P. Selvam

Synthesis of Mg2FeH6, Mg2CoH5 and Mg2NiH4 by high-pressure sintering of the elements

Abstract The title compounds were synthesized by sintering mixtures of fine metal powders at optimized experimental conditions of temperature (450–500°C) and hydrogen pressure (90 bar). In contrast to previous work, nearly single-phase samples of up to 10 g were obtained in a single-step process.

New ternary and quaternary metal hydrides with K2PtCl6-type structures

Abstract The ternary alkaline earth transition metal hydrides Ca 2 FeH 6 , Sr 2 FeH 6 , Eu 2 FeH 6 , Mg 2 RuH 6 , Mg 2 OsH 6 and Ca 2 OsH 6 were synthesized from metal mixtures at 450–500°C and 70–130 bar H 2 pressure. They crystallize with the f.c.c. K 2 PtCl 6 structure type and contain octahedral [TH 6 ] 4− (T = Fe, Ru, Os) hydride complexes with bond distances [T-D] = 1.618(5) A (Ca 1.98 Mg 0.02 FeD 6 ), 1.673(4) A (Mg 2 RuD 6 ), 1.682(2) A (Mg 2 OsD 6 ) and 1.728(5) A (Ca 1.92 Mg 0.08 OsD 6 ) as refined from neutron powder diffraction of magnesium-containing deuterides. In systems containing two alkaline earth elements quaternary hydrides with primitive cubic structures are formed in which the alkaline earth atoms are ordered. Preliminary neutron data on iron-based CaMgFeD 6 and nickel-based CaMgNiD 4 confirm the presence of octahedral [FeD 6 ] 4− and tetrahedral [NiD 4 ] 4− complexes respectively.

XPS studies of the surface properties of CaNi5

Abstract The “air-exposed” sample of CaNi 5 contains various oxygenated species on the surface. The surface properties of CaNi 5 are analogous to those of LaNi 5 with surface enrichment of calcium similar to lanthanum in LaNi 5 .

Surface properties of LaNi5: A reinvestigation

Abstract Although a number of surface analysis reports are available on the LaNi5 system, yet a clear picture of the actual species involved and their consequences on the activation and deactivation processes has not been reached. Moreover, differences of opinion have been seen among the researchers. The present results distinctly indicate that the air exposed samples are completely contaminated with a variety of oxygenated species (such as oxides, hydroxides and carbonates) of the constituent elements, hitherto not reported by others. The protective nature of such species in regard to the durability of the alloy and the formation of supported-metal type species upon activation accounts for its excellent behaviour. The relative higher stability of LaNi5 over CaNi5 under cyclic operating conditions has been explained based on surface energy criteria of the constituent elements rather than the deterioration initiated by hydrogen and/or gaseous impurities alone. In addition, the specific importance of X-ray induced Auger electron spectroscopy (XAES) technique in conjunction with X-ray photoelectron spectroscopy (XPS) has been highlighted in the identification of different species.

Surface properties and their consequences on the hydrogen sorption characteristics of certain materials

Abstract Surface analyses (X-ray photoelectron spectroscopy, X-ray-induced Auger electron spectroscopy and Auger electron spectroscopy) results of some hydrogen storage intermetallic compounds of the type AB, AB5, A2B and the hydride, A2BH4 (A ≡ Ti, La, CaorMg; B ≡ Fe, NiorCu) show the presence of numerous oxidation (aerial) products, such as oxides, hydroxides and carbonates. In general, the hydroxide and/or carbonate species are present in small amounts and persist only on the outer passivated surface. However, there are cases where they seem to be predominant, at least in the top few layers. Therefore such species are also expected to play an important role in the surface chemistry of the alloys and hydrides similar to that observed with surface oxides. Accordingly, the activation procedure varies for the different materials. Upon activation the surface may be described as a supported metal system which in turn increases the activity thus explaining the rapid kinetics of the hydriding/dehydriding reactions upon cycling processes. The easy activation of certain alloys could also be explained on the basis of the reducibility and the thickness of the surface oxide layers. In other words, it could be demonstrated based on their oxidation resistance and the segregation behaviour of the metals. The specific reactivity of different systems are related to the segregation, oxidation, and reduction behaviours. Surface enrichment of iron in the TiFe system is demonstrated for the first time.

