Actinide compounds under pressure

Journal of AUoys and Compounds, 181 (1992) 1-12 JAL 8087

1

Actinide compounds under pressure U. B e n e d i c t , S. D a b o s - S e i g n o n * , J. P. D a n c a u s s e , M. G e n s i n i , E. G e r i n g * * , S. H e a t h m a n a n d H. Luo* Commission of the European Communities, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, W-7500 Karlsmehe 1 (FRG) J. S t a u n O l s e n Physics Laboratory, H.C. Orsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Kcbenhavn O (Denmark) L. G e r w a r d Laboratory of Applied Physics, Building 307, Technical University of Denmark, DK-2800 Lyngby (Denmark) R. G. H a i r e Transuranium Research Laboratory (Chemistry Division), Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6375 (USA)

Abstract An overview of pressure-induced structural phase transitions and compressibility of actinide compounds will be given. Systematic trends in the nature of the high-pressure phases, the transition pressures, the hysteresis to retransformation on pressure release, and the compressibility are observed in the family of AnX compounds of B1 (NaC1) structure type. The dioxides studied up to now form high-pressure phases of PbC12 type. UX2 compounds of Fe2As type also tend to have PbC12 type high-pressure phases. The Th3P4 type compounds studied up to now did not transform up to 50 GPa. The same is true for ThOS and UOSe up to about 45 GPa. Comparison with rare earth compounds will be made where possible.

1. I n t r o d u c t i o n H i g h - p r e s s u r e s t r u c t u r a l s t u d i e s of the p u r e l a n t h a n i d e a n d actinide metals has reached a certain completeness [1-3]. New results in the metal field c a n o n l y b e e x p e c t e d f o r p r e s s u r e s a b o v e a b o u t 6 0 G P a a n d i n t h e s t u d y of alloys. In contrast, little w a s k n o w n , a few years back, a b o u t the behaviour of actinide compounds under pressure. Even for many of the rare *Present address: Institut Curie, Section de Physique et Chimie, Physicochimie des Elements Transuraniens, U.R.A.C.N.R.S. 448, 11 rue Pierre et Marie Curie, F-75231 Paris, Cedex 05, France. **Present address: Robert Bosch GmbH, Postfaeh 1160, W-8600 Bamberg, FRG. tPresent address: Cornell University, Department of Materials Science and Engineering, Bard Hall. Ithaca, NY 14853-1501, USA.

0925-8388/92/$5.00

© 1 9 9 2 - Elsevier Sequoia. All rights reserved

earth compounds, no high-pressure studies have been reported up to now. It was only in the 1980s that a few laboratories started high-pressure investigations on uranium and thorium compounds. The compounds of actinides of higher specific radioactivity, such as neptunium, plutonium, americium and curium, are now being successively included in such studies by laboratories which are equipped for handling strong a-emitters. An early approach [4] divides the compounds of cerium, uranium, neptunium and plutonium into a group with localized 5f electrons (characterized by the existence of ferro- and antiferromagnetism) and a group with itinerant 5f electrons, on the basis of Ce-Ce, or An-An distances. A relatively narrow range of m e t a l - m e t a l distances, extending over a few tens of picometres ("Hill limit" or "Hill transition zone") separates these two groups. Pressure reduces the m e t a l - m e t a l distance and can thus create itinerancy in compounds which have localized f electrons at ambient pressure. This "delocalization" of f electrons can be accompanied by changes in crystal structure. The Hill approach has been used to identify compounds which have a certain probability of undergoing f delocalization under pressure because their metal-metal distance is already near the Hill transition zone at ambient pressure. Pressure-induced structural phase transitions have indeed been observed for some of these compounds, but it is not possible from structural study alone to prove or disprove delocalization. Complementary study by pressure-dependent study of optical, magnetic or electrical properties is necessary to decide upon that issue. There is one case where structural study alone gives a strong hint to the occurrence of f delocalization. This is the case of the isostructural volume collapses under pressure which were observed in cerium metal and in some rare earth compounds. Evidence from theory and from experimental study of physical properties indicates that in these so-called "electronic phase transitions" the f electrons indeed undergo delocalization and start to participate in the bonding. There are different ways of describing the mechanism of this delocalization. Two of the most commonly used are the so-called " p r o m o t i o n " picture, where an f electron is promoted to the conduction band, increasing the metallic valence by one, and the hybridization picture, where f electrons are considered to become itinerant, i.e. band-like, in a first step, and to hybridize with conduction electrons in a second stage. These stages are merely thinking schemes which facilitate calculation of the mechanism by certain codes, and have not to be viewed as successive stages in time. In the following, Section 2 deals with the B1 type compounds where the largest number of results is available and where systematic trends begin to emerge. This is also, among the compounds treated in this paper, the only class of materials where sufficient data on the corresponding rare earth compounds exists to start comparison between the 4f and the 5f series. For the other classes of compounds (dioxides, Section 3; Fe2As type compounds, Section 4; ThaP4 type compounds, Section 5) no high pressure X-ray diffraction

