Hydrogen in porous Vycor glass ͑pore radius R p ϭ3.0 nm͒ has been investigated with a torsional oscillator technique at 7.5 K рTр 22 K. H 2 molecules which are adsorbed in Vycor at TϾT 3 (T 3 , triple point of bulk H 2 ) leave the Vycor when decreasing the temperature to below a characteristic value T c рT 3 ; T c depends on the amount of H 2 in the Vycor. This interpretation of the data is supported by simultaneous measurements of the H 2 vapor pressure. A similar phenomenon is observed with a capacitor filled with Vycor into which H 2 is condensed. We conclude that the free energy of solid H 2 in the Vycor is larger than that of bulk solid H 2 . Information on the free energy of H 2 confined in the Vycor is important to understand the depression of its freezing temperature in restricted geometries. We also discuss the properties of solids and the depression of their freezing temperature in restricted geometries.

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We investigated the low-temperature properties of polycrystalline HoNi2 11 B2C by means of ac-susceptibility and neutron powder diffraction in the temperature range of 1.5 to 10 K and zero magnetic field. A sample with pronounced re-entrant behaviour was chosen since it exhibits a superconducting state which is strongly affected by the intrinsic magnetism, the formation of long range magnetic order, via pair-breaking. The evolution and volume fractions of the three different magnetic structures were observed and correlated to the superconducting behaviour.

The low-temperature properties of HoNi 2 B 2 C strongly depend on its thermodynamic state established via thermal treatment. We present high resolution x-ray powder diffraction data taken on a pair of polycrystalline samples with identical chemical composition (HoNi 2 B 2.1 C) but annealed at different temperatures, namely, at 800 and at 1100°C. Their superconducting transition temperatures differ by more than 10%. Rietveld refinement and difference Fourier analysis reveal subtle differences in the atomic parameters and in the electron density on the carbon site. Furthermore, a new atomic site can be identified for both samples, which is partially occupied with the lighter atoms boron or carbon.