Phase formation, crystal structures and magnetic properties of perovskite-type phases in the system La2Co1+z(MgxTi1−x)1−zO6

Abstract

Perovskite-type cobaltates in the system La 2 Co 1+z (Mg x Ti 1-x ) 1-z O 6 were studied for z = 0 ≤ x ≤ 0.6 and 0 ≤ x < 0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption nearedge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350°C. The space group symmetry of the structure changes from P2 1 /n via Pbnm to c R3 with both increasing Mg content and increasing Co content. The

Crystal structure, thermal expansion and high-temperature electrical conductivity of A-site deficient La2−Co1+(Mg Nb1−)1−O6 double perovskites

“…This is similar to that observed for the La 2 Co 1+x (Mg y Ti 1-y ) 1-x O 6 system [19]. The High contents of Nb in the compounds gives relatively high t-factor values (Goldschmidt tolerance factor) and an orthorhombic a -a -c + tilting of the octahedra, leading to the orthorhombic space group Pnma (However, some of the compounds are monoclinic, thus allowing B-cation ordering, see Figure 2a), however, the compositions with high Mg 2+ /Nb 5+ ratios have even larger t-values due to the presence of Co 3+ and an a -a -a -tilting which leads to an rhombohedral R-3c symmetry, similar to what was found for LaCoO 3 [28] The syntheses of nominal compositions with slightly La-deficient compositions at 1450°C…”

“…It seems, however, that perovskites with weaker B-O bonds and high oxide-ion conductivities, like ferrates, cobaltates, nickelates or cuprates, have negligible A-site deficiencies. One of the ways to create A-deficiency in such perovskites could be by partial replacement of the 3d-metal cation by a cation belonging to groups 4-6 in the Periodic [19,20]. These perovskites were found to be an excellent model system for studies of correlations between the oxidation state of cobalt, and various high-temperature properties important for cathode in IT-SOFC, like thermal expansion and electrical conductivity.…”

Tracking of high-temperature thermal expansion and transport properties vs. oxidation state of cobalt between +2 and +3 in the La2Co1+z(Ti1−xMgx)1−zO6-system

“…Thermal expansion curves, from RT up to 1273 K, for La 2 Co(Ti 1-x Mg x )O 6 (0 ≤ x ≤ 0.5) and The semiconductor-like behaviour of these samples and the results from XANES measurements 11 indicate that the cobalt ions exist in two discrete oxidation states, i.e. +2 and +3, and suggest that the electronic conduction in these samples can be described in terms of small polaron hopping model.…”

“…La 2 O 3 (Aldrich, 99.99 %), MgO (Merck, analytical grade) (both powders were pre-dried at 1223 K for 24h) cobalt(II)acetate tetra hydrate (Sigma-Aldrich, reagent grade) and titanium(IV)butoxide (Sigma-Aldrich, 97.0 wt%) were dissolved in stoichiometric amounts in concentrated nitric acid with anhydrous citric acid (Merck, 99%) as a complex binder, to get a well dispersed mixing of the elements in the precursor gel. More details about the synthesis is described in reference 11 . The gels were calcinated at 1173 K in air for 2 h. The samples were annealed in air at a final temperature of 1623 K for 24 h with intermediate grindings.…”

“…11 For La 2 Co(Ti 1-x Mg x )O 6 (0 ≤ x ≤ 0.6) being compounds with fixed cobalt content, the formal Co oxidation state is changed from +2 to +3.2 with a substitution of Ti 4+ by Mg 2+ . The symmetry of the perovskite phase in the system changes from monoclinic (P2 1 /n) for 0 ≤ x ≤ 0.2, via orthorhombic (Pnma) (0.3 ≤ x ≤ 0.4) to rhombohedral (R3c) (0.5 ≤ x ≤ 0.6).…”