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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�
JOURNAL DE PHYSIQUE
Colloque C8, supplement au n ° l l , Tome k5, novembre 198* page C8-145
P R O P E R T I E S OF A C T I N I D E M E T A L S UNDER HIGH P R E S S U R E
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.
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.
Résumé - Les propriétés des actinides sous pression sont discutées sur la base du dualisme entre électrons 5f localisés et délocalisés.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984826
JOURNAL DE PHYSIQUE
Fig.1
-
Atomic volumes o f a c t i n i d e m e t a l s .0 ~ ' " ' " " " ' ~
Ac Th Po U N p Pu Am Crn Bk C f Es
F i g . 2 - B u l k m o d u l i o f t h e a c t i n i d e m e t a l s , compared t o t h o s e o f t h e l a n t h a n i d e m e t a l s .
The second subgroup, americium t o c a l i f o r n i u m , i s c h a r a c t e r i s e d by l o c a l is e d 5 f e l e c t r o n s . I n terms o f e l e c t r o n energy, t h i s means t h a t t h e s e e l e c t r o n s have s h a r p e n e r g y l e v e l s a n d do n o t c o n t r i b u t e t o t h e m e t a l l i c bond. I n a s p a t i a l sense, i t means t h a t a p a r t i c u l a r 5f e l e c t r o n i s f i x e d ( " l o c a l i s e d " ) t o a p a r t i c u l a r a c t i n i d e atom. I n c o n t r a s t t o t h e f i r s t subgroup, t h e l o c a l i s e d 5 f e l e c t r o n s c o n t r i b u t e t o t h e appearance o f magnetic o r d e r i n curium, b e r k e l i u m and c a l i f o r n i u m . The atomic volumes a r e l a r g e r ( F i g . l ) , t h u s c l o s e r t o t h o s e o f t h e t r i v a l e n t l a n t h a n i d e m e t a l s The c o h e s i v e e n e r g i e s a r e i n g e n e r a l l o w e r t h a n t h o s e o f t h e " i t i n e r a n t " 5 f m e t a l s . The c o m p r e s s i b i l i t i e s a r e a l s o o f t h e same o r d e r as t h o s e of t h e t r i v a l e n t l a n t h a - n i d e s ( F i g . 2 ) . The c r y s t a l s t r u c t u r e o f t h e f o u r m e t a l s o f t h e second subgroup i s double-hexagonal close-packed (dhcp), t h u s o f r e l a t i v e l y h i g h symmetry.
The l i m i t between t h e subgroups i s n o t sharp. Americium has i n some r e s p e c t a n i n t e r - mediate p o s i t i o n . A l t h o u g h i t s 5 f e l e c t r o n s a r e l o c a l i s e d , i t does n o t e x h i b i t magnetic o r d e r , and as we w i l l see below, i t s 5 f e l e c t r o n s can go i t i n e r a n t by r e l a - t i v e l y moderate p r e s s u r e . T h i s makes a m e r i c i u m a p a r t i c u l a r l y i n t e r e s t i n g m e t a l t o study. Thorium, t h e f i r s t member o f t h e s e r i e s , i s i n f a c t n o t a r e a l a c t i n i d e m e t a l because i t s 5 f l e v e l s a r e p r a c t i c a l l y unoccupied i n t h e ground s t a t e c o n f i g u r a t i o n . F i l l i n g o f t h e 5 f s h e l l , which a c c o r d i n g t o a t o m i c number s h o u l d b e g i n a t thorium, i s d e l a y e d and s t a r t s o n l y a t t h e f o l l o w i n g element, p r o t a c t i n i u m . Thorium s h o u l d t h u s be c o n s i d e r e d as a subgroup o f i t s own, d i f f e r i n g f r o m t h e f o l l o w i n g elements ( f i r s t subgroup) by e.g. a h i g h symmetry ( c u b i c close-packed, c c p ) c r y s t a l s t r u c t u r e , h i g h a t o m i c volume and h i g h e r c o m p r e s s i b i l i t y . (Figs.1 and 2 ) .
