Perovskite structure of rare earth titanate oxides are of growing interest in the recent year. Ln₂Ti₂O₇ belongs to the family of perovskite layer structure compounds which is one of interesting ferroelectrics substances characterized by high Curie temperature (Tc > 1500 °C), high coercive filed and a low dielectric constant. In addition, compared with conventional ferroelectric materials, such as lead zirconium titanat or strontium bismuth tantalate, these materials do not contain volatile element which may cause leakage problem due to a possible increase of defect density. Recently, much attention has been focused on the photocatalytic activity in the water splitting with the help of these compounds. It has been shown that the photocatalytic activities are highly dependent on the electronic band structure for these materials.    In the present work, we have studied the structural and the electronic properties of the layered titanate photocatalysts by mean of first-principles calculations. These calculations were performed in the framework of density functional theory (DFT), within the generalized gradient approximation (GGA). The total energy calculation and the analysis of the electronic structure are performed within the projector augmented wave (PAW) method as implemented in the Vienna Ab-initio Simulation package (VASP)  and the full-potential linearized augmented planewave (FPLAPW) as implemented in the WIEN2K package. It is found that the monoclinic P2₁ phase of the Ln2Ti2O7 compounds is stable in energy compared to the orthorhombic ones. This phase sustains ferroelectric layers of strongly distorted TiO6 octahedra. The ferroelectric behavior of this phase persists in the orthorhombic Cmc21 one whereas the Cmcm phase becomes paraelectric. We have also predicted the existence of a paraelectric phase, for the La₂Ti₂O₇ compound, with the P2/m symmetry. Four the electronic structure of these compounds, we have established that the conduction band of La₂Ti₂O₇ consisted mainly of Ti 3d and La 5d, whereas the valence band consisted mainly of O 2p and Ti 3d. The empty La 4f level was found to be located in the conduction band. The occupied and unoccupied Nd 4f level in Nd₂Ti₂O₇ was shifted to lower energy. The unoccupied Nd 4f level located between the conduction band and the valence band was found to be detrimental to photocatalytic activity in water splitting because it could act as an electron-trapping site.

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