Properties of Actinide Metals Under High Pressure U

PROPERTIES OF ACTINIDE METALS UNDER HIGH PRESSURE U. Benedict

To cite this version:

U. Benedict. PROPERTIES OF ACTINIDE METALS UNDER HIGH PRESSURE. Journal de Physique Colloques, 1984, 45 (C8), pp.C8-145-C8-148. 10.1051/jphyscol:1984826. jpa-00224326

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PROPERTIES OF ACTINIDE METALS UNDER HIGH PRESSURE

U. Benedict

Commission of the European Communities, Joint Research Centre, Karlsruhe Establishment, European Institute for Transuranium Elements, Postfach 2266, D-7500 Karlsruhe, F.P.G.

Résumé - Les propriétés des actinides sous pression sont discutées sur la base du dualisme entre électrons 5f localisés et délocalisés. Abstract - High pressure properties of the actinide metals are discussed on the basis of the dualism between localised and itinerant 5f electrons.

INTRODUCTION Together with the gaseous elements, actinides are among those materials whose in tense study under high pressure has started only recently. Interest of solid state physicists in the high pressure properties of actinides arose mainly on the basis of high pressure studies of the so-called 4f elements, the lanthanides (Ln). The 5f shell being filled in the actinide (An) series, comparison of both series pro mised to give information on similarities and differences between the behaviour of the 4f and the 5f electrons. As in the high pressure study of other materials, the introduction of the diamond anvil cell (DAC) has greatly increased the accessible pressure range for the study of actinides. But with the exception of certain isotopes of thorium and uranium, actinides exhibit strong radioactivity. This property has in the past restricted their study to a few specialized laboratories. For actinides heavier than einsteini um (fermium, mendelevium, nobelium and lawrencium), the available amounts are too small to allow preparation of pure solids suitable for solid state studies. Ein- steinium-235, having a half-life of 20 days, can in principle be studied in the solid state immediately after its isolation, but no high pressure studies were made up to now. Actinium, which is in general discussed together with the actinides, is extremely difficult to handle due to its particularly high radioactivity; no high pressure studies were reported for this element either. The available experimental data on high pressure behaviour are thus limited to the remaining 9 elements: thorium, protactinium, uranium,neptunium, plutonium, americium, curium, berkelium and californium.

ELECTRONIC STRUCTURE and PROPERTIES at AMBIENT PRESSURE and TEMPERATURE The solid state properties of the actinide metals are controlled by the dualism of the localised and the itinerant configuration of the 5f electrons /1-4/. Under ambient pressure, this dualism leads to the distinction of two main subgroups in the actinide series.

The first subgroup, protactinium to plutonium, has its 5f electrons in an itinerant (delocalised) state. This means they &re of band type, hybridize with the conduction electrons and thus contribute to the metallic bonding. Magnetic order, which in the lanthanide metals is limited to the presence of localised 4f electrons,is con sequently not observed in this subgroup. The strengthening of the metallic bond by the 5f participation leads to small atomic volumes (Fig. 1), a high cohesive energy and low compressibility (Fig.2). Low symmetry (orthorhombic and monoclinic) crystal structures are found whose formation is probably related to the particular direc tional properties of the hybridized orbitals including a contribution from 5f electrons.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984826 JOURNAL DE PHYSIQUE

0~'"'"""'~ Ac Th Po U Np Pu Am Crn Bk Cf Es

Fig.1 - Atomic volumes of actinide Fig.2 - Bulk moduli of the actinide metals, metals. compared to those of the lanthanide metals.

The second subgroup, americium to cal ifornium, is characterised by localised 5f electrons. In terms of electron energy, this means that these electrons have sharp energylevelsand do not contribute to the metallic bond. In a spatial sense, it means that a particular 5f electron is fixed ("localised") to a particular actinide atom. In contrast to the first subgroup, the localised 5f electrons contribute to the appearance of magnetic order in curium, berkelium and californium. The atomic volumes are larger (Fig.l), thus closer to those of the trivalent lanthanide metals The cohesive energies are in general lower than those of the "itinerant" 5f metals. The compressibilities are also of the same order as those of the trivalent lanthanides (Fig.2). The crystal structure of the four metals of the second subgroup is double-hexagonal close-packed (dhcp), thus of relatively high symmetry.

The limit between the subgroups is not sharp. Americium has in some respect aninter- mediate position. Although its 5f electrons are localised, it does not exhibit magnetic order, and as we will see below, its 5f electrons can go itinerant by relatively moderate pressure. This makes americiuma particularly interesting metal to study. Thorium, the first member of the series, is in fact not a real actinide metal because its 5f levels are practically unoccupied in the ground state configuration. Filling of the 5f shell, which according to atomic number should begin at thorium, is delayed and starts only at the following element, protactinium. Thorium should thus be considered as a subgroup of its own, differing from the following elements (first subgroup) by e.g. a high symmetry (cubic close-packed, ccp) crystal structure, high atomic volume and higher compressibility. (Figs.1 and 2).