THE EFFECT o f PRESSURE
The most remarkable e f f e c t o f p r e s s u r e on t h e a c t i n i d e m e t a l s i s t h a t due t o c l o s e r c o n t a c t between t h e l a t t i c e atoms, l o c a l i s e d 5 f e l e c t r o n s c a n become i t i n e r a n t , hy- b r i d i s e w i t h t h e c o n d u c t i o n e l e c t r o n s , and p a r t i c i p a t e i n t h e m e t a l l i c bond. The subgroup w i t h l o c a l i s e d 5 f e l e c t r o n s can t h u s under p r e s s u r e a c q u i r e p r o p e r t i e s which, a t ambient pressure, a r e c h a r a c t e r i s t i c f o r t h e subgroup Pa t o Pu. Most o f t h e d a t a i n t h i s c h a p t e r a r e t a k e n f r o m a r e v i e w p u b l i s h e d r e c e n t l y by t h e p r e s e n t a u t h o r /5/.
a )
Th~rjum
Compression d a t a f o r t h o r i u m a r e a v a i l a b l e f r o m shock wave t e s t s up t o 140 GPa and f r o m X-ray d i f f r a c t i o n up t o 68 GPa. No phase t r a n s i t i o n was observed i n t h e p r e s - s u r e range s t u d i e d by X-ray d i f f r a c t i o n . The e l e c t r i c a l r e s i s t i v i t y a t room tempe-
r a t u r e does n o t show an anomaly i n the pressure range i n v e s t i o a t e d ( 5 16 GPa). An unusual pressure dependence o f the superconducting t r a n s i t i o n temperature was ob- served and b e l i e v e d t o be due t o e i t h e r a change i n t h e Fermi surface topology o r a c r y s t a l l o g r a p h i c phase change near 7 GPa.
5f e l e c t r o n s being i t i n e r a n t i n these metals a t ambient pressure, no d r a m a t i c e f f e c t s a r e expected t o occur under pressure a t room temperature. T h e i r room temperature c r y s t a l s t r u c t u r e s a r e conserved up t o r a t h e r h i g h pressures. P a r t i c u l a r l y h i g h pressures have t o be a p p l i e d t o these metals t o provoke a phase t r a n s i t i o n : i n u r a - nium t h e r e a r e i n d i c a t i o n s f o r a phase change around 71 GPa.
p,T phase diagrams between room temperature and t h e m e l t i n g p o i n t and up t o moderate pressures havebeen determined, mainly by d i f f e r e n t i a l thermal a n a l y s i s , f o r uranium, neptunium and plutonium. A common f e a t u r e o f t h e t h r e e phase diagrams i s t h a t a t pressures above 3 GPa o n l y two phases continue t o e x i s t , w h i l e 3 (U, Np) o r 6(Pu) phases are observed a t ambient pressure. Less dense h i g h temperature phases a r e replaced by denser ones under the e f f e c t o f pressure.
Several groups of authors r e p o r t e d on t h e v a r i a t i o n o f the superconducting t r a n s i t i o n temperature T o f uranium w i t h pressures n o t exceeding 8.5 GPa. P a r t i c u l a r f e a t u r e s observed i n t 6 e Tc(p) c u r v e o f s i n g l e c r y s t a l a-U were a s c r i b e d t o t h e low tempe- r a t u r e phase t r a n s i t i o n s which occur below 43 K a t ambient pressure.
