title: Effect of co-substitution of nitrogen and  fluorine in BaTiO3 on ferroelectricity and  other properties.
abstract: BaTiO3, with both nitrogen and fluorine substituted for oxygen, has been prepared and the properties of the doped material with the composition BaTiO2:8N0:1F0:1 have been studied. The color of the sample changes to light green on doping, accompanying a broad visible absorption band with its edge at 2.5 eV. The first-order ferroelectric transition changes to a broad transition on doping along with a decrease in the dielectric constant. We have examined the local structure, electronic structure and polar lattice dynamical properties of N and F co-substituted BaTiO3 using first-principles density functional theory-based calculations and derived the mechanism of the diffuse ferroelectric transition observed here with a broad peak in the temperature-dependent dielectric response. The calculated Born charges clearly reveal a strong disparity in the interaction of N, O and F with Ti: N being the most ferro-active, with a highly anomalous charge, and F being the least active, with an almost nominal charge. This originates from an electronic structure in which the top-most valence band is constituted of the 2p states of N, while the electronic states of F lie deep in the energy band, resulting in a local structure with short covalent Ti–N and long ionic Ti–F bonds. Disorder in these hetero ferro-active anions leads to a relaxor-like diffuse phase transition and a reduction in polarization. Calculations confirm the reduction of 0.65 eV in the bandgap of BaTiO3, which changes from indirect to direct type.
authors.affiliation: Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur PO, Bangalore-560064, India.
authors.forename: Nitesh
authors.initials: N
authors.lastname: Kumar
authors.forename: Jaysree
authors.initials: J
authors.lastname: Pan
authors.forename: N
authors.initials: N
authors.lastname: Aysha
authors.forename: Umesh V
authors.initials: UV
authors.lastname: Waghmare
authors.forename: A
authors.initials: A
authors.lastname: Sundaresan
authors.forename: C N R
authors.initials: CN
authors.lastname: Rao