This study performs O K-and Ti L 3,2-edge x-ray absorption near-edge structure ͑XANES͒ measurements and first-principles pseudopotential calculations for the electronic structures of ABO 3-type Pb 1Ϫx Ca x TiO 3 (xϭ0-1) perovskites. The features in the O K-edge XANES spectra are found to be contributed primarily by hybridization between O 2p and Ti 3d, Pb 6p, and Ca 3d orbitals. The O K-edge XANES spectra reveal that partial substitution of A cations, Pb, by Ca not only decreases O 2p-Pb 6p but also O 2p-Ti 3d hybridization. The Ti L 3,2-edge measurements find that the off-center displacement of Ti, and hence, ferroelectricity persist up to a Ca concentration between 0.3 and 0.4.

We report O and Ca K-edges x-ray absorption near edge structure (XANES) spectra of Ba 1−x Ca x TiO 3 (x = 0.01 and 0.08), BaTiO 3 and CaTiO 3 and the electronic structure of Ba 0.875 Ca 0.125 TiO 3 obtained by first-principles calculation. The characteristic features in the O K-edge XANES spectra of these ferroelectric perovskites are influenced by the Ca concentration. They differ substantially from those of the reference TiO 2 . The O K-edge spectra suggest that the combination of the alkaline-earth-metal oxides, CaO and/or BaO, with TiO 2 enhance the effective charge of the O ions. Thus, a large dipole moment may result from the displacement of the Ti ion from the centre of the TiO 6 octahedron leading to collective displacement of Ti ions through attractive dipole-dipole couplings and may give rise to ferroelectricity. In the Ca K-edge XANES spectra there is a pre-edge feature similar to those found in other 3d transition-metal perovskites, which may provide information about hole doping.

No abstract

Ab initio electronic structure calculations employing the fixed-spin moment procedure and the generalized gradient correction are used to explore the mechanical stability of ␣-iron and related magnetic materials, including body-centered-cubic cobalt and ziconium. The mechanical stability is found to depend sensitively on the magnetization. At relatively low values of the magnetization, specific vibrational and elastic instabilities are identified and are interpreted in terms of the behavior of the electronic density of states at the Fermi level. These results also raise the possibility of influencing the stability of bulk magnetic phases by external fields.

No abstract