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Crystal chemistry of NpNi5 compound
Journal of
ALLOYS AND COMFOb~D$ ELSEVIER
Journal of Alloys and Compounds 257 (1997) 268-272
Crystal chemistry of NpNi 5 compound M. Akabori a'*, R.G. Haire b, J.K. Gibson b, Y. Okamoto ~, T. Ogawa ~ "Department of Chemicals and Fuels Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan bTransuranium Research Laboratory, Chemical and Analytical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Received 7 January 1997; accepted 24 January 1997
Abstract
The compound, NpNi5, was prepared and its crystal properties studied by means of powder X-ray diffraction. The existence and properties of this compound were pursued as they are important in developing further the Np-Ni phase diagram. The crystal symmetry of this material was determined to be hexagonal and have the D2d CaCu5 structure type, which is also observed for ThNi 5 and PuNis. The lattice parameters of NpNi 5 were calculated to be: ao =0.4859+0.0002 and co=0.3991+-0.0003 nm. Based on these lattice parameters and the apparent radius of Np in the NpNi 5 compound, it appears that Np can be considered as being tetravalent, which has certain bonding implications. The apparent valence states for Np and other actinides in these actinide-Ni 5 compounds are discussed, as well as the variation in cell volumes noted for the known actinide- and Ianthanide-Ni 5 compounds. © 1997 Elsevier Science S.A. Keywords: Intermetallic compound:Neptunium-based alloy; X-Ray diffraction; Crystal structure
1. Introduction
The alloying behaviors of actinide elements are of interest for both technological and fundamental science. Technological interest stems mainly from applications in nuclear science and technology, while fundamental interest concerns bonding, changes in electronic nature and structural facets. Recently, neptunium and americium alloys have been selected as candidate fuels and/or targets for transuranium element transmutation processes via reactors or accelerators [1]. Some of these thermodynamic properties of such alloys have been studied systematically; for example, N p - A m [21, Np-Zr [3-5] and Np with selected 3d transition elements (Cr, Mn, Fe, Co and Ni) [6,7]. In particular, the alloying behavior of minor actinides with 3d transition elements is important to predict the certain aspects of reactor fuel performance, as the transition elements are major components of cladding materials (e.g., stainless steel, HT-9, etc.) and the reactions between the fuels and the claddings induce limiting factors for the fuel's lifetime. In the case of U - P u - Z r fuels [8] for fast breeder reactors, uranium is likely to react with cladding materials to form some intermetallic compounds such as UFe z and UNi 5. In the Np-3d transition systems, Np6X [9] and NpX 2 [10] compounds have been observed for N p *Corresponding author. 0925-8388/97/$17.00 © 1997Elsevier Science S.A. AII rights reserved. PII S0925-8388(97)0003 1-5
Mn, Fe and Co systems. However, for the Np-Ni alloy system, only limited experimental data exist for the crystal structures and properties of the known NpNi 2 compound, whereas the Th-, U - and Pu-Ni systems are well established and many intermetallic compounds are known for these actinide nickel systems. For the N p - N i system, the existence of a NpNi 5 compound was expected based on the existence of Th-, U - and PuNi 5 compounds. However, these three actinide compounds are not isostructural. The ThNi 5 and PuNi 5 compounds have the hexagonal (D2d) CaCu 5 type structure, whereas UNi 5 has the cubic (Cl5b) structure "known for AuB%. Therefore, the crystal structure of NpNis, assuming it exits, would be one question depending on which near-neighbor (U or Pu) Np would mimic. A prediction is complicated by the trend in metallic radii for these lighter actinide metals; the radii decrease in going from Th to Np and then increase at Pu (Np has the smallest radius of these actinides). Based on the fact that NpNi: exhibits a cubic C15 structure like PuNi a, it could be expected that NpNi 5 might have a hexagonal (D2d) structure, as does PuNi 5. The RENi 5 (RE: rare earth) compounds also form the hexagonal (D2d) structure, although these RE metals have much larger radii. These RE materials have been extensively studied because of their potential as materials for hydrogen storage, purification, etc. [11,12]. Studies of such actinide alloys are also informative for elucidating changing trends in 5f-electron bonding,
M. Akabori et aI. / Journal o f Alloys and Compounds 257 (1997) 2 6 8 - 2 7 2
whether or not alloys form with different transition metals and the structure and compositions of the products can be indicative of electronic interactions in the materials. In general, the 5f-electron nature in light actinide compounds, including Np, is influenced by 5f-electron bands of varying width. It is known that the 5f-electrons of these lighter actinide metals are less-localized than the 4f-electrons of the RE metals, which have narrow bands but are not as broad banded as the transition metal's 3d electrons. With regard to intermetallic actinide compounds, Hill [13] has established an empirical rule (Hill limit) where the 5felectrons in intermetallic compounds of uranium, neptunium and plutonium would be expected to be localized if the actinide interatomic distances exceed certain critical values (0.35, 0.325 and 0.34 nm for U, Np and Pu compounds, respectively). However, the rule is limited to compounds with nonmagnetic partners, as reflected for the neptunium cubic Laves compounds with magnetic and nonmagnetic partners, NpX 2 (X=AI, Os, Ir and Ru as nonmagnetic partners; Fe, Ni, Co and Mn as magnetic ones) [14,15]. The magnetic properties of the neptunium3d transition metals Laves compounds are strongly influenced by the material combining with the actinideactinide distances that are formed. The main purpose of the present study was to determine: (1) whether the NpNi 5 compound could be prepared, (2) its crystal symmetry by X-ray powder diffraction techniques and (3) ascertain whether it would be isostructural with the UNi 5 or the PuNi 5 compound.