3 (HPXRD) studies are known for the corresponding rare earth compounds, e x cep t for CeO2.

2. B1 type c o m p o u n d s The stable r o o m temperature, ambient pressure phase of the 1:1 actinide c o m p o u n d s AnX is of the NaC1 (B1) type. The only known exceptions are ThTe and ThBi, which are of the CsC1 (B2) type, and a few c o m p o u n d s of truly intermetallic character. This large family of isostructural c o m p o u n d s lends itself particularly well to systematic study of physical properties as a function of the nature of the anion, the nature of the cation, the pressure and the temperature. A large n u m b e r of B1 type c o m p o u n d s of thorium, uranium, neptunium and plutonium have b e e n investigated by HPXRD [5-19]. Systematic trends in the nature of the high-pressure phases, the transition pressures, the hysteresis to retransformation on pressure release, and the compressibility were observed for this group of compounds. The known LnX c o m p o u n d s also crystallize in the B1 structure type. A certain n u m b e r of t h e m has been studied under high pressure by X-ray diffraction and by ot he r methods. This allows us to discuss resemblances and differences between 4f and 5f transition metals in this class of compounds.

2.1. High-pressure phases and volume compression Table 1 gives the phase transitions observed in AnX com pounds of the B1 type for An Th to Pu. The table does not include c o m p o u n d s of heavier actinides where only a few results (for CmBi, AmN, CmN and CfN) are available up to now. CmBi was studied up to 48 GPa [15]. It undergoes a first-order phase transition to the B2 type around 12 GPa, with a volume decrease of about 11%, and then a second transition, probably of second order, to a third phase with tetragonal symmetry. The isothermal bulk modulus of B1 type CmBi was determined as Bo = 53(2) GPa, its pressure derivative as B~)= 8(3). AmN, CmN and CfN do not undergo structural changes up to about 50 GPa [16, 17]. In the case of AmN [16] there are indications of a 2 % - 3 % isostructural volume collapse around 34 GPa. Whether a volume decrease of ap pr oxi m a t e l y 1% around 20 GPa in ThN [18] can be interpreted in the same way is not clear. Comparative bar diagrams of the stability ranges of the different phases for the mono-arsenides, -antimonides, -selenides, and -tellurides were given in ref. 10. The bulk moduli and their pressure derivatives, determined by fitting the p r e s s u r e - v o l u m e data to semi-empirical equations such as those of Birch and of Murnaghan, are listed in Table 2.

2.2. Hysteresis to retransforrnation In general, the B1 t ype actinide c o m p o u n d s exhibit a large difference between the transition pressure on pressure increase and the transition pressure on pressure decrease [ 11 ]. This hysteresis is probably due to delayed

TABLE 1 P r e s s u r e - i n d u c e d s t r u c t u r a l t r a n s i t i o n s in m o n o c a r b i d e s , m o n o p n i c t i d e s and m o n o c h a l c o g e n i d e s of thorium, u r a n i u m , n e p t u n i u m a n d p l u t o n i u m (adapted from ref. 20) X

An Th

U

Np

Pu

C

(B1 up to 65 GPa)

Orthorhombic ~-27 GPa 26%

N

(B1 u p to 47 GPa)

