10:30 AM - *SF11.09.01
Nitride Tuning of Transition Metal Oxides
Institut de Ciència de Materials de Barcelona (CSIC)1
The introduction of nitrogen in transition metal oxides involves changes in the electronic structure affecting the physical properties. The lower electronegativity and higher polarizability of nitrogen compared to oxygen increases the bond covalency, decreasing the interelectronic repulsion. [1, 2] The larger electrical charge of nitride anion increases the crystal field splitting and the polarization, and allows the stabilization of structures with new cation compositions and properties. Because nitrides are less stable than oxides, the development of new synthetic approaches is necessary for the search of new compounds. This lecture will present different aspects of our recent research on nitride-based transition metal compounds focussing on the relationships between the synthesis conditions, the oxidation states of the cations, the ordering of anions and the electronic properties. LaTaON2 is a highly nitrided perovskite that shows a centrosymmetric, pseudocubic structure derived from the Pm-3m aristotype. A new preparation method at high temperature in N2 produces samples with enhanced grain growth and sintering that show a large dielectric permittivity, ascribed to partial, local order of N3- and O2- induced by covalency. The anion order in this compound, that has been also prepared by ammonolysis, has been found to be dependent on the synthetic approach.  The topotactic nitridation of cation ordered, tetragonal Sr2FeMoO6 in NH3 at moderate temperatures leads to cubic, Fm-3m double perovskite oxynitride Sr2FeMoO4.9N1.1 that shows ferromagnetic order and negative magnetoresistance below c.a. 100 K.  BaWON2 is the first example of a hexagonal perovskite oxynitride, resulting from the combination of a large A-site cation and the stabilization of the small W6+ to compensate the charge of the two nitride anions.  It crystallizes in the 6H polytype, in the non-centrosymmetric space group P63mc, and the anions N3- and O2- are completely ordered in the vertex shared and face shared positions, respectively, of the WX6 octahedra. The anion order, together with second order Jahn-Teller effect of W6+ cations and electrostatic repulsions along the sharing faces of the octahedra, induce high distortions in the coordination polyhedra leading a polar structure, and a large dielectric permittivity is observed.
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