## 3 Results
This reddish colour, not observed for the films sintered at 680 \({}^{\circ}\)C, may be indicative of some reaction with the alumina substrate. Indeed, in our previous work [33] we proposed that the formation of 1:1 stoichiometry binary Al\({}_{2}\)O\({}_{3}\)-TeO\({}_{2}\) compound results from the oxidation of TeO\({}_{2}\), which occurs at temperatures \(>\) 600 \({}^{\circ}\)C to form Te\({}_{4}\)O\({}_{9}\) and TeO\({}_{3}\), that triggers the formation of Al\({}_{2}\)TeO\({}_{6}\). Under vacuum the oxidation of TeO\({}_{2}\) does not take place and there is no reaction between Al\({}_{2}\)O\({}_{3}\) and TeO\({}_{2}\). To understand what gives rise to these observations and to support our predictions we conducted XRD analysis to examine the evolution of the formed phases under different sintering conditions (Figure 3). For the sintering temperature of 680 \({}^{\circ}\)C, the XRD pattern reveals only peaks of TiTe\({}_{3}\)O\({}_{8}\) phase (JCPDS 70-2439) identical to those of calcined powders [30]. However, as hypothesised, for films sintered at 700 \({}^{\circ}\)C and 720 \({}^{\circ}\)C, a new phase of Al\({}_{2}\)TeO\({}_{6}\) (JCPDS 15-0689) can be identified. For films sintered at 700 \({}^{\circ}\)C, Al\({}_{2}\)TeO\({}_{6}\) is residual, but for films sintered at 720 \({}^{\circ}\)C, Al\({}_{2}\)TeO\({}_{6}\) is clearly visible and even peaks of TiO\({}_{2}\) can be detected, indicative of TiTe\({}_{3}\)O\({}_{8}\) decomposition. Thus, the reddish colour observed in TiTe\({}_{3}\)O\({}_{8}\) thick films sintered at temperatures higher than 700 \({}^{\circ}\)C is related to the formation Al\({}_{2}\)TeO\({}_{6}\) phase, which results from the reaction between TeO\({}_{2}\) from TiTe\({}_{3}\)O\({}_{8}\) and alumina substrates. Figure 2: Optical images of TiTe\({}_{3}\)O\({}_{8}\) films deposited on carbon coated 25.4 \(\times\) 25.4 \(\times\) 0.0254 mm\({}^{3}\) alumina substrates and sintered at 680 (a), 700 (b) and 720 \({}^{\circ}\)C (c) for 5 h, respectively. Films sintered at 680 \({}^{\circ}\)C are not fully dense and some films sintered at 720 \({}^{\circ}\)C exhibit an irregular surface with a reddish colour resulting from a reaction with the alumina substrate. We have also inspected the microstructure of the sintered TiTe\({}_{3}\)O\({}_{8}\) films deposited on carbon coated alumina substrates, for the films sintered at 700 \({}^{\circ}\)C for 5 h (Figure 4). Films present a uniform and dense surface microstructure with the typical cubic shaped grains of TiTe\({}_{3}\)O\({}_{8}\) and well visible grain boundaries (Figure 4a). In agreement, a dense microstructure (with only some residual porosity), a thickness of ~19 \(\upmu\)m as well as a good adhesion to the substrate can be observed from the cross-section SEM micrographs (Figure 4b). There is also a thin reaction layer between the film and the substrate interface with a thickness of <200 nm (Figure 4c). EDS was used for the analysis of the interface between TiTe\({}_{3}\)O\({}_{8}\) films and Al\({}_{2}\)O\({}_{3}\) substrates (Figure 4c). The interlayer at the interface between the film and the substrate (region A) is Al and Te rich, indicating the presence of Al\({}_{2}\)TeO\({}_{6}\) and confirming the previous XRD data. On the other hand, for region B, which corresponds to the main bulk of the thick film, the detected elements are mainly Te and Ti, from TiTe\({}_{3}\)O\({}_{8}\), being the peak for Al considerably low. Thus, under the sintering conditions of 700 \({}^{\circ}\)C for 5 h in air rather dense 19-\(\upmu\)m-thick TiTe\({}_{3}\)O\({}_{8}\) films are prepared with the presence of a thin (<200 nm) reaction interlayer of Al\({}_{2}\)TeO\({}_{6}\).