Rhombohedral ~-29 GPa =3%

P

B2 - - 3 0 GPa 12%

Rhombohedral - - 1 0 GPa 0% Orthorhombic -- 28 GPa O%

As

B2 18-26 GPa ~- 10%

B2 1 7 - 3 0 GPa -- 11%

B2 25--40 GPa = 9%

B2 3 5 - 3 8 GPa =9%

Sb

B2 9 - 1 2 GPa -- 9%

B2 9 - 1 0 GPa ~ 12%

Tetragonal 1 0 - 1 8 GPa -- 12%

B2 = 20 GPa 4% Tetragonal 40 GPa 5%

Bi

B2 ~<0 GPa

B2 ~ 5 GPa ~11%

B2 = 8 GPa

S

Hexagonal 23--33 G P a 0%

Rhombohedral -- 12 GPa 0%

Se

B2 --15 GPa

B2 2 0 - 2 6 GPa

B2 ~ 2 3 GPa

= 9%

-- 8%

-- 9%

B2 ~<0 GPa

B2 9 - 2 0 GPa -- 8%

B2 1 2 - 2 0 GPa ~ 7%

Te

Rhombohedral -- 20 GPa 0% B2 35 GPa 11% B2 1 5 - 1 9 GPa ~-9%

F o r e a c h c o m p o u n d , t h e table fists t h e following f r o m top to b o t t o m : s t r u c t u r e of t h e highp r e s s u r e p h a s e ; p r e s s u r e ( r a n g e ) of transition; v o l u m e d e c r e a s e o n transition (0% m e a n s n o v o l u m e d e c r e a s e detected). ThC, ThN, no t r a n s i t i o n in t h e p r e s s u r e r a n g e studied. ThBi, ThTe, B2 a s t h e a m b i e n t p r e s s u r e p h a s e , no transition.

5 TABLE 2 Isothermal bulk moduli B0 (GPa) (upper lines), and pressure derivatives B~ (lower lines) for the actinide carbides, pnictides and chalcogenides AnX (adapted from ref. 20) Th

U

c

109(4) 4.0(3)

160(4) 3.6(5)

N

175(15) 4.0(4)

203(6) 6.3(6)

P

137(7) 5(1)

102(4) 4(1)

As

118(4) 3.4(10)

Sb

84(8) 5(2)

Bi

Np

Pu

100(4) 4.4(4)

70(1) 6.2(6)

69(3) 3.3(3)

62(3) 4(1)

55(2) 8(2)

68(2) 3.3(5)

75(5) <0

134(8) <0

S

145(6) 5(1)

105(8) 5(1)

Se

125(10) 4 (constrained)

74(4) 5(1)

60(3) 2.5(5)

98(4) 2.6(5)

Te

102(4) B2 type 3.8(4)

48(3) 4.9(3)

62(2) 1.8(4)

34(3) 12(4)

e s t a b l i s h m e n t o f t h e r m o d y n a m i c equilibrium; the equilibrium transition pressure is likely t o h a v e s o m e i n t e r m e d i a t e value. In UTe, USb, a n d UBi, r e t r a n s f o r m a t i o n to the B1 type p h a s e o n p r e s s u r e release d o e s n o t o c c u r at r o o m t e m p e r a t u r e . The B2 type h i g h - p r e s s u r e p h a s e is m a i n t a i n e d in a m e t a s t a b l e c o n d i t i o n at a m b i e n t pressure. It s h o u l d be recalled, too, that ThBi a n d ThTe have a stable B2 type p h a s e at a m b i e n t p r e s s u r e or, o t h e r w i s e e x p r e s s e d , their B 1 - B 2 t r a n s f o r m a t i o n o c c u r s at negative pressures.

2.3. D i s c u s s i o n The B 1 - B 2 p h a s e t r a n s i t i o n is the d o m i n a t i n g m e c h a n i s m in the highp r e s s u r e s t r u c t u r a l b e h a v i o u r o f t h e s e c o m p o u n d s . A l t h o u g h interactinide d i s t a n c e s r e a c h the Hill t r a n s i t i o n z o n e u n d e r p r e s s u r e in s o m e o f t h e s e c o m p o u n d s , this p h a s e t r a n s f o r m a t i o n d o e s n o t s e e m to have any link t o the 5f e l e c t r o n s b e c a u s e it iS also the typical p r e s s u r e - i n d u c e d structural t r a n s i t i o n f o r the alkali halides. In addition, for a structural p h a s e transition linked to 5f delocalization, o n e w o u l d e x p e c t that the h i g h - p r e s s u r e s t r u c t u r e h a s s o m e l o w e r s y m m e t r y t h a n cubic. This a s s u m p t i o n is b a s e d on an empirical c o r r e l a t i o n b e t w e e n itinerant f states a n d low crystal s y m m e t r y ,

