W.C.M. Mattens

On the valence state of Yb and Ce in transition metal intermetallic compounds

Abstract In the pure state Yb is a divalent metal, similar to Ca; in alloys it can become trivalent like the majority of the rare earth metals. Using a value of 38 kJ (mol Yb)−1 for the energy difference between divalent and trivalent Yb metal and using model calculations for the heat of formation of intermetallic compounds, we are able to account for the existing information on the valence state of Yb in transition metal compounds. A similar analysis of compounds of Ce with transition metals shows that a model in which the 4f electron is treated as a core electron, i.e. being absent in the tetravalent modification of Ce and present as a fully localized electron in trivalent Ce, does not apply.

Nuclear resonance and relaxation in intermediate‐valent YbCuAl

27Al NMR shifts and spin‐lattice relaxation times have been measured in the intermediate‐valent compound YbCuAl over the temperature range 4.2 K to 295 K. Above Tmax?25 K, the temperature for which the bulk susceptibility χ exhibits a maximum, a linear relation was observed between K and χ: K=7.5(5) ×10−4−0.19(1) χ (emu/mole). For T<Tmax this relation was no longer obeyed; at 4.2 K the shift was about 20% more positive than predicted from the linear relation and the measured χ. Satellite NMR in the Kondo alloy CuFe exhibits no such nonlinearity. It is tentatively attributed to a temperature‐dependent effect of intermediate valence on the transferred hyperfine coupling between Yb moments and 27Al nuclei. Relaxation data yield a 4f spin fluctuation rate which decreases by about a factor of 2 between Tmax and 100 K. A simple ionic model for the intermediate‐valent state accounts qualitatively for this observation. Further corroborating experiments are suggested.

The 40-T facility of the University of Amsterdam

Abstract An overview is given of the high-field installation of the University if Amsterdam now in operation; this installation can generate fields up to 40 T, constant within 8 mT for about 80 ms. Recent improvements of the regulation of the field are discussed, as well as some examples of recent measurements.

Thermal expansion and magneto-volume effects in the mixed-valent compound YbCuAl

Abstract Thermal expansion, longitudinal magnetostriction and magnetization under high pressures of YbCuAl are presented. All results are in complete accordance with what is expected for a compound containing Yb in the mixed-valent state.

Neutron inelastic scattering experiments on the mixed-valent compound YbCuAl

Abstract The dynamical susceptibility of YbCuAl has been established by means of time of flight neutron scattering experiments. Non-Korringa behaviour of the quasi-elastic line-width has been found and comparison with NMR data will be given.

Design of magnet coils for semi-continuous magnetic fields up to 60 T

Abstract Two constraints on semi-continuous magnet coils are the strength of the winding material, and the maximum power which the rectifier must deliver for a short time. Here we assume that for a 60-T coil wire of sufficient strength (about 100 kg/mm2 is available. It was shown earlier that a single, homogeneous coil needs a power of nearly 150 MW, which calls for huge capital investments in power lines, transformers, switches and rectifiers. By using two nested coils, wound from wires with different strengths and current densities, and energized successively by the same rectifier, we can reduce the power to less than 50 MW.

Magnetic properties of ytterbium intermetallic compounds with intermediate valency

Out of 17 binary and 2 ternary ytterbium intermetallics, in the five systems, YbGa2, YbCu2, YbAl2, YbAl3 and YbCuAl an intermediate valency was found. The χ(T) curves for these compounds have a common functional form with a characteristic temperature Tmax. No tendency to saturation is observed in the magnetization at 1.4 K in fields up to 330 kOe.

Intermediate-valence state of Yb in Y- and Gd-substituted YbCuAl

Abstract Yb 1− x Y x CuAl with x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.9, 0.99 and 1.0 and Yb 0.99 Gd 0.01 CuAl samples were prepared by the methods known to produce perfectly ordered, single phase YbCuAl. The susceptibility between helium and room temperatures and the magnetization at helim temperature in fields up to 34 T were measured for the whole series. The results are consistent with a single-ion description of the intermediate-valence state: Y substitution leads to a proportional reduction of susceptibility and magnetization, without loss of the characteristic intermediate-valence features, and Gd substitution gives an additive Curie-Weiss contribution without any sign of giant-moment formation.

Valency and Magnetic Behaviour of Ytterbium in Intermetallic Compounds

We have investigated 17 binary and some ternary intermetallic compounds of Yb in order to determine the factors controlling its valency. From susceptibility measurements between 1.4 and 1060 K the trivalent state can be firmly established by the validity of the Curie‐Weiss law in 7 systems. Diamagnetism or weakly temperature dependent paramagnetism, indicative of divalent Yb, is found in 7 systems. In YbGa2 and especially YbCu2 the temperature dependence is too strong to classify these systems as simple Pauli paramagnets, and in YbAl2, YbAl3 and YbCuAl the susceptibility goes through a maximum and becomes Curie‐Weiss like at high temperatures. Yb in the latter five systems is considered having an intermediate valency. A correlation is established between the valency of Yb and the electronegativity of its metallic environment. The temperature dependence of the susceptibility of the intermediate‐valency systems can be reasonably described by invoking a 4f resonance on each Yb ion, but the complete lack of s...

On the valence state of Yb, Eu and Ce in non-transition metal compounds

Abstract By means of a semi-empirical model to calculate the heats of formation of intermetallic compounds, the valence states of Yb, Eu and Ce in intermetallic compounds with non-transition metals are predicted. The results are compared with existing experimental information on the valence states from magnetic properties or lattice parameter data.