Energy and environment—An all time search☆

Abstract While worldwide concern for the environment grows, some promising international initiatives take shape. This report gives an outlook of the current problems and summarizes the recent progress. It also outlines a concrete step and some proposals for action. In this context, the study evaluates the potential of hydrogen as a non-polluting fuel and as a true energy carrier for the future. The various other possible approaches are briefly described.

Hydrogen-stabilized Mg2RhH1.1 with filled Ti2Ni-type structure

Mg2RhH1.1 is the first example for a hydrogen stabilized binary metal compound with filled Ti2Ni-type structure. Its composition was refined from X-ray and neutron powder diffraction data on the deuteride and found to be Mg2RhD1.1. The deuterium atoms in the structure occupy octahedral holes formed by Mg atoms and tetrahedral holes formed by Mg and/or Rh atoms. Retrieval of hydrogen by desorption destabilizes the structure and leads to a hitherto unknown binary metal compound of composition Mg2Rh which crystallizes with the Ti2Pd-type structure, a branch of the MoSi2-type structure.

Evidence for the formation of surface carbonates on some hydrogen storage intermetallic compounds: an XPS study

Abstract X-Ray photoelectron spectroscopic (XPS) studies of air exposed samples of LaNi 5 , CaNi 5 , Mg 2 Ni, Mg 2 Cu and FeTi indicate that the interaction of atmospheric carbon dioxide results in the formation of surface carbonates in addition to the usual oxides and hydroxides. The reaction of carbon dioxide, in fact, is favoured by the oxidized surface which in turn produces the carbonate species on the top few layers, hitherto not reported. In particular, the carbonates seem to be predominant in the case of CaNi 5 . The adsorption of carbon dioxide also gives an idea of the segregated elements of the different systems. Even though such carbonate species on the alloy surfaces seem to be expected rather than surprising, there is no evidence available in the literature. To our knowledge, this forms the first report on such a problem. The results also provide valuable information concerning the understanding of various surface reactions and the nature of activation processes which often differ between materials involved in hydrogen uptake.

Ion-induced carbide formation of TiFe : evidence from XPS and AES studies

X-ray photoelectron and Auger electron spectroscopic studies on TiFe demonstrate that ion bombardment of the alloy not only results in sputter cleaning or even destruction of the surface but also leads to the formation of compact carbide layers. Such carbide species are of considerable importance in elucidating the reaction mechanism of hydrocarbon synthesis. They are also expected to play a significant role in the hydrogen sorption properties of materials. A better understanding of the nature of sputtered carbon is needed for its use as protective (against corrosion) thin films in magnetic recording applications. A comparison of photoelectron and Auger electron spectra of the surface carbides of TiFe would also be of interest since it is unusual for both types of data to be produced on the same samples where chemical shifts are involved.

Note on the formation of surface carbides

Abstract Features of the effect of ion bombardment and its influence on the composition of solid surfaces are considered. During ion bombardment, not only does cleaning (sputtering) of the surface take place, but the formation of carbides through the implantation of carbon from contamination also occurs. Such species were recognized clearly from their chemical shifts of XPS on intermetallic compounds, namely LaNi 5 , CaNi 5 , TiFe, MgaNi and Mg 2 Cu and on the hydride Mg 2 NiH 4 . These surface carbides, for example, can seriously restrict data analysis owing to their contribution to the core level alterations, and the rate of sputtering owing to their extreme hardness. However, ion bombardment would serve as an unusual method of preparation of some compounds which cannot be obtained by conventional means. Types of problems, methods and results of their investigation are described.