6

TABLE 3 Isostructural volume collapses observed under pressure in lanthanide pnictides and ehalcogenides LnX (adapted from ref. 20) Ce P

Sm

Eu

Tm

Yb

10 GPa 3%

Bi

2.6 GPa ~- 10%

O

0-3 GPa -- 8%

S

12.5 GPa 4.5%

30 GPa =5%

>/8 GPa

0.65 GPa -- 11%

Se

~ 4 GPa -- 8%

Te

2-8 GPa -- 17%

11-16 GPa 2.5%

--1.5 GPa --5%

5 GPa

15-17 GPa -- 7%

1.5-3 GPa --9%

13-18 GPa = 10%

For each compound, the table lists the following from top to bottom: pressure (range) of collapse, and volume decrease on collapse.

a n d is t h o u g h t to be linked t o a s t r o n g directionality o f the h y b r i d i z e d (s, p, d, f) orbitals. In t h e AnX c o m p o u n d s , t h e B1--*B2 t r a n s i t i o n is a c c o m p a n i e d b y a v o l u m e d e c r e a s e o f t h e o r d e r o f 10%. A n t i m o n y a n d b i s m u t h as a n i o n s f a v o u r t e t r a g o n a l h i g h - p r e s s u r e s t r u c t u r e s . M o n o c a r b i d e s , -nitrides, -phosphides, a n d - s u l p h i d e s o f t h o r i u m a n d u r a n i t u n either did n o t t r a n s f o r m in the p r e s s u r e r a n g e s t u d i e d (ThC, ThN) o r t r a n s f o r m e d to s t r u c t u r e s o f relatively low s y m m e t r y (UC, UN, UP, US, ThS). In this g r o u p of t h o r i u m a n d u r a n i u m c o m p o u n d s with light anions, only ThP exhibits the B 1 - B 2 s t r u c t u r a l transition. The B 1 - B 2 t r a n s i t i o n p r e s s u r e s g e n e r a l l y i n c r e a s e with the a t o m i c n u m b e r o f the actinide. T h e y d e c r e a s e w i t h the a t o m i c n u m b e r of the anion. The h y s t e r e s i s zones, i.e. the p r e s s u r e r a n g e s b e t w e e n transitions at i n c r e a s i n g a n d d e c r e a s i n g p r e s s u r e , h a v e a t e n d e n c y to w i d e n with i n c r e a s i n g a t o m i c n u m b e r o f t h e actinide. PuAs h a s the l a r g e s t hysteresis z o n e o f t h e s e c o m p o u n d s , o f the o r d e r o f 3 0 GPa. In USb, UBi, a n d UTe, h y s t e r e s i s to r e t r a n s f o r m a t i o n e n a b l e s us to o b t a i n m e t a s t a b l e B2 t y p e p h a s e s at a m b i e n t pressure. T h e r e is a t r e n d t o i n c r e a s i n g c o m p r e s s i b i l i t y with i n c r e a s i n g a t o m i c n u m b e r o f t h e actinide element. This t e n d e n c y is m o s t clearly m a r k e d for t h e m o n o a r s e n i d e s . T h e r e is also a t e n d e n c y for m o n o a n t i m o n i d e s to be m o r e c o m p r e s s i b l e t h a n m o n o a r s e n i d e s of the s a m e actinide.

7 TABLE 4 Pressure-induced structural transitions in lanthanum and lanthanide pnictides and chalcogenides LnX of the B1 structure type (adapted from ref. 20) La

Ce

P

B2 19 GPa 10.5%

As

B2 16 GPa 14%

Sb

Bi

Tetragonal 11 GPa -- 10%

Pr

Sm

Eu

Tm

Tetragonal 11 GPa -- 10% Tetragonal ÷ B2 13 GPa 5.5% B2 40 GPa =8%

O

B2 21.5 GPa -- 12% B2 14.5 GPa -- 13%

Se

Te

B2 8(1) GPa --8.5%

B2 9(1) GPa --11.5%

B2 --11 GPa --9%

B2 11(1) GPa -~13%

Hexagonal 15 GPa --4%

For each compound, the table lists the following from top to bottom: structure of the highpressure phase, pressure (range) of transition; volume decrease on transition.

