Temperature-dependent Raman spectra of polycrystalline La 1Ϫx Mn 1Ϫx O 3 samples, referred to as LaMnO 3ϩ␦ for convenience, were correlated to their structural and magnetic properties for different ␦. For ␦р0.05, a softening of the ϳ610 cm Ϫ1 Raman mode is observed below the magnetic ordering temperature. This softening is reduced as ␦ increases from 0.00 to 0.05. This behavior is not associated with lattice anomalies, and is interpreted in terms of a spin-phonon coupling caused by a phonon modulation of the superexchange integral. A quantitative analysis of this effect is presented. ͓S0163-1829͑99͒10737-9͔

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Li1 - x Ni1 - y Co y O2 - δ oxides have been synthesized for 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1 by chemically extracting lithium at ambient temperature from LiNi1 - y Co y O2 with an oxidizing agent NO2PF6 in acetonitrile medium. The samples have been characterized by X-ray diffraction, wet-chemical analyses to determine lithium and oxygen contents, and infrared spectroscopy. While the nickel-rich end members Ni1 - y Co y O2 - δ have the O3 structure similar to the initial LiNiO2, but with smaller lattice parameters, the cobalt-rich end members Ni1 - y Co y O2 - δ consist of a mixture of P3 and O1 phases that are formed from the initial O3 structure of LiCoO2 by a sliding of the oxide-ion layers. The nickel-rich phases Li1 - x Ni1 - y Co y O2 - δ have an oxygen content close to 2 with a negligible amount of oxygen vacancies, while the cobalt-rich phases have considerable amount of oxygen vacancies (δ = 0.33 in CoO2 - δ). Fourier transform infrared spectroscopy indicates a semiconductor to metal transition in Li1 - x CoO2 - δ for (1 − x) < 0.77. On the other hand, Li1 - x Ni1 - y Co y O2 - δ oxides with 0 ≤ y ≤ 0.5 remain as semiconductors for the entire 0 ≤ (1 − x) ≤ 1. The differences in oxygen loss behavior and electrical conduction are explained on the basis of qualitative band diagrams.

The effect of Sr 2+ substitution for Ba 2+ on the crystal chemistry, oxygen content, thermal expansion, electrical conductivity, and catalytic activity for oxygen reduction reaction ͑ORR͒ of the double perovskite oxides GdBa 1−x Sr x Co 2 O 5+␦ has been investigated for 0 ഛ x ഛ 1.0. The GdBa 1−x Sr x Co 2 O 5+␦ system exhibits a structural change from orthorhombic ͑x = 0͒ to tetragonal ͑0.2 ഛ x ഛ 0.6͒ to orthorhombic ͑x = 1͒ with increasing Sr content. The difference in ionic radii between ͑Ba 1−x Sr x ͒ 2+ and Gd 3+ plays a dominant role in determining the oxygen-content value in GdBa 1−x Sr x Co 2 O 5+␦ , and the oxygen content and the oxidation state of cobalt increase with increasing Sr content. The electrical conductivity of the GdBa 1−x Sr x Co 2 O 5+␦ system increases with Sr content due to an increasing oxygen content and a straightening of the O-Co-O bonds as evidenced by the structural change from orthorhombic to tetragonal. Sr substitution also improves the chemical stability of the GdBa 1−x Sr x Co 2 O 5+␦ cathodes in contact with the Ce 0.9 Gd 0.1 O 1.95 and La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 2.8 electrolytes at 1100°C. The x = 0.2 and 0.6 samples with a tetragonal structure exhibit higher catalytic activity for ORR in single-cell solid oxide fuel cell than the x = 0 and 1.0 samples with an orthorhombic structure.

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The structural stability and oxygen permeation properties of Sr 3Ϫx La x Fe 2Ϫy Co y O 7Ϫ␦ with 0 р x р 0.3 and 0 р y р 1.0 have been studied at high temperature (800 р T р 900°C) and in the oxygen partial pressure range 10 Ϫ5 р pO 2 р 0.21 atm. These phases have a perovskite-related intergrowth structure with tetragonal symmetry, which does not change with temperature up to 1000°C in the range 10 Ϫ5 р pO 2 р 0.21 atm. The oxygen permeation flux of Sr 3Ϫx La x Fe 2Ϫy Co y O 7Ϫ␦ membranes increases with increasing Co content, decreases with increasing lanthanum content, and does not change with time. Measurements of oxygen permeation flux as a function of membrane thickness for Sr 2.7 La 0.3 Fe 1.4 Co 0.6 O 7Ϫ␦ at 900°C indicates the oxygen transport is bulk limited for this composition. Assuming that this observation is also valid for other compositions, the ionic conductivity, i , and the vacancy diffusion coefficient D v have been estimated. The structural stability and permeation properties of Sr 3Ϫx La x Fe 2Ϫy Co y O 7Ϫ␦ are compared with those of the perovskite phases SrFe 0.2 Co 0.8 O 3Ϫ␦ and Sr 0.6 La 0.4 Fe 0.2 Co 0.8 O 3Ϫ␦ .

The high temperature crystal chemistry and oxygen permeation properties of the cation-ordered LnBaCo2normalO5+δ perovskite oxides [lanthanide (Ln)=Pr , Nd, and Sm] have been investigated in comparison with the cation-disordered La0.5Ba0.5CoO3−δ perovskite. The LnBaCo2normalO5+δ ( Ln=Pr , Nd, and Sm) oxides exhibit a metal-insulator transition at T<200°C , as evidenced by total conductivity measurements and high temperature X-ray diffraction data and an oxygen vacancy order–disorder transition at T350°C in air, as evidenced by an orthorhombic to tetragonal transition. At a given temperature, the oxygen permeation flux decreases from Ln=La to Nd to Sm due to the changes in crystal symmetry and lattice strain. The oxygen permeation mechanism in the Ln=Nd is bulk-diffusion-limited rather than surface-exchange-limited for membrane thickness L≥1.1mm .