2. Experimental N p - N i alloys with a nominal composition of Np-83.7 at.% Ni were prepared by arc-melting mixtures of pure neptunium and nickel. The neptunium (purity of 99.99 wt.%) was obtained via an electrorefining process [16], and the Ni metal was a commercial product with a purity of >99,9 wt.%. The metals were weighed to within 0.01 mg and the mixture was arc-melted in a helium atmosphere glove box. The arc-melting was repeated 6 - 8 times to ensure complete homogeneity of the alloy button. After the arc-melting, the weight loss of the alloy button was examined to confirm retention of the components was >-95%. The products obtained were very brittle and easily broken into small pieces and/or powder for microscopic examinations, samples for annealing and X-ray diffraction measurements. Portions of the arc-melted alloy preparations were sealed in partially evacuated (0.1 atm helium for thermal conductance) quartz capillaries for X-ray diffraction, while other portions were annealed at 1423 K for 4 h under high vacuum ( < 1 0 .7 torr) in a tantalum crucible; after annealing, these were also sealed under a 0.1 atm helium atmosphere for analysis. The as-prepared alloy samples
269
were also annealed at 773 and 973 K in the quartz capillaries. X-Ray diffraction analyses at room temperature were performed using a conventional 114.6 mm Debye-Scherrer camera together with Mo KcL radiation. The diffraction lines were analyzed with the program POWLES [17] to calculate lattice parameters for each phases observed. High-temperature X-ray diffraction (up to 923 K) analyses were carded out using a special, in-house modified 57.3 mm Debye-Scherrer camera, where the alloy sample was heated by a platinum coil mounted in the camera. The sample temperatures were calibrated by employing a platinum chip in a capillary and using the thermal expansion of the platinum lattice for calculating temperatures. Based on the uncertainty of the lattice parameters measured and the reported thermal expansion coefficient for pure Pt, the controlled sample temperatures are considered accurate to about - 10~20 °C.
3. Results and discussion 3.1. NpNi 5 system
X-Ray data for the NpNi 5 samples were analyzed on the basis of two potential crystal structures; the hexagonal D2d and the cubic C15b. The best fit between the room temperature experimental and the calculated data was obtained with the hexagonal assignment. The best diffraction patterns were obtained from a sample annealed in vacuum at 1473 K. Table 1 compares the observed d values and diffraction intensities for NpNi s for this sample with those calculated on the basis of atomic arrangements of the D2d type structure based on structures for ThNi 5 and PuNi 5. The lattice parameter refinement using a least squares refinement yielded the following hexagonal lattice parameters: a = 0.4859+0.0002 nm c = 0.3991_+0.0003 rim. The observed diffraction data agreed well with the calculated lines, although some very weak diffraction lines were not observed experimentally. These findings were taken to support the contention that the alloy products represented a single phase NpNi 5 compound that was isostructural with ThNi 5 and PuNi 5. Alloy samples were also annealed in quartz capillaries at temperatures up to 973 K. However, with these samples, faint diffraction lines arising from trace quantities of NpO 2 were observed. The neptunium oxide was formed during the annealing at the higher temperatures, although there was no visible evidence, by microscopic examinations, of a reaction between the samples and quartz walls, nor was there signs of oxide on the alloy surfaces. This minor oxidation was not believed to have a major effect on the
M. Akabori et al. / Journal of Alloys and Compounds 257 (1997) 268-272
270
Table 1 X-Ray powder data for NpNi 5 annealed at 1423 K
Table 2 The crystallographic data for the hexagonal actinide-Ni s compounds
dob,. (rim)
d~,t~" "(nm)
hkl
1 fobs.)
III~ (catc.)
0.4203 0.3982 0,2894 0,2423 0.2105 0.2076 0,1994 0.1863
0.4208 0.3991 0.2896 0,2430 0.2104 0.2075 0.1996 0,1861 0.1803 0.1591 0,1542 0.1478 0,1448 0.1403 0,1331 0.1324 0.1269 0.1244 0.1215 0.1 t67 0,1162 0,1t48 0,1120 0.1052 0,1038 0.1021 0.1008 0.0998 0.13965 0,0938 0.0931 0.0923 0,0918 0.0902 0.0895
100 (301 101 110 200 111 002 201 102 210 112 211 202 300 003 301 103 212 220 113 221 302 311 400 222 213 312 004 303 321 402 114 410 204 411
vw vw m+ m m vs w vvw
8 10 62 35 36 I00 26 2 2 3 18 19 23 7 i_ 24 6 2 15 15 2 7 7 4 16 5 1 2 7 4 4 3 6 4 10
0.1589 0.1539 0.1477 0.1446 0.1402 0,1323 0.1268 0.1244 0,1214 0.1169 0.1145 0.1120 0,1054 0.1038
0.0998 0.0965 0.0933 0,0917 0.0895
vvw vw w w+ wmvw vvw w vw vvw vw vvw vw vvw vw ~ vvw vvw vw
Calculated using the lattice parameters (a = 0,4859 nm, c = 0.39914 nm).
formation of the NpNi 5 compound or its lattice parameters. The alloy data from samples annealed in the quartz capillaries were consistent with X-ray data obtained from samples free of trace amounts of oxides. Analyses of the high-temperature samples showed the absence of phase transitions for the NpNi 5 up to 973 K. This conclusion is in accord with DTA results [18] for N p - N i alloys with the same composition as studied here. The only previously detected transition is at 1570 K, which is assigned to be the congruent melting of the NpNi 5 compound. This temperature is in excellent accord with the congruent melting points of UNi s and PuNi 5 in these respective alloy systems [19], where those compounds have narrow composition ranges (variation is only a few at.% of Ni). Crystallographic data for selected hexagonal-type, actinide-Ni 5 compounds are summarized in Table 2. Beahm et al. [21] have studied the effect of partial substitution of uranium for thorium or plutonium in ThNi 5 and PuNi 5. These authors concluded that the plutonium in PuNi 5 can be substituted by uranium up to 50% and
Compound
ThNi; Uo 2Tho 8Nibs NpNi~ Uo.~Puo sNib5 Uo.3PUo.TNi~ PuNi~
Lattice parameter
V~ot~(urn 3)
c/a
a (rim)
c (nm)
Th-rich Ni-rich
0.4953 0.4915
0.399t 0.4026
0.08479 0.08423
0.806 0.819
Pu-rich Ni-rich
0.4950 0.4859 0.4893 0.4863 0.4872 0.4861
0.4017 0.3991 0.3968 0.3993 0.3980 0.3982
0.08524 0.08162 0.08227 0.08178 0.08181 0.08149
0.812 0.821 0.811 0.821 0.817 0.820
Ref. [20]. b Ref. [21], c This work; slightly Np rich (83.7 vs. 83.33 at.% for stoichiometric compound). d Ref. [22].
