On-line delta(18)O measurement of organic and inorganic substances

A method for the automated sample conversion and on-line oxygen isotope ratio (delta(18)O) determination for organic and inorganic substances is presented. The samples are pyrolytically decomposed at 1400 degrees C in the presence of nickelized graphite. With the system presented organic as well as inorganic samples such as nitrates, sulphates and phosphates of 50-100 &mgr;g O can be analyzed for their delta(18)O values with a standard deviation usually better than 0.5 per thousand. Additionally, carbon isotope ratios of organic substances and nitrogen isotope ratios of inorganic nitrogenous compounds are available in the same sample run. Data for international and some inter-laboratory reference materials are presented to show the accuracy and reliability of the method. The effect of some additives on the CO yield was checked for substances which do not pyrolyze completely. Copyright 1999 John Wiley & Sons, Ltd.     

The frequency dependence of the ac susceptibility of the PdMn spin glass

We have measured the frequency dependence of the ac susceptibility _X(T) for three alloys of $\text{Pd}$Mn in the spin-glass regime. A strong dependence of _X(T) on frequency is observed near and below the freezing temperature in contrast to the archetypal $\text{Cu}$Mn spin glass. We discuss possible reasons for these differences.     

Adsorption of hydrogen on rhodium; Comparison with hydrogen adsorption on platinum and iridium

The adsorption of hydrogen on Rh has been studied (i) on a single crystal tip using field electron microscopy, and (ii) on a filament carrying this tip, using thermal desorption spectroscopy. The results are compared to those of other Group VIII metals. An isosteric heat of adsorption of 19 kcal/mole was found at low coverage, decreasing slightly with increasing coverage. This heat is substantially lower than that on Ru and Ir, determined by the same method. The work function increases by 0.4 eV, a value comparable to data reported for Ni and Ru, but significantly larger than those of Ir and Pt. An electropositive state of hydrogen as observed for Pt and Ir was not found for Rh. A small fraction of the adsorbed hydrogen is not desorbed at temperatures where other transition metal surfaces are completely denuded. This β₂-hydrogen which is desorbed only at 600–800 K, is tentatively assigned to a subsurface species.     

Magnetic and transport properties of some U3T3X4-compounds (T = Co, Ni, Cu and X = Sb, Sn)

We have prepared and studied a new series of isostructural U₃T₃X₄-compounds with T = Co, Ni, Cu for X = Sb and T = Ni for X = Sn. Ferromagnetism is found for U₃Co₃Sb₄ and U₃Cu₃Sb₄ below 10 and 88 K, respectively. In contrast, no magnetic order was observed down to 1.7 K for the semiconductor U₃Ni₃Sb₄ and the “moderately” heavy-fermion system U₃Ni₃Sb₄.     

The ferromagnetic transition temperature of dilute Pd-Co,Pd-Fe and Pd-Mn alloys

Results of resistivity measurements on dilute Pd-Mn alloys with concentrations from 0.15 at .% to 0.7 at .% are discussed. A calculation of the transition temperature of Pd-Co, Pd-Fe and Pd-Mn alloys with concentrations between 0.05 at .% and 3 at .% is presented and compared with the available experimental data.     

Magnetization measurements for AuFe spin-glass alloys in fields up to 40 T

High-field magnetization curves are presented for AuFe alloys of Fe concentrations: c = 3.1, 5.1, 6.5 and 10.4 at.%, at T = 4.2 K in pulsed fields up to 40 T. For c = 10.4 at.% additional data were taken at T = 1.2 K and 77 K. From these results we conclude that the nearest-neighbour interaction between the Fe atoms in Au is ferromagnetic, of order of 80 K. The behaviour of the magnetization curves can be qualitatively explained in terms of ferromagnetic clusters that are antiferromagnetically coupled.     

The beneficial effect of hydrogen on CO oxidation over Au catalysts. A computational study

Density functional theory calculations have been carried out to explore the effect of hydrogen on the oxidation of CO in relation to the preferential oxidation of CO in the presence of excess hydrogen (PROX). A range of gold surfaces have been selected including the (100), stepped (310) surfaces and diatomic rows on the (100) surface. These diatomic rows on Au(100) are very efficient in H-H bond scission. O(2) hydrogenation strongly enhances the surface-oxygen interaction and assists in scission of the O-O bond. The activation energy required to make the reaction intermediate hydroperoxy (OOH) from O(2) and H is small. However, we postulate its presence on our Au models as the result of diffusion from oxide supports to the gold surfaces. The OOH on Au in turn opens many low energy cost channels to produce H(2)O and CO(2). CO is selectively oxidized in a H(2) atmosphere due to the more favorable reaction barriers while the formation of adsorbed hydroperoxy enhances the reaction rate.     

Two-dimensional magnetic and superconducting phases in metal-insulator La(2-x)Sr(x)CuO(4) superlattices measured by muon-spin rotation

We show, by means of low-energy muon-spin rotation measurements, that few-unit-cells thick La(2)CuO(4) layers synthesized digitally by molecular beam epitaxy are antiferromagnetically ordered. Below a thickness of about 5 CuO(2) layers the long-range ordered state breaks down, and a magnetic state appears with enhanced quantum fluctuations and a reduced spin stiffness. This magnetic state can exist in close proximity (few ?) to high-temperature superconducting layers, without transmitting supercurrents.