G.Ranga Rao

Nature of nitrogen adsorbed on transition metal surfaces as revealed by electron spectroscopy and cognate techniques

The nature of the chemisorbed states of nitrogen on various transition metal surfaces is discussed comprehensively on the basis of the results of electron spectroscopic investigations augmented by those from other techniques such as LEED and thermal desorption. A brief discussion of the photoemission spectra of free N2, a comparison of adsorbed N2 and CO as well as of physisorption of N2 on metal surfaces is also presented. We discuss the chemisorption of N2 on the surfaces of certain metals (e.g. Ni, Fe, Ru and W) in some detail, paying considerable attention to the effect of electropositive and electronegative surface modifiers. Features of the various chemisorbed states (one or more weakly chemisorbed gamma-states, strongly chemisorbed alpha-states with bond orders between 1 and 2. and dissociative chemisorbed beta-states) on different surfaces are described and relations between them indicated. While the gamma-state could be a precursor of the alpha-state, the alpha-state could be the precursor of the beta-state and this kind of information is of direct relevance to ammonia synthesis. The nature of adsorption of N2 on the surfaces of some metals (e.g. Cr, Co) deserves further study and such investigations might as well suggest alternative catalysts for ammonia synthesis.

Superconductivity in the Bi2(Ca, Sr)n+1CunO2n+4 (n=1, 2, or 3) series: Synthesis, characterization and mechanism

Abstract The n =1, 2 and 3 members of the Bi 2 (Ca, Sr) n+1 Cu n O 2n+4 series of superconductors have been synthesized and characterized by a variety of techniques including electrical resistivity, magnetic susceptibility and non-resonant microwave absorption. Based on studies with a number of compositions, the T c ranges for the n =1, 2 and 3 members are suggested to be 60±20 K, 82±8 K, and 107±3 K, the rather broad ranges arising from differences in sample stoichiometry and homogeneity. Electron spectroscopic studies show that Cu is mainly in the 2 + and 1 + states suggesting the importance of oxygen holes.

Evidence for holes on oxygen in some nickel oxides

Core-level spectroscopic studies show the presence of holes on oxygen in LaNiO3 and LiNiO2 Nickel in these oxides seems to be essentially in the 2+ state instead of the 3+ state-where it would formally be expected to be on the basis of the stoichiometry.

Elusive superconductivity in polycrystalline samples of layered lanthanum nickelates

La2-xNiO4, La2-xSrxNiO4 and related layered nickelates have been investigated for possible presence of superconductivity. While there is clear onset of diamagnetism around 20 K in many of these nickelates, we do not, however, find any anomaly in the electrical resistivity, magnetoresistance or thermopower around 20 K. High energy spectroscopic studies show Ni to be in the 2+ oxidation state accompanied by a substantial proportion of oxygen holes.

A combined XPS-UPS-EELS study of nitrogen adsorbed on clean and barium-promoted iron surfaces: The nature of the precursor to dissociation

For N2 on a clean Fe surface, the adsorbed precursor in a parallel orientation becomes predominant around 110 K, while at lower temperatures it coexists with a weakly adsorbed species. On a Ba-promoted Fe surface, however, N2 is present exclusively in the precursor state in the temperature range 80–150 K following moderate exposure. Besides exhibiting a low N-N stretching frequency of 1530 cm−1, the precursor shows a clear separation between the 5σ and 1π levels in the UPS; the precursor dissociates to give a nitridic species around 160 K.

Adsorption of nitrogen on clean and modified single-crystal Ni surfaces

Abstract N2 chemisorbs on (100), (110) and (111) single-crystal faces at 80 K without any dissociation. A molecular precursor species at the threshold of dissociation is formed at 80 K on a Ni(110) surface covered with atomic carbon or nitrogen; the precursor species dissociates at higher temperatures. At low C concentrations, however, dissociative adsorption occurs on Ni(110) at 80 K without forming any precursor species, accompanied by molecular chemisorption on the open Ni sites.

Nitrogen adsorbed on clean and promoted Ni surfaces

Abstract The temperature dependence of the N(1s) spectrum of N 2 adsorbed on a polycrystalline Ni surface suggests the presence of two species. N 2 adsorbed on Ba- or Al-promoted Ni is more strongly bound to the surface as evidenced by the decrease in the intensity of the N(1s) signal due to the unscreened final state relative to the screened final state as well as an increased separation between the 5 σ and 4 σ levels; intra-molecular bonding on promoted surfaces is weaker as also found by EELS studies. Electron-withdrawing modifiers, on the other hand, decrease the strength of the metal-nitrogen bonding.

Adsorption of CO and N2 on modified transition-metal surfaces

Electron spectroscopic studies clearly demonstrate that modification of the surfaces of Mn, Fe and Ni metals by chlorine significantly decreases the strength of interaction between the metal and adsorbed molecules such as CO and N2. This is in contrast to the effect of electropositive additives such as Ba and Al which increase the adsorption bond strength significantly.

Investigations of oxide superconductors by x-ray absorption, photoemission and cognate spectroscopies

Abstract X-ray absorption spectra, X-ray photoelectron spectra and Auger spectra of cuprate superconductors are discussed. The studies establish the absence of Cu3+ for all practical purposes, but point out the importance of oxygen holes. X-ray photoelectron spectra of BaBi0.25Pb0.75O3 and related compounds are also examined.

A comparative study of CO and N2 adsorbed on clean and promoted transition metal surfaces by a combined use of EELS, XPES and UVPES

CO and