C. N. Ramachandra Rao

Promotion of the metal–oxide support interaction in the Ni/TiO2 catalyst. Crucial role of the method of preparation, the structure of TiO2 and the NiTiO3 intermediate

Based on in situ X-ray diffraction and EXAFS studies, it is shown that NiTiO3 formed by the reaction of TiO2(anatase) with Ni2+ during the calcination of Ni/TiO2, gives rise to Ni metal and TiO2(rutile) on reduction, a process that may be responsible for the strong interaction between the metal and the oxide support.

Structure and magnetotransport properties of the layered manganites Re1.2Sr1.8Mn2O7(RE = La, Pr, Nd)

We have examined the magnetotransport properties and the structure, by Rietveld refinement of powder X-ray data, of the phases RE1.2Sr1.8Mn2O7(RE = La, Pr, Nd). We find that on cooling, La2Sr1.8Mn2O7 undergoes a transition to a nearly perfect ferromagnet with 90% magnetization at 1.45 T, as reported by earlier workers, but the Pr and Nd phases show only a small magnetization that grows gradually as the temperature is decreased. There seems to be significant correlation between electrical transport and the Jahn-Teller elongation of the apical Mn–O bonds in these systems. The elongation of the apical Mn-O bonds forces the nine-coordinate rock-salt site to be occupied preferentially by the smaller rare-earth-metal cations. This preferential occupation is reliably obtained from the X-ray refinement. All three title phases show a magnetoresistance ratio of about 4 (corresponding to a magnetoresistance, [R(0)–R(H)]/R(0), of about 75%) at a field of 7 T and temperatures around 100 K.

Investigations of transition-metal clusters deposited on solid substrates by photoelectron spectroscopy, bremsstrahlung isochromat spectroscopy and related techniques

The 2p core-level binding energies of Cu and Ni clusters increase while the kinetic energies of the LMM Auger lines decrease with decreasing cluster size. The shifts in the core-level binding energy and the Auger kinetic energy relative to the bulk metal are largest in the case of clusters deposited on an insulating substrate such as Al2O3 and least on amorphous carbon; shifts on TiO2 are intermediate between those on Al2O3 and carbon. The increase in core-level binding energies with decreasing cluster size signifies a metal to non-metal transition. The occurrence of such a transition is borne out more vividly by BIS studies. Accordingly, BIS studies on Pd clusters deposited on amorphous carbon clearly show that electron states are progressively introduced near the Fermi level with increase in cluster size. The chemical reactivity of transition-metal clusters varies with the size. Thus, the relative proportion of the atomic oxygen species increases with decreasing size of Ag clusters.

Structure and superconducting properties of Ga-substituted YBa2Cu3O7–δ and YBaSrCu3O7–δ systems

In YBa2Cu3–yGayO7–δ, Ga can be substituted at the Cu(1) site up to y= 0.1 without change in structure, but this is accompanied by a slight decrease in the hole concentration and Tc; the same is true when Y is partly substituted by Ca, as in Y1–xCaxBa2Cu3–yGayO7–δ(0.0 0.2. The y= 0.3 and 0.4 compositions show metal–semiconductor transitions at 60 and 120 K, respectively, while the y= 0.6 composition is a semiconductor. When Y is partly substituted by Ca as in Y1–xCaxBaSrCu3–yGayO7–δ, the material is superconducting even when y= 0.3. All these Ga-substituted cuprates are in the underdoped region and accordingly Tc increases with increase in hole concentration.

Interaction of nitrogen and oxygen with C60

Evidence is presented for the strong interaction of nitrogen and oxygen with buckminsterfullerene.

An electron spectroscopic study of the adsorption of nitrogen on a Ni/TiO2 catalyst prepared in situ in the spectrometer: evidence for dissociative adsorption in the strong-metal-support-interaction (SMSI) state

Nitrogen is dissociatively adsorbed on an annealed Ni/TiO2 surface just as on a Ti–Ni alloy surface while it is molecularly adsorbed on a Ni/Al2O3 surface.

Oxygen chemistry in copper oxides: evidence for O– species

Core-level spectroscopic studies suggest that cuprates nominally supposed to contain Cu3+ ions are likely to have the excess positive charge on oxygen instead, giving rise to O– type species (oxygen holes).

Electronic transitions in hydrogen bonded dimers of carboxylic acids in the vapour phase: an electron energy loss spectroscopic study

An electron energy loss spectroscopic study of the formic acid dimer has shown bands centred around 7.2, 8.5, 9.8, and 11.1 eV, of which the first and the third bands are assigned to n–π* transitions and the other two to π–π* transitions; similar transitions are found in the acetic acid dimer.