suggested that, if NpNi 5 would form the hexagonal D2d structure, its lattice parameters would be close to those determined for Uo.~Pu0.sNi 5 (a=0.4893+_0.00t3 and c = 0.3968+--0.0005 nm). Our experimental values recently obtained for NiNp5 differ more than the assigned uncertainties but are within 1% of the proposed parameters; our parameters give a slightly smaller unit cell volume. As shown in Table 2, the actinide-Ni 5 compounds have homogeneity ranges from 8 2 - 8 3 at.% Ni for UNi 5 and 8 3 - 8 6 at.% Ni for ThNi 5 or PuNis; the range increases with increasing temperatures [19]. Higher Ni contents appear to decrease the a parameters and increase the c parameters of the AnNi 5 compounds. Fig. t shows the dependence of the hexagonal AnNi 5 lattice parameters for different actinide elements, including those for the RENi 5 compounds [23]. The parameters plotted for ThNi 5 and PuNi 5 were mean values for the An- and Ni-rich compounds (see Table 2). The variation of the lattice parameter with the actinide present is much greater for the "a" than the "c" parameter, and the trend is very similar to that for the RENi s. This suggests that the interactions between Rare earth elements
0.505
La Ce Pr Nd Pm Sm Eu Gd Tb I ~ 14o Er Tm Yb Lu ,
,
.
.
.
A ~AnNi5 0.500 ~ E~0.495
.
.
.
.
,
,
)
~ ~ ,'~° ~ @
a
0.485 a(o~Ni6) ~a (AnNi5) .... '
'
'
'
~
Np Pu
0.405
0,395 ,~-E (RENi5)
Th
,
0,400
0.490
0.480
•
~
~ ~
~
'
0.390 o 0.385
'
0,380
Actinide elements Fig. 1. The lattice parameters of actinide and rare-earth Ni s compounds.
M. Akabori et al. / Journal o f Alloys and Compounds 2 5 7 (1997) 2 6 8 - 2 7 2
actinides are more significant along the " a " axis than the " c " axis, reflecting the larger interatomic distances (AnAn, An-Ni) along the c-axis (~0.40 nm) compared tO those on the basal plane (~0.28 nm). Fig. 2 shows the variation of the unit cell volume of lanthanide and actinide MNi 5 (M: f-element) compounds for the f-element which formed the hexagonal D2d MNi 5 structure. It is seen that the cell volumes for elements of both f-series decrease with increasing atomic number, where an anomaly is observed with CeNi 5. The tendency is for a sharp decline in volume with atomic number (steeper for the actinides) reflecting contractions in bond length/ radii [24]. A similar actinide contraction has not been observed with the actinide (U, Np and Pu)-3d transition (Mn, Fe and Co) Laves phases having the AnX 2 stoichiometry [10]. 3.2. Apparent actinide metallic valence in AnNi s compounds
(1)
where VAn and VNi are the atomic volumes of actinide and nickel atoms at the atomic fractions of XAn and XNi, respectively. In practice, these atomic volumes would change with composition, crystal structure, bonding, etc.. Elliott et al. considered the composition dependence of atomic volumes in N p - G a solid solutions. In the present analysis, it was necessary to assess the atomic volumes of the actinides in the AnNi 5 compounds due to a lack of composition-dependent data of the lattice parameters. The
Rare earth elements 0.088 ,ace p~ ~PmSmEuGd'CbQyHoErTmYbLo 0.087
T~gl~ 3 Nickel interatomic distances and partial atomic volumes in actinide-Ni 5 corn-pOunds Compounds
d~, (nm)
V~, (nm 3)
VA~(rim "~)
Ra. (nm)
ThNi 5 Uo2ThusNi 5 UNi; NpNi 5 Uo3Pu07Ni 5 PuNi 5
0.2459 0.2465 0.2398 0.2439 0.2441 0.2436
0.01051 0.01059 0.00975 0.01026 0.01028 0.01023
0.03194 0.03229 0.02926 0.03030 0.03038 0.03052
0.1781 0.1787 0.1729 0.1750 0.1751 0.1754
The cubic AuBe~ type structure.
atomic volumes of nickel atoms and the metallic radii of the actinides in AnNi 5 compounds were estimated from the nearest interatomic distances in the AnNi 5 crystals using a relationship given by Zachariasen for 12 coordinated atoms [24]. Specifically, (2)
R = V1/3/25/6
The partial atomic volumes of actinide elements in AnNi 5 compounds can be calculated by a simple approach suggest by Elliott et al. [25] for N p - G a solid solutions. In this approach, the average atomic volume, Va,., can be partitioned into two components at each composition. Thus,
Vav' =XAn "VAn "~ XNi "VNi
27t
"
where R and V are the metallic radius and the volume per atom, respectively. The nearest interatomic distances of nickel (Nii-6Ni 2, Ni2-4Ni I and Niz-4Ni 2) and the metallic radii of actinides are summarized in Table 3. All the nickel interatomic distances derived were smaller than that in pure nickel (0.2492 nm), whereas the metallic radii of actinides were significantly larger than those of pure metals, except for the case of ThNi 5. In general, the atomic sizes can be closely related to their valences. For instance, as the valence for neptunium changes from + 3 (5f 4) to + 7 (5f°), the metallic radius decreases from 0.188 to 0.1503 nm [24]. The increases in the metallic radii of actinides in AnNi 5 compounds could then be considered as being due to increased localization of their 5f electrons. It is well known that the 5f electrons in uranium, neptunium and plutonium are normally itinerant and participate in bonding, which produces smaller than expected atomic volumes and several physical properties. Fig. 3 shows the atomic number dependence of the calculated and predicted metallic radii, as proposed by 0.20
A 0.19
.--- 0 . 0 8 6
~= 0.085
+3
0.18
E 0.084
E
•
t4
•- - 0,17 .
0,083
i
rY 0,16
rO
0.082 0.15
0.081
0.14
0.080 Th
Np Pu
Actinide elements Fig. 2, The unit celt volumes of actinide and rare-earth Ni 5 compounds.