2.4. Comparison with LnX compounds Isostructural volume collapses under pressure were observed for monochalcogenides of cerium, samarium, europium, thulium and ytterbium, and f o r C e P a n d S m B i ( T a b l e 3). P r e s s u r e - i n d u c e d s t r u c t u r a l t r a n s i t i o n s w e r e o b s e r v e d for m o n o p n i c t i d e s of l a n t h a n u m a n d cerium, for the m o n o c h a l cogenides of europium, and for the monotellurides of cerium, praeseodymium, s a m a r i u m a n d t h u l i u m ( T a b l e 4). In contrast to the lanthanide compounds, isostructural volume collapses were not detected until recently in the AnX compounds. Recent work on t h e m o n o b i s m u t h i d e s o f u r a n i u m a n d n e p t u n i u m [ 15, 21 ] r e v e a l e d a n e g a t i v e c u r v a t u r e o f t h e V(p) c u r v e s f o r t h e s e t w o c o m p o u n d s . T h i s p h e n o m e n o n h a d b e e n t a k e n i n l a n t h a n i d e c o m p o u n d s s u c h a s SINS, a n d S m S e a s t h e sign of a c o m m e n c i n g 4f p a r t i c i p a t i o n in the b o n d i n g , or otherwise expressed,

1.009,x

.

.

.

.

.

.

.

t ' ~ B1

~o 0.g0 ~

0.80

UBi

~x'x'~x'~×

0

B2 ~ ~ ' ~ x

0.70

0.60

.

[ 5

[ 10

I 15

I 20

I 25

×.......×~ x I 30

J 35

I /-,0

I

h5 50 p, GPa

Fig. 1. Relative volume of UBi vs. pressure (from ref. 15).

TABLE 5 Pressure-induced structural transitions in CeO2 and actinide dioxides AnO2 (adapted from ref. 2O) High-pressure structure

Transition pressure (GPa) (upstroke)

Volume decrease on transition (96)

Reference

CeOe

Orthorhombic PbC12-1ike

31.5(10)

7.5(7)

22

ThO2

Orthorhombic PbCI~ type

40-50

=8

23

UO2

Orthurhombic PbCl2 type

Between 33 and 40

=5

24

Orthorhombic PbClz or space group C m v m

29-38

= 2.5 or =5

25

NpO2

Orthorhombic space group C m c m

34-36

=6

26

PuOz

Orthorhombic PbCI~ type

39-43

= 12

23

of a so-called electronic transition which was often thought to be linked to an increase in the valence of the cation. O n t h e b a s i s o f t h e s e r e s u l t s , it m u s t b e d i s c u s s e d w h e t h e r a n e l e c t r o n i c t r a n s i t i o n a l s o t a k e s p l a c e i n U B i a n d NpBi. If it o c c u r s , it is l e s s s h a r p t h a n , f o r e x a m p l e , i n c e r i u m m e t a l , a n d is i m m e d i a t e l y f o l l o w e d b y t h e s t r u c t u r a l t r a n s i t i o n t o t h e B 2 t y p e , a s s h o w n f o r U B i i n Fig. 1.

9 TABLE 6 Isothermal bulk modulus B 0 and its pressure derivative B~ for CeO2 and actinide dioxides An02 (adapted from ref. 20) B0 (GPa)

B~

Reference

Ce02

230(10)

4.00 constrained

22

Th02

262(4)

6.7(5)

23

U02

210(10) 230(8) 207(10)

7(2) 7(2)

24 27 26

Np02

200(2)

3.8(5)

26

Pu02

379(4)

2.4(4)

23

TABLE 7 Isothermal bulk modulus, pressure derivative of the bulk modulus, transition pressure, and volume decrease on transition for uranium compounds of the Fe2As type (from ref. 29) Compound

Bo (GPa)

B~

P~ (GPa)

AV (%)

UP2

124(15)

9(2)

22

4

UAs2

101(8)

4.7(7)

15

7

UAsS

105(7)

3.7(5)

46

7

UAsSe

99(6)

3.8(5)

-

-

The uncertainties given in parentheses are the standard errors in units of the last decimal place. TABLE 8 Isothermal bulk moduli and pressure derivatives of ThOS and UOSe (adapted from ref. 30) B0 (GPa), B~) Birch

