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2014 Le comportement du bioxyde d'uranium sous des pressions jusqu'à 40 GPa a été étudié par diffraction X sur poudre dans des cellules à enclumes de diamant. L'oxyde cfc se transforme progressivement en UO2 II, phase moins symétrique, entre 29 et 38 GPa. Lorsqu'on diminue la pression, la transformation inverse se fait seulement entre 15 et 5 GPa. Il s'agit probablement d'une transformation purement structurale qui n'est pas liée à une délocalisation d'électrons 5f.Abstract. 2014 Uranium dioxide was studied by X-ray powder diffraction under pressure in diamond anvil cells up to 40 GPa. A gradual transition to a low symmetry high pressure phase (UO2 II) occurs in the range 29 to 38 GPa. On pressure release, UO2 II is conserved by hysteresis below 15 GPa, but retransforms completely to fcc UO2 I above 5 GPa. The transformation is likely to be a purely structural one which is not due to delocalization of 5f electrons.

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High‐pressure X‐ray diffraction studies have been performed on ThC powder up to 36 GPa. No structural phase transformation has been observed, the rock‐salt structure of ThC being stable over the whole pressure range. The equation of state can be described by the Murnaghan equation with the bulk modulus B0 = 109(7) GPa and its pressure derivative B′0 = 3.1(6). The high‐pressure behaviour of ThC is compared with that of other actinide carbides and nitrides and correlated with the occupation of the 5f electron shell.