Th
, Pa
r U
~ Np
~ Pu
t Am
Actinide elements Fig. 3. Radius of actinides in actinide-Ni 5 compounds,
272
M. Akabori et al. / Journal of Alloys and Compounds 257 (1997) 268-272
Zachariasen [24]. Although the Zachariasen concept may have limitations, this approach (see Fig. 3) does suggest that the metallic valences of these actinide elements in the AnNi 5 compounds are likely to be less than in the metals themselves, possibly being tetravalent. This would suggest localized electron configurations for U, Np and Pu of 5f 2, 5f 3 and 5f 4, respectively. Van Daal et al. [26] reported that in UNi 5, the uranium is U 4÷ (Sf 2 state), from the lattice constant, susceptibility, specific heat etc. of this compound. However, X-ray-excited photoelectron spectroscopy data obtained from UNi 5 [27] suggested the configuration is 5f 3 (e.g., a trivalent state). This contradicts the tetravalent configuration suggested by Fig. 3. It is important to recognize the limitations of these assignments although they allow a comparative picture. The valence state of rare earths in the RENi 5 compounds is also estimated from the lattice parameters, using the same method. As a result, it is seen that the radii of rare earths in the RENi 5 decrease from 0.182 for La to 0.174 nm for Lu gradually with increasing the atomic number except Ce (0.176 nm). The radii are very similar to those of rare earth metals [24], suggesting that the valence state of rare earths in RENi 5 compounds is trivalent. On the other hand, the radius of Ce in CeNi5 is less than that of ",/-Ce (0.1824 nm) and larger than that of a-Ce (0.1707 nm), suggesting that the valence of Ce in CeNi s is between +3.1 ('y-Ce) and +3.8 (~-Ce). In contrast to the AnNi 5 compounds, the magnetic properties of the actinide cubic Laves phases, AnX 2, have extensively been studied. From the interatomic spacing of neptunium in the NpX,, the 5f electrons of Np are expected to be itinerant more than localized, because the N p - N p distances are smaller than the Hill limit (-0.325 nm) for neptunium [28]. However, in the case of Np-3d transition metals (Mn, Fe, Co and Ni) systems [14,15], the magnetic properties of NpX z suggest localization of neptunium's 5f-electrons and strong interaction between these 5f-electrons and the 3d-electrons of the transition metals. This suggests that the 5f-electrons of neptunium atoms in NpNi s compounds could also be influenced by or interact with the 3d-electrons of the transition elements. Alternatively, the interatomic spacing of actinides in AnNi s compounds are of the order of about 0.40 nm, significantly larger than the Hill limits for actinides, which would suggest that the actinide's 5f-electrons in the AnNi s compounds would be localized. Additional investigations of these AnNi s compounds will be needed to clarify and understand the electronic interactions between components, the electronic configurations and the magnetic properties of the AnNi 5 compounds.
Acknowledgments This work was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department
of Energy, under Contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation and by the Japan Atomic Energy Research Institute (JAERI) under the Japan-US Actinide Program. The authors gratefully acknowledge Drs. S. Raman of LMER, T. Mukaiyama, T. Hoshi and Muromura of JAERI for their ongoing support and contributions.
References [ll T. Takizuka et al., Systems study on partitioning and transmutation at JAERI, OECD/NEA 3rd International Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation, Cadaraehe, France, Dec. 12-i4, 1994. [2] J.K. Gibson, R.G. Haire, J. Nucl. Mater. 195 (1992) 156. [3] J.K. Gibson, R.G. Haire, J. Nucl. Mater. 201 (1992) 225. [4] J.K. Gibson, R.G. Haire, M.M. Gensini, T. Ogawa, J. Alloys Comps. 2t3-214 (1994) 106. [5] M.M. Gensini, J.K. Gibson, R.G. Haire, J. Alloys Comps. 213-214 (1994) 402. [6] J.K. Gibson, R.G. Hake, E.C. Beahm, M.M. Gensini, A. Maeda, T. Ogawa, J. Nucl. Mater. 21t (1994) 215. [7] J.K. Gibson, R.G. Haire, Y. Okamoto, T. Ogawa, J. Alloys and Compounds, (in press). [8] C. Sad, C.T. Walker, M. Kurata, T. Inoue, J. Nuet. Mater. 208 (1994) 201. [9] B.C. Giessen, R.B. Roof, A.M. Russell, R.O. Eltiott, J. LessCommon Metals 53 (1977) 147. [10] D.J. Lam, A.W. Mitchell, J. Nucl. Mater. 44 (1972) 279. [11] K.H.J. Buschow, H.H. Van Mal, J. Less-Common Metals 29 (1972) 203. [12] K. Yamaguchi, D.Y. Kim, M. Ohtsuka, K. Itagaki, J. AlIoys Comps. 221 (1995) I61. [13] H. Hill, in: W.N. Miner tEd.), Plutonium I970 and Other Actinides, AIME, New York, 1971, p, 2. [14] A.T. Aldred, B.D. Dunlap, D.J. Lain, GM. Lander, M.H. Mueller, I. Nowik, Phys. Rev. B I1(1) (1975) 530. [15] G.M. Kalvius, W. Potzel, S. Zwirner, J. GaI, L Nowik, J. Alloys Comps. 213-214 (1994) 138. [16] R.G. Haire, J. Less-Common Metals 121 (1986) 379. [17] D.E. Williams, Ames Laboratory Report IS-t052, 1964. [18] LK. Gibson and R.G. Haire, unpublished work, 1994. [19l T.B. Massalski, Binary Alloy Phase Diagrams, Vol. 3, ASM, Metals Park, Ohio, 1990, pp. 2878-2880 (Ni-U) and pp. 2845-2847 (Ni-Pu). [20] J.R. Thomson, J. Less-Common Metals 29 (1972) 183. [21] E.C. Beahm and C.A. Culpepper, unpublished work, 1990. [22] F.H. Etlinger, J. Metal. Soc. AIME 206 (1959) 1256. [23] P. Villars and L.D. Calvert, Pearson's Hand Book of Crystallogaphic Data for Intermetallic Phases, 2nd ed., The Materials Information Society, Materials Park, OH, 1991. [24] W.H. Zachariasen, J. Inorg. Nut1. Chem. 35 (1973) 3487. [25] R.O. Elliott, A.M. Russell, B.C. Giessen, Scr. Metall. 8 (1974) 1335. [26] H.J. van Daal, K.H.J. Bushow, EB. van Aken, M.H. yam Marren, Phys. Rev. Lett. 34 (1975) 1457. [27] H. Grohs, H. H6chst, F. Steiner, S. Hfifner, K.H.J. Buschow, Solid State Commun. 33 (1980) 573. [28] J.M. Foumier and L. Manes, Actinide Solids: 5f Dependence of Physical Properties, in: L. Manes tEd.), Structure and Bonding, Vol. 59/60, Springer-Verlag, Berlin, 1985, pp. 1-56.