Murnaghan

Average

ThOS

201, 3.1

202, 2.9

201.5, 3.0

UOSe

153, 2.1

155, 1.5

154, 1.8

3. D i o x i d e s Of this group of compounds, the high-pressure structural behaviour was i n v e s t i g a t e d f o r CeO2 [ 2 2 ] , ThO2 [23], UOe [24, 2 5 ] , NpO2 [ 2 6 ] , a n d P u O z [23 ]. I n all o f t h e s e , p r e s s u r e i n d u c e s a t r a n s i t i o n t o a n o r t h o r h o m b i c s t r u c t u r e o f PbC12 t y p e . F o r UO2 a n d NpO2, a n o t h e r p o s s i b l e s t r u c t u r e a s s i g n m e n t is

10 TABLE 9 AI~3X4, bulk modulus and its pressure derivative for ambient pressure (from ref. 31) Compound

Bo (GPa)

B'o

Th3P 4

126(5)

4.0(4)

U3P4

160(15)

4.1(9)

U3As4

121(10)

4.3(8)

U3Sb4

93(4)

4.2(9)

The uncertainties in parentheses are given in units of the last decimal place. in space group C m c m with an atomic a r r a n g e m e n t similar to that existing for the PbC12 type structure, whose space group is P n m a . Transition pressures and volume decreases are given in Table 5. Table 6 shows the compressibility data of these compounds. The m e a s u r e m e n t of optical reflectivity of UO2 u n d e r pressure [28] allowed the narrowing o f the f - d gap with increasing pressure to be followed, and showed that even be f or e the structural transition occurs, pressure induces f - d mixing in this compound.

4. Thorium and uranium compounds o f Fe2As (PbFCI) type Many binary and t er na r y c o m p o u n d s of actinides and non-metallic elements with the composition AnX2 or AnXY crystallize under normal conditions in the Fe2As or PbFC1 t ype structures. X-ray p o w d e r diffraction in pressure ranges below 60 GPa was p e r f o r m e d for six of these compounds: UPe, UAS2, UAsS, UASSe [29], ThOS, and UOSe [30]. UAsSe, ThOS and UOSe did not exhibit a phase transformation in the pr es sure range investigated (up to 54, 43, and 47 GPa respectively). The ot her t hree c o m p o u n d s transformed to structures similar to the PbCI2 type: UP2 around 22 GPa with a volume decrease of approximately 4%, UAs2 a r ound 15 GPa with a volume decrease o f approximately 7%, and UAsS ar ound 46 GPa with a volume decrease of approximately 7%. Compressibility data for these c o m p o u n d s are shown in Tables 7 and 8.

5. Th3P4 type compounds The group VA elements phosphorus, arsenic, antimony and bismuth form cubic Th3P4-type c o m p o u n d s with t he actinides. The space group of the crystal structure i s I 4 3 d (No. 220) with Z = 4. E ach actinide atom is surrounded by eight n ear es t neighbours of the non-actinide element. Compressibility and crystal structure of ThaP4 and the uranium pnictides U3X4, where X--P, As and Sb, were studied in the pr es s ur e range up to 50 GPa [31|. Phase

ll

t r a n s i t i o n s w e r e n o t o b s e r v e d in this p r e s s u r e range. Table 9 s h o w s the c o m p r e s s i b i l i t y data. References 1 T. Kriiger, B. Merkau, W. A. Grosshans and W. B. Holzapfel, High Pressure Res., 2 (1990) 193-236. 2 U. Benedict, W. A. G r o s s h a n s and W. B. Holzapfel, Physica B, 144 (1986) 1 4 - 1 8 . 3 U. Benedict, in A. J. F r e e m a n and G. H. Lander (eds.), Handbook on the Physics and Chemistry of the Actinides, Vol. 5, North Holland, Amsterdam, 1987, pp. 2 2 7 - 2 6 9 . 4 H. H. Hill, i n W . N. Miner (ed.), Plutonium 1070 and OtherActinides, Metallurgical Society of AIME, New York, 1970, p. 2 5 L. Gerward, J. Staun Olsen, U. Benedict, S. Dabos, H. Luo, J. P. Iti~ and 0. Vogt, High Temp. High Pressures, 20 (1988) 5 4 5 - 5 5 2 . 6 J. Staun Olsen, L. Gerward, U. Benedict, H. Luo and 0. Vogt, High Temp. High Pressures, 20 (1988) 5 5 3 - 5 5 9 . 7 L. Gerward, J. Staun Olsen, U. Benedict, S. Dabos and O. Vogt, High-Pressure Res., ] (1989) 235-251. 8 J. Staun Olsen, L. Gerward, U. Benedict, S. Dabos, J. P. Itid and O. Vogt, High-Pressure Res., 1 (1989) 2 5 3 - 2 6 6 . 9 S. Dabos-Seignon, U. Benedict, S. Heathman, J. C. Spirlet and M. Pages, J. Less-Common Met., 160 (1990) 3 5 - 5 2 . 10 M. Gensini, E. Gering, S. Heathman, U. Benedict and J. C. Spirlet, High-Pressure Res., 2 (1990) 3 4 7 - 3 5 9 . 11 S. Dabos-Seignon and U. Benedict, High Pressure Res., 4 (1990) 3 8 4 - 3 8 6 . 12 L. Gerward, J. Staun Olsen, U. Benedict and H. Luo, J. Less-Common Met., 161 (1990) L11-L14. 13 J. Staun Olsen, L. Gerward, U. Benedict, J. P. Iti~ and K. Richter, J. Less-Common Met., 121 ( 1 9 8 6 ) 4 4 5 - 4 5 3 . 14 I. Vedel, A. M. Redon and J. M. L~ger, Physica 13, 144 (1986) 6 1 - 6 5 . 15 M. Gensini, R. G. Haire, U. Benedict, F. Hulliger and J. Rebizant, p a p e r presented at the

