other hosts from the same (or similar) spinel family have been explored extensively. The research in this direction was extended from imaging probes sensitive in the near infrared (NIR) region around 700 nm (also known as the first biological window) to higher second and/or third biological windows lying deep in the wavelength domain. The important point in all these investigations was to synthesize hosts with crystal fields that can affect the Cr ions (present as dopants) in such a way that there is a shift in the emission maximum from the NIR to a higher wavelength region. This strategy was reported for a magnesium gallate host where the Cr emission shifted to 712 nm as compared to 693 nm for a zinc gallate host [2]. The technological key point of spinels that makes them suitable for such an application is the presence of antisite defects in the host lattice. Antistic defects make it possible for the dopant ion to sit at a distorted site as compared to the actual site where the ion is expected to be present in the host lattice. Spinels can be categorized as normal spinels, which have the general formula of \(AB_{2}\)O\({}_{4}\), and an inverse spinels, which have a general formula, \(B(AB)\)O\({}_{4}\). Another class of spinels is complex spinels, which have a mixture of normal and inverse spinels. In recent reports, spinels have also found technological importance as suitable materials for magnetic [3], high-temperature ceramics [4], as superhard materials [5], and as materials suitable for high pressure sensing applications [6]. The common synthesis procedure of spinel-type materials is a solid-state method. The materials in this procedure are synthesized at high temperatures under a reductive or inert atmosphere. Such an environment is required to keep the host and dopant in the desired valence state, for example, +3 for Cr. From the literature, one can also observe that most of the researchers' work on normal spinel-type materials, while very little, is published on inverse spinels, probably due to complex synthesis conditions. Another important observation is the recent developments of researchers on UV-emitting persistent luminescent (PersL) materials. UV-emitting PersL materials have found application in anticounterfeiting, photocatalysis, photodynamic therapy, and others, as discussed in a recent review article [7]. The UV region can be sub-categorized as UV-A, UV-B, and UV-C regions, and some smart techniques are required to prepare such materials. Based on these two aspects, in the present work, we investigated the influence of synthesis conditions, annealing temperature and ZnO concentration in zinc orthothanate (Zn\({}_{2}\)TiO\({}_{4}\)). Zn\({}_{2}\)TiO\({}_{4}\) is an inverse spinel that exhibits interesting dielectric and optical properties, making it potentially useful in electronics, photonics, and optoelectronics [8; 9; 10; 11]. A Zn\({}_{2}\)TiO\({}_{4}\) application for photocatalysis involves light to drive chemical reactions, which can be used for environmental remediation and water purification [11]. It has a band gap of 3.30 eV (375 nm) with a prominent photoluminescence (PL) emission in the wavelength region of 470-800 nm [12]. The 375 nm band also corresponds to ZnO-related host band gap. Hence, based on this overview and these objectives, we prepared twelve samples using the solid-state synthesis method, modulating the ZnO concentration with the objective to alter the ZnO-related absorption band at 375 nm or to modify the Zn\({}_{2}\)TiO\({}_{4}\) band gap.