The microscopic insight into how and why catalytically active nanoparticles change their shape during oxidation and reduction reactions is a pivotal challenge in the fundamental understanding of heterogeneous catalysis. We report an oxygen-induced shape transformation of rhodium nanoparticles on magnesium oxide (001) substrates that is lifted upon carbon monoxide exposure at 600 kelvin. A Wulff analysis of high-resolution in situ x-ray diffraction, combined with transmission electron microscopy, shows that this phenomenon is driven by the formation of a oxygen-rhodium-oxygen surface oxide at the rhodium nanofacets. This experimental access into the behavior of such nanoparticles during a catalytic cycle is useful for the development of improved heterogeneous catalysts.

Magnetization measurements of a Gd 0.5 Ba 0.5 CoO 3 perovskitelike compound have revealed an anomalous behavior at T i 240 K and T C 277 K that corresponds to appearing and disappearing spontaneous magnetization. The transition at T i is accompanied by the jump of conductivity and a giant magnetoresistance. Below T i , Gd 0.5 Ba 0.5 CoO 3 exhibits metamagnetic behavior. At about T M 370 K, a transition from a semiconductive to a quasimetallic state has been observed. It is supposed that a ferromagnetic low-spin cobalt state occurs in the temperature range between T i and T C .[S0031-9007(98)05789-5] 72.15.Gd, 75.70.Pa La 12x A x CoO 3 (A Sr, Ba) cobaltites are of considerable interest because of the peculiar way their magnetic and transport properties change with composition and temperature [1][2][3][4][5]. While LaCoO 3 shows a high resistivity and antiferromagnetic exchange interaction, the La 12x Sr x CoO 3 solid solutions evolve toward a ferromagnetic intermediate-spin state with itinerant 3d electrons as x increases [4][5][6][7]. The composition with x 0.5 is a metallic ferromagnet with a moment approximately 1.5m B per formula unit and the Curie temperature of 220 K. The size of the Ln ion is well known to influence strongly the magnetic and transport properties of the compounds with the perovskite structure. However, the data on cobaltites of rare earth elements doped by Ba or Sr are quite limited. The studies of Ln 12x Sr x CoO 3 (Ln Pr, Nd, Sm) have shown these materials to be similar to La 12x Sr x CoO 3 with T C increasing with x as well as with the size of the rare earth ion [3].This paper reports the discovery of a new family of magnetic semiconductors exhibiting both antiferromagnetferromagnet and metal-insulator first-order phase transitions. The nature of these transitions and properties of Gd 0.5 Ba 0.5 CoO 3 compound differs from those observed in manganites as well as other related materials studied earlier. We have examined structural, magnetic, and transport properties of the Gd 12x Ba x CoO 3 perovskites in order to investigate the cobaltite properties as a function of lanthanide ionic radii and a variation of the spin state of the cobalt ions with temperature. We have found that Gd 0.5 Ba 0.5 CoO 3 exhibits a first-order phase transition accompanied by dramatic changes of magnetic properties and an anomalous behavior of electrical transport at about T i 240 K.The studied ceramic samples were prepared by a solid state reaction. Mixtures of Gd 2 O 3 , BaCO 3 , Co 3 O 4 were pressed into pellets, sintered at 1473 K for 5 h in air and then cooled to room temperature at a rate of 100 K͞h. This process was repeated in order to obtain a homogeneous solid solution. According to the powder x-ray diffraction (XRD) patterns, the specimens were single phase. The XRD data at room temperature were indexed on the basis of a distorted perovskite-type structure with orthorhombic symmetry. For Gd 0.5 Ba 0.5 CoO 3 the lattice parameters were calculated to be a 3.909 Å, b 3.876 Å, and c 3.768 Å. As oxygen ...

We have investigated the dissolution of a natural oxide layer on a Nb(110) surface upon heating, combining x-ray reflectivity, grazing incidence diffuse scattering, and core-level spectroscopy. The natural oxide reduces after heating to 145°C partially from Nb2O5 to NbO2, and an enrichment in subsurface interstitial oxygen by ∼70% in a depth of 100Å is observed. After heating to 300°C, the oxide reduces to NbO and the surplus subsurface oxygen gets dissolved into the bulk. Our approach can be applied for further investigation of the effect of subsurface interstitial oxygen on the performance of niobium rf cavities.