ALLOYS AND COMFOb~D$ ELSEVIER
Journal of Alloys and Compounds 257 (1997) 268-272
Crystal chemistry of NpNi 5 compound M. Akabori a'*, R.G. Haire b, J.K. Gibson b, Y. Okamoto ~, T. Ogawa ~ "Department of Chemicals and Fuels Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan bTransuranium Research Laboratory, Chemical and Analytical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Received 7 January 1997; accepted 24 January 1997
Abstract
The compound, NpNi5, was prepared and its crystal properties studied by means of powder X-ray diffraction. The existence and properties of this compound were pursued as they are important in developing further the Np-Ni phase diagram. The crystal symmetry of this material was determined to be hexagonal and have the D2d CaCu5 structure type, which is also observed for ThNi 5 and PuNis. The lattice parameters of NpNi 5 were calculated to be: ao =0.4859+0.0002 and co=0.3991+-0.0003 nm. Based on these lattice parameters and the apparent radius of Np in the NpNi 5 compound, it appears that Np can be considered as being tetravalent, which has certain bonding implications. The apparent valence states for Np and other actinides in these actinide-Ni 5 compounds are discussed, as well as the variation in cell volumes noted for the known actinide- and Ianthanide-Ni 5 compounds. © 1997 Elsevier Science S.A. Keywords: Intermetallic compound:Neptunium-based alloy; X-Ray diffraction; Crystal structure
1. Introduction
The alloying behaviors of actinide elements are of interest for both technological and fundamental science. Technological interest stems mainly from applications in nuclear science and technology, while fundamental interest concerns bonding, changes in electronic nature and structural facets. Recently, neptunium and americium alloys have been selected as candidate fuels and/or targets for transuranium element transmutation processes via reactors or accelerators [1]. Some of these thermodynamic properties of such alloys have been studied systematically; for example, N p - A m [21, Np-Zr [3-5] and Np with selected 3d transition elements (Cr, Mn, Fe, Co and Ni) [6,7]. In particular, the alloying behavior of minor actinides with 3d transition elements is important to predict the certain aspects of reactor fuel performance, as the transition elements are major components of cladding materials (e.g., stainless steel, HT-9, etc.) and the reactions between the fuels and the claddings induce limiting factors for the fuel's lifetime. In the case of U - P u - Z r fuels [8] for fast breeder reactors, uranium is likely to react with cladding materials to form some intermetallic compounds such as UFe z and UNi 5. In the Np-3d transition systems, Np6X [9] and NpX 2 [10] compounds have been observed for N p *Corresponding author. 0925-8388/97/$17.00 © 1997Elsevier Science S.A. AII rights reserved. PII S0925-8388(97)0003 1-5
Mn, Fe and Co systems. However, for the Np-Ni alloy system, only limited experimental data exist for the crystal structures and properties of the known NpNi 2 compound, whereas the Th-, U - and Pu-Ni systems are well established and many intermetallic compounds are known for these actinide nickel systems. For the N p - N i system, the existence of a NpNi 5 compound was expected based on the existence of Th-, U - and PuNi 5 compounds. However, these three actinide compounds are not isostructural. The ThNi 5 and PuNi 5 compounds have the hexagonal (D2d) CaCu 5 type structure, whereas UNi 5 has the cubic (Cl5b) structure "known for AuB%. Therefore, the crystal structure of NpNis, assuming it exits, would be one question depending on which near-neighbor (U or Pu) Np would mimic. A prediction is complicated by the trend in metallic radii for these lighter actinide metals; the radii decrease in going from Th to Np and then increase at Pu (Np has the smallest radius of these actinides). Based on the fact that NpNi: exhibits a cubic C15 structure like PuNi a, it could be expected that NpNi 5 might have a hexagonal (D2d) structure, as does PuNi 5. The RENi 5 (RE: rare earth) compounds also form the hexagonal (D2d) structure, although these RE metals have much larger radii. These RE materials have been extensively studied because of their potential as materials for hydrogen storage, purification, etc. [11,12]. Studies of such actinide alloys are also informative for elucidating changing trends in 5f-electron bonding,
M. Akabori et aI. / Journal o f Alloys and Compounds 257 (1997) 2 6 8 - 2 7 2
whether or not alloys form with different transition metals and the structure and compositions of the products can be indicative of electronic interactions in the materials. In general, the 5f-electron nature in light actinide compounds, including Np, is influenced by 5f-electron bands of varying width. It is known that the 5f-electrons of these lighter actinide metals are less-localized than the 4f-electrons of the RE metals, which have narrow bands but are not as broad banded as the transition metal's 3d electrons. With regard to intermetallic actinide compounds, Hill [13] has established an empirical rule (Hill limit) where the 5felectrons in intermetallic compounds of uranium, neptunium and plutonium would be expected to be localized if the actinide interatomic distances exceed certain critical values (0.35, 0.325 and 0.34 nm for U, Np and Pu compounds, respectively). However, the rule is limited to compounds with nonmagnetic partners, as reflected for the neptunium cubic Laves compounds with magnetic and nonmagnetic partners, NpX 2 (X=AI, Os, Ir and Ru as nonmagnetic partners; Fe, Ni, Co and Mn as magnetic ones) [14,15]. The magnetic properties of the neptunium3d transition metals Laves compounds are strongly influenced by the material combining with the actinideactinide distances that are formed. The main purpose of the present study was to determine: (1) whether the NpNi 5 compound could be prepared, (2) its crystal symmetry by X-ray powder diffraction techniques and (3) ascertain whether it would be isostructural with the UNi 5 or the PuNi 5 compound.
