## 1 Introduction  
Over the last decades, novel phenomena and functionalities at artificial heterointerfaces have been attracting extensive interest in both materials science and fundamental condensed matter physics. Thanks to the considerable advancements in thin-film growth, the electrostatic boundary conditions at oxide surfaces and interfaces can be used to epitaxially design artificial heterointerfaces with atomic precision, to form new electronic phases, or novel low-dimensional states usually inaccessible in the bulk oxides. Two-dimensional free-electron gases (2 DEG) [1, 2] ferromagnetism between two non-magnetic materials [3, 4] and interface electronic reconstruction [5] are prominent examples of exotic phenomena emerging at heterointerfaces between oxide layers with properties differing from either constituent, thus suggesting new possible platforms for a future generation of technological applications [6]. In this regard, understanding the physical mechanisms responsible for the formation of a 2 DEG at the surface of insulating oxides or at their interface remains one of the most challenging issues. For instance, in the case of SrTiO\({}_{3}\)-based interfaces,many reports have, indeed, pointed towards the building of an internal electrical potential as a mandatory condition to drive the formation of the 2 DEG. Such a scenario appears to be confirmed by the existence of a threshold thickness of the oxide layers as the onset of the highly conductive state [7, 8]. However, alternatively, extrinsic mechanisms attributed to local structural imperfections (mainly oxygen vacancies) have been suggested to be responsible for the arising of the 2 DEG, with no need of a critical thickness [9, 10, 11], thus still leaving the debate open. This is also the case of anatase TiO\({}_{2}\). Some reports point to a three-dimensional character of the TiO\({}_{2}\) surface [12], while others support a two-dimensional nature [13, 14] or the coexistence of both 2D and 3D characters [15], similar to other oxide systems. Having access to a very high-quality single-unit-cell (and sub-unit-cell) layer of a given material is the only way to provide conclusive evidences on the dimensionality issue. In this framework, very recently, some of the authors have demonstrated that no critical thickness is required to stabilize the 2 DEG at the anatase TiO\({}_{2}\) surface and implying its purely 2D nature after optimization of the synthesis process, which demands the use of a functional buffer layer of LaNiO\({}_{3}\)[16]. However, the growth of anatase TiO\({}_{2}\) directly on LaAlO\({}_{3}\) substrate has required the deposition of a TiO\({}_{x}\) extra-layer to have a 2 DEG established, thus questioning the validity of the scenario in which no critical thickness is required [16]. In the present work, we provide an in-depth investigation of the film/substrate interface of anatase TiO\({}_{2}\) thin films grown on LAO (001) substrates by pulsed laser deposition (PLD). By resorting to spherical aberration-corrected scanning transmission electron microscopy (STEM) high angle annular dark field imaging (HAADF) complemented by multi-slice simulations based on relaxed structures obtained from ab initio calculations, we determined the intrinsic structure of the TiO\({}_{2}\)/LAO interface. With our approach, we validated a new model which strongly relies on the fact that the intrinsic reduced nature of TiO\({}_{2}\) anatase films, demonstrated in our previous work [17], is prone to stabilize a Ti\({}_{2}\)AlO\({}_{4}\) alloy in the very initial layers of the film, as a result of the competition between the structural parameters of the two interfacing oxides. To the best of our knowledge, no previous report considered the presence of an oxygen-deficient anatase structure to describe the formation of a new phase near the film/substrate interface. By means of density functional theory (DFT) calculations on oxygen deficient structures, we relate the evolution of the interfacial structure to the ionic relaxation at the interface.