Density functional calculations of the structural, electronic, and ferroelectric properties of high-k titanate Re2Ti2O7 (Re=La and Nd)

We have studied the structural, electronic, and ferroelectric properties of La2Ti2O7 (LTO) and Nd2Ti2O7 (NTO) by first-principles density functional theory calculations. The computed structural parameters are found to be in good agreement with experimental findings. In particular, the P2(1) phase is confirmed to be energetically preferred over the other configurations for both titanates. The calculations revealed the possible existence of an unidentified phase, namely, the P2(1)/m paraelectric structure. From the modern theory of polarization the spontaneous polarization of LTO and NTO was calculated to be 7.72 mu C/cm(2) and 7.42 mu C/cm(2), respectively, in accordance with experimental findings. Its origin is ascribed to the displacement of the rare earth (Re) cations in the [100] cleavage plane and parallel to the b axis. Electronic charge density distributions and Bader's topological analysis indicates that the bonding interactions between Re and O as well as Ti and O are not purely ionic, a noticeable covalent bonding is identified between Ti and O. Band structure calculations using a generalized gradient approximation (GGA) approach predicted insulating ground state for LTO with band gap energy of about 2.84 eV. Conversely, a half-metallic ground state was predicted for NTO. Using GGA+U approach, an appropriate insulating ground state was found with band gap energy of 1.63 eV. Density of states analysis suggest that the localized Nd 4f levels act as charge-trapping sites, explaining the reduced photocatalytic activity of NTO. (C) 2010 American Institute of Physics. [doi:10.1063/1.3459891]