2. Experimental N p - N i alloys with a nominal composition of Np-83.7 at.% Ni were prepared by arc-melting mixtures of pure neptunium and nickel. The neptunium (purity of 99.99 wt.%) was obtained via an electrorefining process [16], and the Ni metal was a commercial product with a purity of >99,9 wt.%. The metals were weighed to within 0.01 mg and the mixture was arc-melted in a helium atmosphere glove box. The arc-melting was repeated 6 - 8 times to ensure complete homogeneity of the alloy button. After the arc-melting, the weight loss of the alloy button was examined to confirm retention of the components was >-95%. The products obtained were very brittle and easily broken into small pieces and/or powder for microscopic examinations, samples for annealing and X-ray diffraction measurements. Portions of the arc-melted alloy preparations were sealed in partially evacuated (0.1 atm helium for thermal conductance) quartz capillaries for X-ray diffraction, while other portions were annealed at 1423 K for 4 h under high vacuum ( < 1 0 .7 torr) in a tantalum crucible; after annealing, these were also sealed under a 0.1 atm helium atmosphere for analysis. The as-prepared alloy samples
269
were also annealed at 773 and 973 K in the quartz capillaries. X-Ray diffraction analyses at room temperature were performed using a conventional 114.6 mm Debye-Scherrer camera together with Mo KcL radiation. The diffraction lines were analyzed with the program POWLES [17] to calculate lattice parameters for each phases observed. High-temperature X-ray diffraction (up to 923 K) analyses were carded out using a special, in-house modified 57.3 mm Debye-Scherrer camera, where the alloy sample was heated by a platinum coil mounted in the camera. The sample temperatures were calibrated by employing a platinum chip in a capillary and using the thermal expansion of the platinum lattice for calculating temperatures. Based on the uncertainty of the lattice parameters measured and the reported thermal expansion coefficient for pure Pt, the controlled sample temperatures are considered accurate to about - 10~20 °C.
3. Results and discussion 3.1. NpNi 5 system
X-Ray data for the NpNi 5 samples were analyzed on the basis of two potential crystal structures; the hexagonal D2d and the cubic C15b. The best fit between the room temperature experimental and the calculated data was obtained with the hexagonal assignment. The best diffraction patterns were obtained from a sample annealed in vacuum at 1473 K. Table 1 compares the observed d values and diffraction intensities for NpNi s for this sample with those calculated on the basis of atomic arrangements of the D2d type structure based on structures for ThNi 5 and PuNi 5. The lattice parameter refinement using a least squares refinement yielded the following hexagonal lattice parameters: a = 0.4859+0.0002 nm c = 0.3991_+0.0003 rim. The observed diffraction data agreed well with the calculated lines, although some very weak diffraction lines were not observed experimentally. These findings were taken to support the contention that the alloy products represented a single phase NpNi 5 compound that was isostructural with ThNi 5 and PuNi 5. Alloy samples were also annealed in quartz capillaries at temperatures up to 973 K. However, with these samples, faint diffraction lines arising from trace quantities of NpO 2 were observed. The neptunium oxide was formed during the annealing at the higher temperatures, although there was no visible evidence, by microscopic examinations, of a reaction between the samples and quartz walls, nor was there signs of oxide on the alloy surfaces. This minor oxidation was not believed to have a major effect on the
M. Akabori et al. / Journal of Alloys and Compounds 257 (1997) 268-272
270
Table 1 X-Ray powder data for NpNi 5 annealed at 1423 K
Table 2 The crystallographic data for the hexagonal actinide-Ni s compounds
dob,. (rim)
d~,t~" "(nm)
hkl
1 fobs.)
III~ (catc.)
0.4203 0.3982 0,2894 0,2423 0.2105 0.2076 0,1994 0.1863
0.4208 0.3991 0.2896 0,2430 0.2104 0.2075 0.1996 0,1861 0.1803 0.1591 0,1542 0.1478 0,1448 0.1403 0,1331 0.1324 0.1269 0.1244 0.1215 0.1 t67 0,1162 0,1t48 0,1120 0.1052 0,1038 0.1021 0.1008 0.0998 0.13965 0,0938 0.0931 0.0923 0,0918 0.0902 0.0895
100 (301 101 110 200 111 002 201 102 210 112 211 202 300 003 301 103 212 220 113 221 302 311 400 222 213 312 004 303 321 402 114 410 204 411
vw vw m+ m m vs w vvw
8 10 62 35 36 I00 26 2 2 3 18 19 23 7 i_ 24 6 2 15 15 2 7 7 4 16 5 1 2 7 4 4 3 6 4 10
0.1589 0.1539 0.1477 0.1446 0.1402 0,1323 0.1268 0.1244 0,1214 0.1169 0.1145 0.1120 0,1054 0.1038
0.0998 0.0965 0.0933 0,0917 0.0895
vvw vw w w+ wmvw vvw w vw vvw vw vvw vw vvw vw ~ vvw vvw vw
Calculated using the lattice parameters (a = 0,4859 nm, c = 0.39914 nm).
formation of the NpNi 5 compound or its lattice parameters. The alloy data from samples annealed in the quartz capillaries were consistent with X-ray data obtained from samples free of trace amounts of oxides. Analyses of the high-temperature samples showed the absence of phase transitions for the NpNi 5 up to 973 K. This conclusion is in accord with DTA results [18] for N p - N i alloys with the same composition as studied here. The only previously detected transition is at 1570 K, which is assigned to be the congruent melting of the NpNi 5 compound. This temperature is in excellent accord with the congruent melting points of UNi s and PuNi 5 in these respective alloy systems [19], where those compounds have narrow composition ranges (variation is only a few at.% of Ni). Crystallographic data for selected hexagonal-type, actinide-Ni 5 compounds are summarized in Table 2. Beahm et al. [21] have studied the effect of partial substitution of uranium for thorium or plutonium in ThNi 5 and PuNi 5. These authors concluded that the plutonium in PuNi 5 can be substituted by uranium up to 50% and
Compound
ThNi; Uo 2Tho 8Nibs NpNi~ Uo.~Puo sNib5 Uo.3PUo.TNi~ PuNi~
Lattice parameter
V~ot~(urn 3)
c/a
a (rim)
c (nm)
Th-rich Ni-rich
0.4953 0.4915
0.399t 0.4026
0.08479 0.08423
0.806 0.819
Pu-rich Ni-rich
0.4950 0.4859 0.4893 0.4863 0.4872 0.4861
0.4017 0.3991 0.3968 0.3993 0.3980 0.3982
0.08524 0.08162 0.08227 0.08178 0.08181 0.08149
0.812 0.821 0.811 0.821 0.817 0.820
Ref. [20]. b Ref. [21], c This work; slightly Np rich (83.7 vs. 83.33 at.% for stoichiometric compound). d Ref. [22].