THE EFFECT of PRESSURE

The most remarkable effect of pressure on the actinide metals is that due to closer contact between the lattice atoms, localised 5f electrons can become itinerant, hy- bridise with the conduction electrons, and participate in the metallic bond. The subgroup with localised 5f electrons can thus under pressure acquire properties which, at ambient pressure, are characteristic for the subgroup Pa to Pu. Most of the data in this chapter are taken from a review published recently by the present author /5/. a) Th~rjum Compression data for thorium are available from shock wave tests up to 140 GPa and from X-ray diffraction up to 68 GPa. No phase transition was observed in the pressure range studied by X-ray diffraction. The electrical resistivity at room temperature does not show an anomaly in the pressure range investioated (5 16 GPa). An unusual pressure dependence of the superconducting transition temperature was observed and believed to be due to either a change in the Fermi surface topology or a crystallographic phase change near 7 GPa.

5f electrons being itinerant in these metals at ambient pressure, no dramaticeffects are expected to occur under pressure at room temperature. Their room temperature crystal structures are conserved up to rather high pressures. Particularly high pressures have to be applied to these metals to provoke a phase transition: in uranium there are indications for a phase change around 71 GPa. p,T phase diagrams between room temperature and the melting point and up to moderate pressures havebeen determined, mainly by differential thermal analysis, for uranium, neptunium and plutonium. A common feature of the three phase diagrams is that at pressures above 3 GPa only two phases continue to exist, while 3 (U, Np) or 6(Pu) phases are observed at ambient pressure. Less dense high temperature phases are replaced by denser ones under the effect of pressure.

Several groups of authors reported on the variation of the superconducting transition temperature T of uranium with pressures not exceeding 8.5 GPa. Particular features observed in t6e Tc(p) curve of single crystal a-U were ascribed to the low temperature phase transitions which occur below 43 K at ambient pressure.

C) M~L~!~-!~~~-!~E~!~S_E~-~~~~I~E~'"_~!S_I-AC~-C!~-E~~-C~ These four metals have recently been shown to undergo a phase transition under pressure which marks a transition from localised to itinerant 5f electrons. Starting from their dhcp forms, they first transform to the ccp structure, and an orthorhombic a-uranium type phase was reported for all of them as the phase existing at the highest pressures attained (Figs. 3, 4 and 5). An intermediate monoclinic phase was observed in americium, and an intermediate distorted ccp phase in californium.

Low-symmetry orthorhombic and monoclinic phases were described above as being linked to 5f itinerancy in uranium, neptunium and plutonium. This correlation, together with the sharp volume decrease observed in Cm, Bk and Cf upon formation of the low- symmetry phases (Figs. 4 and 5), leads to the conclusion that the 5f electrons are itinerant in the orthorhombicandmonoclinic phases of Am, Cm, Bk and Cf. The pressure at which the low-symmetry structure forms ("delocalisation pressure") is particularly high in curium. The 5f shell being half-filled in curium, this metal has the largest possible number of unpaired 5f electrons and thus a particularly large (in absolute value) spin-polarisation energy /3/. Spin-polarisation energy accounts for most of the energy gained when an actinide metal assumes the localised configuration; thus curium has the most stable localised 5f configuration, and this ex- plains why particularly high pressure has to be applied in this metal to delocalise the 5f electrons. In Cm Bk and Cf this delocalisation takes place when an atomic volume of 18-21 -10-3 nm3 has been reached by compression of the precursor (ccp or distorted ccp) phase. But in americium the atomic volume at delocalisation is still much higher (% 25.10-~ nm3), indicating a greater ease of delocalisation for this metal which is on the limit to the subgroup of 5f-itinerant metals. d) Co!earlson-to-the-!a_!t_ha_!I!e_-!~ta_1s_ The dhcp,+ ccp transition under pressure, described above for Am, Cm, Bk and Cf, was also reported for the lanthanides Pr, Nd, Sm and Gd, and in lanthanum itself. This underlines the similarity between some of the heavier actinides and the trivalent metals in the first half of the lanthanide series. The dhcp -. ccp transition is not linked to the f electrons, butis thoughttobecontrolledby anincreaseof the d occu- pation. The similarity in behaviour of 4f and 5f electrons is demonstrated by tk appearance of low-symmetry high pressure phases in Ce, Pr, Nd and Sm. For the first two metals, the a-U type structure was observed as in the heavier actinides, for Nd and Sm the exact structure was not yet determined. C8-148 JOURNAL DE PHYSIQUE

1 Cm

CCP -AV 21% V

orthorh . 10 20 30 40 50 60

Fig.3 - Phase transitions under Fig.4 - Phase transitions under pressure in cu- pressure in americium (after Roof rium (R.G. Haire, U. Benedict, J.R. Peterson, /6/ 1. C. Dufour, J.P. ItiP, unpublished) .

V: unit cell volume

Fig.5 - Phase transitions under pressure in berkelium and californium /7,8/.

OO 10 zo 30 co 50 p, GPa

REFERENCES

1. Johansson B., Proc. 2nd Internat. Conf. on the Electronic Structure of the Acti- nides, Wroclaw (1976) 49. 2. Freeman A.J., Physica 102 (1980) 3. 3. Johansson B., Skriver H.L., ~srtenssonN., Andersen O.K. and Glotzel , D., Physica 102 B (1980) 12. 4. Johansson B. and Skriver H.L., J. Magn. Magn. Mat. 9 (1982) 217. 5. Benedict U., J. Less-Common Metals 100 (1984) 153. 6. Roof R.B., Z. Krist. -158 (1982) 307. 7. Haire R.G., Peterson J.R., Benedict U. and Dufour C., J. Less-Common Metals 102 (1984) 119. 8. Benedict u., Peterson J.R., Haire R.G. and Dufour C., J. Phys. F: Metal Phys. -14 (1 984) L43.