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Specific heat and resistivity measurements were performed on polycrystalline samples of the solid-solution Y x Lu 1Ϫx Ni 2 B 2 C in order to determine thermodynamic properties such as the specific-heat difference ⌬C, the thermodynamic critical field H c (T), as well as the upper critical field H c2 (T). These properties were analyzed within the Eliashberg theory including anisotropy effects, yielding electron-phonon coupling anisotropy parameters ͗a k 2 ͘ ranging between 0.02 and 0.03 for the whole series, and Fermi velocity anisotropy parameters of ͗b k 2 ͘ϭ0.245-0.3. Excellent agreement between theory and experiment was achieved for these parameters, the Sommerfeld constant ␥ and model phonon spectra determined from specific heat measurements. An analysis of the previously investigated boronitride La 3 Ni 2 B 2 N 3Ϫ␦ for comparison revealed the electron-phonon anisotropy to be of great significance in describing its thermodynamic properties and the calculations yielded ͗a k 2 ͘ Ӎ0.08 and ͗b k 2 ͘Ӎ0.245. The T c behavior within the series Y x Lu 1Ϫx Ni 2 B 2 C is discussed in terms of the density of states at the Fermi level N(0).

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Weak itinerant ferromagnetism in YCo9Si4 below about 25 K is studied by means of magnetisation, specific heat, and resistivity measurements. Single crystal X-ray Rietveld refinements at room temperature reveal a fully ordered distribution of Y, Co and Si atoms within the tetragonal space group I4/mcm isostructural with LaCo9Si4. The latter exhibits itinerant electron metamagnetism with an induced moment of about 1 µB/f.u. above 6 T, whereas YCo9Si4 exhibits a spontaneous magnetisation M0 ≃ 12 Am 2 /kg at 2 K which corresponds to an ordered moment of about 1.6 µB/f.u. indicating weak itinerant ferromagnetism.

The phase relations of the GdCo 13−x Si x system have been studied by means of scanning microscopy, microprobe analysis and x-ray diffraction. Single-phase samples GdCo 9±δ Si 4∓δ (structure type LaFe 9 Si 4 with space group I 4/mcm) are formed in a narrow composition range |δ| 0.2, where GdCo 9 Si 4 forms as a fully ordered ternary compound. The magnetic properties of GdCo 9 Si 4 have been investigated by ac susceptibility, magnetization, specific heat and resistivity measurements. These experiments reveal ferrimagnetism below about 47 K, which is analysed in terms of a two-sublattice molecular field model with a local moment Gd subsystem coupling antiparallel to the itinerant ferromagnetic Co 3d sublattice. The 3d-3d exchange of the latter is the driving force for the onset of long range magnetic order. The validity of the twosublattice model is demonstrated by high field measurements, showing that the ferrimagnetic coupling starts to break up at a lower critical field µ 0 H l 27 T. The magnetic ground state of GdCo 9 Si 4 has been analysed on a microscopic basis via LSDA electronic structure calculations.