suggested that, if NpNi 5 would form the hexagonal D2d structure, its lattice parameters would be close to those determined for Uo.~Pu0.sNi 5 (a=0.4893+_0.00t3 and c = 0.3968+--0.0005 nm). Our experimental values recently obtained for NiNp5 differ more than the assigned uncertainties but are within 1% of the proposed parameters; our parameters give a slightly smaller unit cell volume. As shown in Table 2, the actinide-Ni 5 compounds have homogeneity ranges from 8 2 - 8 3 at.% Ni for UNi 5 and 8 3 - 8 6 at.% Ni for ThNi 5 or PuNis; the range increases with increasing temperatures [19]. Higher Ni contents appear to decrease the a parameters and increase the c parameters of the AnNi 5 compounds. Fig. t shows the dependence of the hexagonal AnNi 5 lattice parameters for different actinide elements, including those for the RENi 5 compounds [23]. The parameters plotted for ThNi 5 and PuNi 5 were mean values for the An- and Ni-rich compounds (see Table 2). The variation of the lattice parameter with the actinide present is much greater for the "a" than the "c" parameter, and the trend is very similar to that for the RENi s. This suggests that the interactions between Rare earth elements
0.505
La Ce Pr Nd Pm Sm Eu Gd Tb I ~ 14o Er Tm Yb Lu ,
,
.
.
.
A ~AnNi5 0.500 ~ E~0.495
.
.
.
.
,
,
)
~ ~ ,'~° ~ @
a
0.485 a(o~Ni6) ~a (AnNi5) .... '
'
'
'
~
Np Pu
0.405
0,395 ,~-E (RENi5)
Th
,
0,400
0.490
0.480
•
~
~ ~
~
'
0.390 o 0.385
'
0,380
Actinide elements Fig. 1. The lattice parameters of actinide and rare-earth Ni s compounds.
M. Akabori et al. / Journal o f Alloys and Compounds 2 5 7 (1997) 2 6 8 - 2 7 2
actinides are more significant along the " a " axis than the " c " axis, reflecting the larger interatomic distances (AnAn, An-Ni) along the c-axis (~0.40 nm) compared tO those on the basal plane (~0.28 nm). Fig. 2 shows the variation of the unit cell volume of lanthanide and actinide MNi 5 (M: f-element) compounds for the f-element which formed the hexagonal D2d MNi 5 structure. It is seen that the cell volumes for elements of both f-series decrease with increasing atomic number, where an anomaly is observed with CeNi 5. The tendency is for a sharp decline in volume with atomic number (steeper for the actinides) reflecting contractions in bond length/ radii [24]. A similar actinide contraction has not been observed with the actinide (U, Np and Pu)-3d transition (Mn, Fe and Co) Laves phases having the AnX 2 stoichiometry [10]. 3.2. Apparent actinide metallic valence in AnNi s compounds
(1)
where VAn and VNi are the atomic volumes of actinide and nickel atoms at the atomic fractions of XAn and XNi, respectively. In practice, these atomic volumes would change with composition, crystal structure, bonding, etc.. Elliott et al. considered the composition dependence of atomic volumes in N p - G a solid solutions. In the present analysis, it was necessary to assess the atomic volumes of the actinides in the AnNi 5 compounds due to a lack of composition-dependent data of the lattice parameters. The
Rare earth elements 0.088 ,ace p~ ~PmSmEuGd'CbQyHoErTmYbLo 0.087
T~gl~ 3 Nickel interatomic distances and partial atomic volumes in actinide-Ni 5 corn-pOunds Compounds
d~, (nm)
V~, (nm 3)
VA~(rim "~)
Ra. (nm)
ThNi 5 Uo2ThusNi 5 UNi; NpNi 5 Uo3Pu07Ni 5 PuNi 5
0.2459 0.2465 0.2398 0.2439 0.2441 0.2436
0.01051 0.01059 0.00975 0.01026 0.01028 0.01023
0.03194 0.03229 0.02926 0.03030 0.03038 0.03052
0.1781 0.1787 0.1729 0.1750 0.1751 0.1754
The cubic AuBe~ type structure.
atomic volumes of nickel atoms and the metallic radii of the actinides in AnNi 5 compounds were estimated from the nearest interatomic distances in the AnNi 5 crystals using a relationship given by Zachariasen for 12 coordinated atoms [24]. Specifically, (2)
R = V1/3/25/6
The partial atomic volumes of actinide elements in AnNi 5 compounds can be calculated by a simple approach suggest by Elliott et al. [25] for N p - G a solid solutions. In this approach, the average atomic volume, Va,., can be partitioned into two components at each composition. Thus,
Vav' =XAn "VAn "~ XNi "VNi
27t
"
where R and V are the metallic radius and the volume per atom, respectively. The nearest interatomic distances of nickel (Nii-6Ni 2, Ni2-4Ni I and Niz-4Ni 2) and the metallic radii of actinides are summarized in Table 3. All the nickel interatomic distances derived were smaller than that in pure nickel (0.2492 nm), whereas the metallic radii of actinides were significantly larger than those of pure metals, except for the case of ThNi 5. In general, the atomic sizes can be closely related to their valences. For instance, as the valence for neptunium changes from + 3 (5f 4) to + 7 (5f°), the metallic radius decreases from 0.188 to 0.1503 nm [24]. The increases in the metallic radii of actinides in AnNi 5 compounds could then be considered as being due to increased localization of their 5f electrons. It is well known that the 5f electrons in uranium, neptunium and plutonium are normally itinerant and participate in bonding, which produces smaller than expected atomic volumes and several physical properties. Fig. 3 shows the atomic number dependence of the calculated and predicted metallic radii, as proposed by 0.20
A 0.19
.--- 0 . 0 8 6
~= 0.085
+3
0.18
E 0.084
E
•
t4
•- - 0,17 .
0,083
i
rY 0,16
rO
0.082 0.15
0.081
0.14
0.080 Th
Np Pu
Actinide elements Fig. 2, The unit celt volumes of actinide and rare-earth Ni 5 compounds.
