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La1- xSr xFe2/3Mo1/3O3 (0 ≤ x ≤ 1) perovskites allow exceptionally wide tuning of the Mo oxidation state from +3 ( x = 0) to +6 ( x = 1) with the FeIII oxidation state virtually unchanged. The end members of this series show antiferromagnetic ordering in LaFe2/3Mo1/3O3 at TN = 520 K and ferrimagnetic ordering in SrFe2/3Mo1/3O3 at TC = 420 K. The crucial role of electron correlations in the Mo 4d shell suggests that LaFe2/3Mo1/3O3 with a half-filled band is a Mott insulator, whereas SrFe2/3Mo1/3O3 is a band insulator. In both cases, the magnetic structure is dictated by antiferromagnetic superexchange between localized magnetic moments. At intermediate compositions, an interplay of antiferromagnetic-superexchange and double-exchange interactions results in nonmonotonous variations of both the magnetic-ordering temperature and saturation magnetization. [ABSTRACT FROM AUTHOR]

The electronic structure single crystals of the new double perovskite Sr2YRu1-xCuxO6 (x = 0-0.05) has been investigated by x-ray absorption nearedge structure (XANES) spectroscopy. The Cu K-edge spectra show that Cu is incorporated as Cu1+ and Cu2+ in the crystals grown in air and in an oxygen atmosphere and Ru5+ is seen in the Ru K-edge spectra. The O K-edge and the Ru L3-edge spectra indicate an increase in the unoccupied states (hole concentration) as x is increased, accompanied by a lattice distortion from an increased O-Ru-O bond angle. These changes observed even with a small Cu addition are interpreted to result from the Cu 3d-O 2p hybridization as Cu replaces Ru in the octahedral sites. [ABSTRACT FROM AUTHOR]

Four perovskite compounds with nominal compositions of (Gd0.5Th0.5)NiO3, (Gd0.5Ce0.5)NiO3, (Yb0.5Th0.5)NiO3 and (Yb0.5Ce0.5)NiO3 were prepared by a polymeric citrate precursor method. They were either slow-cooled in the furnace or quenched to room temperature. All of them have monoclinic symmetry. Quenched samples have a slightly bigger unit cell volume and longer Ni-O bond length, and a smaller grain size than that of the furnace-cooled compounds. All samples are paramagnetic insulators in the range of 2 K to 300 K. The Th containing compounds have larger tolerance factor and Ni(1)–O(1)–Ni(2) bond angle than that of the Ce containing compounds. No mixed valence of Ce, Th and Ni is observed in these samples. [ABSTRACT FROM AUTHOR]

The structures of TaO6 and NbO6 oxygen octahedra in xBa(Mg1/3Ta2/3)O3+(1-x)Ba(Mg1/3Nb2/3)O3 perovskite ceramics with x=0, 0.25, 0.50, 0.75, and 1.0 were investigated by the extended x-ray absorption fine structure method. The decline in the microwave dielectric constant as x increases is caused mainly by the decrease of the mean volume of the oxygen octahedra, regardless of the 1:2 ordered structure and the distortion of the oxygen octahedron. The low Qf values of the TaO6 and NbO6 mixed samples are caused by not only the degrading of the 1:2 ordered structure but also the distortion of oxygen octahedral cages. [ABSTRACT FROM AUTHOR]