J.P. Itié

Delocalisation of 5f electrons in curium metal under high pressure

Curium metal was studied by X-ray diffraction up to 52 GPa. Starting with the double-hexagonal close-packed structure (Cm I), a cubic close-packed structure (Cm II) was formed at 23 GPa and another high-pressure phase (Cm III) at 43 GPa. The latter phase can be indexed as an orthorhombic cell of the alpha -uranium-type structure. The volume difference between the cell of Cm III and that of Cm II, together with the formation of a low-symmetry (orthorhombic) crystal structure, suggests that itineracy of the 5f electrons in curium metal starts at 43 GPa. The fact that the delocalisation pressure is higher in curium than in the neighbouring americium, berkelium and californium metals is explained qualitatively on the basis of a comparison of localisation energies.

Structural stability and equation of state of thorium carbide for pressures up to 36 GPa

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.

X-ray diffraction of curium-248 metal under pressures of up to 52 GPa

Abstract An investigation of the structural behavior of curium-248 metal under pressures of up to 52 GPa has been carried out. Three different metal phases were observed over the range of applied pressure. The initial double hexagonal close-packed structure (Cm I) of the curium metal first transformed at 23 GPa to an f.c.c. structure (Cm II), which then converted to an orthorhombic structure (Cm III). A bulk modulus of 33(5) GPa was derived from the relative volume V V 0 and pressure data for curium metal below 40 GPa.

Delocalisation of 5f electrons in berkelium-californium alloys under pressure

The high-pressure structural behaviour of two berkelium-californium alloys was studied up to about 47 GPa. The alloys had been formed through the ingrowth of 249Cf into 249Bk by beta - decay. The berkelium-rich alloy (about 35% Cf) exhibited the same phases as pure berkelium metal, the californium-rich alloy (about 60% Cf) the same phases as pure californium. An approximately linear increase of transition pressures with californium content was observed. The results are compared with data obtained previously for the high-pressure behaviour of the pure americium, curium, berkelium and californium metals. As in the pure metals, the transformation of the alloys to the alpha-uranium-type structure, accompanied by a sudden volume decrease, is interpreted as the onset of 5f electron itinerancy.

High pressure X-ray diffraction on UX2 compounds

Abstract High pressure X-ray diffraction was performed on several intermetallic uranium compounds UX 2 (X  Co, Fe, Mn, Ir, Al). All the compounds have the same structure at atmospheric pressure (cubic MgCu 2 type). For UMn 2 , UFe 2 , UCo 2 and UIr 2 , this structure remains stable in the pressure range investigated. In the case of UAl 2 , a transformation to a not yet identified new structural phase is observed around 10 GPa. The bulk moduli of all compounds have been determined using the first order Murnaghan equation of state. Differences between these compounds are discussed in terms of 5f electron delocalisation.

Structural study of americium to 52 GPa

Abstract The structural phase transitions of americium under pressure were re-investigated. While 18 GPa was the highest pressure attained in previous work, the pressure range was extended up to 52 GPa in the present study. Important differences with previous studies were observed in three respects: 1. 1) phase Am III cannot be indexed in the monoclinic structure assigned to it previously. The resulting volumes are too large and would correspond to a widening of the lattice by an increasing-pressure phase transition. This contradiction is lifted when Am III is indexed as a trigonal distortion of the cubic close-packed Am II. 2. 2) 5f delocalisation is suggested to occur only at 23 GPa, instead of the 10–12 GPa assumed on the basis of previous theoretical and experimental study. This view is supported by three facts: the absence of a volume discontinuity at the transition from Am II (ccp) to Am III (trigonal) at 13.5 GPa; a volume decrease of approximately 6% at the transition from Am III to Am IV (orthorhombic) at 23 GPa; the crystallographic similarity between Am II and Am III and the fact that the structure assigned to the latter phase also occurs in yttrium and lanthanum under pressure which both have no f electrons. 3. 3) The three phase transitions occured at higher pressures than previously reported. The largest difference is found for the Am III to Am IV transition which was observed at 23 GPa in the present work, while earlier work placed it at 15 GPa. These results contribute to the systematics of the high pressure behaviour of the f metals, in particular if they are discussed in the context of recent work on other heavy actinides, on cerium, and on other light lanthanides.

High-pressure phases of thorium and uranium compounds with the rocksalt structure

Abstract The high-pressure crystal structures of the actinide compounds ThX and UX (X= C, N, P, S, As, Se, Sb, Te) have been studied by X-ray diffraction using synchrotron radiation, in the pressure range up to about 60 GPa Distorted fcc structures were observed for UC (27 GPa), UN (29 GPa), UP (10/28 GPa), US (10 GPa) and ThS (20 GPa). No phase transition has been observed for ThC and ThN. Compounds with As, Se, Sb all transform to the CsCl structure. ThP transform to the CsCl structure at 30 GPa. ThTe has the CsCl structure at ambient pressure and no further phase transition has been observed. UTe transforms to the CsCl structure at 9 GPa.

High-pressure studies of uranium and thorium compounds using synchrotron radiation

This paper reports high-pressure diffraction experiments and absorption-edge measurements on uranium and thorium rocksalt-structure compounds. A description is given of the equipment used in connection with a synchrotron X-ray source. The bulk modulus B0 and its pressure derivative B′0 have been determined. Examples of LIII absorption edge measurements of UC and UP are given.