Th
, Pa
r U
~ Np
~ Pu
t Am
Actinide elements Fig. 3. Radius of actinides in actinide-Ni 5 compounds,
272
M. Akabori et al. / Journal of Alloys and Compounds 257 (1997) 268-272
Zachariasen [24]. Although the Zachariasen concept may have limitations, this approach (see Fig. 3) does suggest that the metallic valences of these actinide elements in the AnNi 5 compounds are likely to be less than in the metals themselves, possibly being tetravalent. This would suggest localized electron configurations for U, Np and Pu of 5f 2, 5f 3 and 5f 4, respectively. Van Daal et al. [26] reported that in UNi 5, the uranium is U 4÷ (Sf 2 state), from the lattice constant, susceptibility, specific heat etc. of this compound. However, X-ray-excited photoelectron spectroscopy data obtained from UNi 5 [27] suggested the configuration is 5f 3 (e.g., a trivalent state). This contradicts the tetravalent configuration suggested by Fig. 3. It is important to recognize the limitations of these assignments although they allow a comparative picture. The valence state of rare earths in the RENi 5 compounds is also estimated from the lattice parameters, using the same method. As a result, it is seen that the radii of rare earths in the RENi 5 decrease from 0.182 for La to 0.174 nm for Lu gradually with increasing the atomic number except Ce (0.176 nm). The radii are very similar to those of rare earth metals [24], suggesting that the valence state of rare earths in RENi 5 compounds is trivalent. On the other hand, the radius of Ce in CeNi5 is less than that of ",/-Ce (0.1824 nm) and larger than that of a-Ce (0.1707 nm), suggesting that the valence of Ce in CeNi s is between +3.1 ('y-Ce) and +3.8 (~-Ce). In contrast to the AnNi 5 compounds, the magnetic properties of the actinide cubic Laves phases, AnX 2, have extensively been studied. From the interatomic spacing of neptunium in the NpX,, the 5f electrons of Np are expected to be itinerant more than localized, because the N p - N p distances are smaller than the Hill limit (-0.325 nm) for neptunium [28]. However, in the case of Np-3d transition metals (Mn, Fe, Co and Ni) systems [14,15], the magnetic properties of NpX z suggest localization of neptunium's 5f-electrons and strong interaction between these 5f-electrons and the 3d-electrons of the transition metals. This suggests that the 5f-electrons of neptunium atoms in NpNi s compounds could also be influenced by or interact with the 3d-electrons of the transition elements. Alternatively, the interatomic spacing of actinides in AnNi s compounds are of the order of about 0.40 nm, significantly larger than the Hill limits for actinides, which would suggest that the actinide's 5f-electrons in the AnNi s compounds would be localized. Additional investigations of these AnNi s compounds will be needed to clarify and understand the electronic interactions between components, the electronic configurations and the magnetic properties of the AnNi 5 compounds.
Acknowledgments This work was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department
of Energy, under Contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation and by the Japan Atomic Energy Research Institute (JAERI) under the Japan-US Actinide Program. The authors gratefully acknowledge Drs. S. Raman of LMER, T. Mukaiyama, T. Hoshi and Muromura of JAERI for their ongoing support and contributions.
References [ll T. Takizuka et al., Systems study on partitioning and transmutation at JAERI, OECD/NEA 3rd International Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation, Cadaraehe, France, Dec. 12-i4, 1994. [2] J.K. Gibson, R.G. Haire, J. Nucl. Mater. 195 (1992) 156. [3] J.K. Gibson, R.G. Haire, J. Nucl. Mater. 201 (1992) 225. [4] J.K. Gibson, R.G. Haire, M.M. Gensini, T. Ogawa, J. Alloys Comps. 2t3-214 (1994) 106. [5] M.M. Gensini, J.K. Gibson, R.G. Haire, J. Alloys Comps. 213-214 (1994) 402. [6] J.K. Gibson, R.G. Hake, E.C. Beahm, M.M. Gensini, A. Maeda, T. Ogawa, J. Nucl. Mater. 21t (1994) 215. [7] J.K. Gibson, R.G. Haire, Y. Okamoto, T. Ogawa, J. Alloys and Compounds, (in press). [8] C. Sad, C.T. Walker, M. Kurata, T. Inoue, J. Nuet. Mater. 208 (1994) 201. [9] B.C. Giessen, R.B. Roof, A.M. Russell, R.O. Eltiott, J. LessCommon Metals 53 (1977) 147. [10] D.J. Lam, A.W. Mitchell, J. Nucl. Mater. 44 (1972) 279. [11] K.H.J. Buschow, H.H. Van Mal, J. Less-Common Metals 29 (1972) 203. [12] K. Yamaguchi, D.Y. Kim, M. Ohtsuka, K. Itagaki, J. AlIoys Comps. 221 (1995) I61. [13] H. Hill, in: W.N. Miner tEd.), Plutonium I970 and Other Actinides, AIME, New York, 1971, p, 2. [14] A.T. Aldred, B.D. Dunlap, D.J. Lain, GM. Lander, M.H. Mueller, I. Nowik, Phys. Rev. B I1(1) (1975) 530. [15] G.M. Kalvius, W. Potzel, S. Zwirner, J. GaI, L Nowik, J. Alloys Comps. 213-214 (1994) 138. [16] R.G. Haire, J. Less-Common Metals 121 (1986) 379. [17] D.E. Williams, Ames Laboratory Report IS-t052, 1964. [18] LK. Gibson and R.G. Haire, unpublished work, 1994. [19l T.B. Massalski, Binary Alloy Phase Diagrams, Vol. 3, ASM, Metals Park, Ohio, 1990, pp. 2878-2880 (Ni-U) and pp. 2845-2847 (Ni-Pu). [20] J.R. Thomson, J. Less-Common Metals 29 (1972) 183. [21] E.C. Beahm and C.A. Culpepper, unpublished work, 1990. [22] F.H. Etlinger, J. Metal. Soc. AIME 206 (1959) 1256. [23] P. Villars and L.D. Calvert, Pearson's Hand Book of Crystallogaphic Data for Intermetallic Phases, 2nd ed., The Materials Information Society, Materials Park, OH, 1991. [24] W.H. Zachariasen, J. Inorg. Nut1. Chem. 35 (1973) 3487. [25] R.O. Elliott, A.M. Russell, B.C. Giessen, Scr. Metall. 8 (1974) 1335. [26] H.J. van Daal, K.H.J. Bushow, EB. van Aken, M.H. yam Marren, Phys. Rev. Lett. 34 (1975) 1457. [27] H. Grohs, H. H6chst, F. Steiner, S. Hfifner, K.H.J. Buschow, Solid State Commun. 33 (1980) 573. [28] J.M. Foumier and L. Manes, Actinide Solids: 5f Dependence of Physical Properties, in: L. Manes tEd.), Structure and Bonding, Vol. 59/60, Springer-Verlag, Berlin, 1985, pp. 1-56.