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The enthalpies of formation are measured in the intermetallic compounds Zr2Cu, ZrCu and the amorphous alloy Zr0.55Cu0.45 by means of a solution calorimeter based on liquid aluminium. The crystallization enthalpies of the amorphous alloys Zr1 —xCux with x = 0.38, 0.45, and 0.55 are determined by means of a differential scanning calorimeter. All these values are found to be relatively small compared with the activation energies for crystallization derived from a study of the crystallization temperature as a function of the heating rate. The enthalpy difference between amorphous and crystalline materials is analysed in terms of Miedema's model. The results indicate that the lower enthalpy content in the former materials is mainly due to a less effective surrounding of the atoms by dissimilar atoms.

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The structural properties of the compound YSi 2 are investigated by means of ab initio simulations based on density-functional theory. More particularly we emphasize the role played by Si vacancies, show that the Th 3 Pd 5 structure can be deduced from the AlB 2 structure by a relaxation process around the Si vacancies within the ͑0001͒ plane. A specific ordered arrangement of the Si vacancies along the ͓0001͔ direction is found to be energetically the most favorable. Geometries obtained from our theoretical calculations are in good agreement with experimental results. ͓S0163-1829͑97͒06920-8͔

High-pressure polymorphs of WO3 have been studied with a first-principles pseudopotential method. The medium-range (0.01–20 GPa) and high-range (20–30 GPa) polymorphs have been characterized and are compared with recent experimental results. The main new feature is the appearance of a sevenfold coordinated tungsten in the high-pressure polymorph. The subtle phase transitions that were induced from Raman spectra evolutions have not been confirmed. However, changes in the W–O distances and O–W–O and W–O–W angles may explain the changes in Raman spectra.

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