C )
M ~ L ~ ! ~ - ! ~ ~ ~ - ! ~ E ~ ! ~ S _ E ~ - ~ ~ ~ ~ I ~ E ~ ' " _ ~ ! S _ I - A C ~ - C ! ~ - E ~ ~ - C ~
These f o u r metals have r e c e n t l y been shown t o undergo a phase t r a n s i t i o n under pres- sure which marks a t r a n s i t i o n from l o c a l i s e d t o i t i n e r a n t 5 f e l e c t r o n s . S t a r t i n g from t h e i r dhcp forms, they f i r s t t r a n s f o r m t o t h e ccp s t r u c t u r e , and an orthorhom- b i c a-uranium type phase was r e p o r t e d f o r a l l o f them as t h e phase e x i s t i n g a t the h i g h e s t pressures a t t a i n e d (Figs. 3, 4 and 5). An i n t e r m e d i a t e monoclinic phase was observed i n americium, and an i n t e r m e d i a t e d i s t o r t e d ccp phase i n c a l i f o r n i u m . Low-symmetry orthorhombic and monoclinic phases were described above as being l i n k e d t o 5 f i t i n e r a n c y i n uranium, neptunium and plutonium. This c o r r e l a t i o n , t o g e t h e r w i t h the sharp volume decrease observed i n Cm, Bk and C f upon formation of t h e low- symmetry phases (Figs. 4 and 5), leads t o the conclusion t h a t t h e 5 f e l e c t r o n s a r e i t i n e r a n t i n t h e orthorhombicandmonoclinic phases o f Am, Cm, Bk and C f . The pres- sure a t which t h e low-symmetry s t r u c t u r e forms ( " d e l o c a l i s a t i o n pressure") i s par- t i c u l a r l y h i g h i n curium. The 5 f s h e l l being h a l f - f i l l e d i n curium, t h i s metal has t h e l a r g e s t p o s s i b l e number o f unpaired 5 f e l e c t r o n s and thus a p a r t i c u l a r l y l a r g e ( i n a b s o l u t e v a l u e ) s p i n - p o l a r i s a t i o n energy /3/. S p i n - p o l a r i s a t i o n energy accounts f o r most o f t h e energy gained when an a c t i n i d e metal assumes t h e l o c a l i s e d c o n f i - guration; thus curium has t h e most s t a b l e l o c a l i s e d 5 f c o n f i g u r a t i o n , and t h i s ex- p l a i n s why p a r t i c u l a r l y h i g h pressure has t o be a p p l i e d i n t h i s metal t o d e l o c a l i s e t h e 5 f e l e c t r o n s . In Cm Bk and C f t h i s d e l o c a l i s a t i o n takes p l a c e when an atomic volume o f 18-21 -10-3 nm3 has been reached by compression o f t h e precursor (ccp o r d i s t o r t e d ccp) phase. B u t i n americium t h e atomic volume a t d e l o c a l i s a t i o n i s s t i l l much h i g h e r (% 2 5 . 1 0 - ~ nm3), i n d i c a t i n g a g r e a t e r ease o f d e l o c a l i s a t i o n f o r t h i s metal which i s on t h e l i m i t t o t h e subgroup o f 5 f - i t i n e r a n t metals.
d)
Co!earlson-to-the-!a_!t_ha_!I!e_-!~ta_1s_
The dhcp,+ ccp t r a n s i t i o n under pressure, described above f o r Am, Cm, Bk and C f , was a l s o r e p o r t e d f o r the lanthanides Pr, Nd, Sm and Gd, and i n lanthanum i t s e l f . This u n d e r l i n e s t h e s i m i l a r i t y between some o f t h e h e a v i e r a c t i n i d e s and t h e t r i v a l e n t metals i n t h e f i r s t h a l f o f t h e l a n t h a n i d e series. The dhcp -. ccp t r a n s i t i o n i s n o t l i n k e d t o t h e f e l e c t r o n s , b u t i s t h o u g h t t o b e c o n t r o l l e d b y a n i n c r e a s e o f t h e d occu- pation. The s i m i l a r i t y i n behaviour o f 4 f and 5 f e l e c t r o n s i s demonstrated by tk ap- pearance o f low-symmetry h i g h pressure phases i n Ce, Pr, Nd and Sm. For t h e f i r s t two metals, t h e a-U type s t r u c t u r e was observed as i n t h e heavier a c t i n i d e s , f o r Nd and Sm t h e exact s t r u c t u r e was n o t y e t determined.
C8-148 JOURNAL DE PHYSIQUE
F i g . 3
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Phase t r a n s i t i o n s under p r e s s u r e i n americium ( a f t e r Roof /6/1.
1 Cm
C C P
-
AV 21%V o r t h o r h
.
10 20 30 40 50 60
Fig.4
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Phase t r a n s i t i o n s under p r e s s u r e i n cu- r i u m (R.G. H a i r e , U. Benedict, J.R. Peterson, C. Dufour, J.P. I t i P , unpublished).
V: u n i t c e l l volume
Fig.5
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Phase t r a n s i t i o n s under p r e s s u r e i n b e r k e l i u m and c a l i f o r n i u m /7,8/.OO 10
zo
30 co 50 p, GPaREFERENCES
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