21~nes Journdes des Actinides, Montechoro, Portugal, April 28-May 1, 1091. 16 S. Dabos-Seignon, U. Benedict and J. C. Spirlet, in preparation. 17 R. G. Haire, in Chemistry Division Annual Progress Report for Period Ending March 31, 1987, Report ORNL-6385, 1987 (Oak Ridge National Laboratory, Oak Ridge, TN), pp. 62-63. 18 L. Gerward, J. Staun Olsen, U. Benedict, J. P. Iti~ and J. C. Spirlet, J. Appl. CrystaUogr., 18 (1985) 3 3 9 - 3 4 1 . 19 U. Benedict, S. Dabos-Seignon, S. Heathman, J. P. Dancausse, M. Gensini, E. Gering and J. C. Spirlet, Proc. Symp. 50th Anniversary of the Discovery of Neptunium and Plutonium, Washington, DC, September 1900, American Chemical Society, in the press. 20 U. Benedict and W. B. Holzapfel, in K. A. Gschneidner and L. Eyring (eds.), Handbook

on the Physics and Chemistry of the Rare Earths, Lanthanide /Actinide Special Volumes, North-Holland, Amsterdam, Chapter 14, in preparation. 21 M. Gensini, R. G. Haire, U. Benedict and F. Hulliger, p a p e r presented at the 21cbnes

Journdes des Actinides, Montechoro, Portugal, April 28-May 1, 1001. K. Vohra, A. L. Ruoff, A. Jayaraman and G. Espinosa, Phys. Rev. B, 37

22 S. J. Duclos, Y. ( 1 9 8 8 ) 4250. 23 J. P. Dancausse, 381-389. 24 T. M. Benjamin, 80 (1981) 280. 25 U. Benedict, G. L171-L177.

E. Gering, S. H e a t h m a n and U. Benedict, High Pressure Res., 2 (1990) G. Zou, H. K. Mao and P. M. Bell, Carnegie Inst. Washington Yearb., D. Andreetti, J. M. Fournier and A. Waintal, J. Phys. Lett., 43 (1982)

12 26 U. Benedict, S. Dabos, C. Dufour, J. C. Spirlet and M. Pages, J. Less-Common Met., 121 (1986) 4 6 1 - 4 6 8 . 27 R. M. Hazen and L. W. Finger, Carnegie Inst. Washington Yearb., 78 (1979) 633. 28 K. Syassen, H. Winzen and U. Benedict, Physica B, 144 ( 1 9 8 6 ) 91. 29 L. Gerward, J. Staun Olsen, U. Benedict, S. Dabos-Seignon and H. Luo, High Temp. High Pressures, in the press. 30 M. Gensini, E. Gering, U. Benedict, L. Gerward, J. Staun Olsen and F. Hulliger, J. LessCommon Met., 171 (1991) L9-L12. 31 L. Gerward, J. Staun Olsen, U. Benedict, H. Luo and F. Hulliger, High Pressure Res., 4 (1990) 3 5 7 - 3 5 9 .