The ternary system: hafnium–silicon–uranium

Journal of Alloys and Compounds 387 (2005) 246–250

The ternary system: hafnium–silicon–uranium F. Weitzera,b , P. Rogla,b,∗ , H. No¨ela a

Laboratoire de Chimie du Solide et Inorganique Mol´eculaire, UMR-CNRS 6511, Universit´e de Rennes I, Avenue du G´en´eral Leclerc, F-35042 Rennes, Cedex, France b Institut f¨ ur Physikalische Chemie, Universit¨at Wien, A-1090 Wien, W¨ahringerstraße 42, Austria Received 1 June 2004; received in revised form 17 June 2004; accepted 17 June 2004

Abstract Phase equilibria in the system Hf–Si–U were established at 1000 ◦ C by optical microscopy, EMPA and X-ray diffraction. No ternary compound was observed; however, at 1000 ◦ C, a rather wide solid solution extends from binary Hf5 Si4 into the ternary system, Ux Hf5−x Si4 , 0 ≤ x ≤ 1.3. In as-cast alloys, the solid solution extends up to x = 1.6. The crystal structure of the solid solution was characterised by X-ray powder-data refinement and found to be isotypic with the Zr5 Si4 -type for the entire homogeneity region. X-ray intensity data were consistent with a random substitution, Hf/U, in one of the Hf-atom sites of Hf5 Si4 . Mutual solubility of U-silicides and Hf-silicides was found to be below about 0.6 at.% except for about 1.6 at.% Hf in U3 Si2 in alloys annealed at 1000 ◦ C. The Hf–U-rich part of the diagram was also investigated at 850 ◦ C establishing the tie-lines to the low temperature compounds U3 Si and Hf2 Si. © 2004 Elsevier B.V. All rights reserved. Keywords: Ternary system Hf–Si–U; Isothermal sections at 850 and 1000 ◦ C; Crystal structure of the solid solution Ux Hf5−x Si4 , 0 ≤ x ≤ 1.3; Rietveld refinement of X-ray powder data

1. Introduction In continuation of our research programme, to provide basic information on the phase relations and the crystal chemistry in the ternary systems actinoid metal–transition metal–silicon [1–3], we presently focus on the ternary uranium silicide system with hafnium. As far as the phase equilibria and compatibility of U3 Si and U3 Si2 with Hf-metal are concerned, the research reported herein is related to low enriched uranium (LEU) proliferation resistant reactor U3 Si2 dispersion fuels widely used in research reactors [4].

2. Experimental All samples, each of the total amount of ca. 1g, were prepared by argon arc-melting the elements. Platelets of depleted

∗

Corresponding author. Tel.: +43-1-4277-52456; fax: +43-1-4277-9524. E-mail address: [email protected] (P. Rogl).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.067

uranium (claimed purity of 99.9% by Merck, Darmstadt, D), pieces of 6N-silicon (99.9999%) and hafnium sponge (99.9%, both from Alfa Ventron, Karlsruhe, D) were used as starting materials. The U-platelets were surface cleaned in diluted HNO3 prior to melting. For homogeneity, the samples were re-melted several times; weight losses were checked to be altogether less than 0.5 mass%. A part of each alloy was contained within a small alumina crucible, sealed in an evacuated silica tube and heat-treated at 850 ◦ C for 500 h or at 1000 ◦ C for 300 h, respectively and finally quenched by submerging the capsule in cold water. Further details of sample preparation, of the X-ray techniques used (including quantitative Rietveld analyses employing the Fullprof program [5]) as well as a general description of the magnetic measurements (SQUID-magnetometer) may be found from our preceding publication on binary uranium silicides [6] or on the ternary system Nb–Si–U [1]. As-cast and annealed samples were investigated by XPD, LOM and quantitative EMPA on SiC-ground and 1/4 ␮m diamond paste polished surfaces, etched by a mixture of 1 cm3 HF + 5 cm3 H2 O2 in 94 cm3 of H2 O. A CAMEBAX SX50

F. Weitzer et al. / Journal of Alloys and Compounds 387 (2005) 246–250

wavelength dispersive X-ray microanalyser (EMPA-XMA) was used for proper identification of the phases operating at an acceleration voltage of 15 kV at 20 nA sample current and with spectrometer crystals such as PET for the U–M␣, Hf–L␣ and TAP for the Si–K␣ radiation. For quantitative analyses, the ZAF correction program was employed [7], comparing the characteristic radiation of the elements from the alloy with that obtained from UO2 , elemental Hf and SiO2 as reference materials. The sum of mass percent for the individual elemental measurements were within (100 ± 0.8) mass% for all alloys investigated.

3. Results and discussion 3.1. The binary boundary systems The boundary systems Hf–U and Hf–Si were accepted from the compilation of binary alloy phase diagrams by Massalski [8]. A recent thermodynamic assessment of the Hf–Si system is due to Zhao [9]. The U–Si system used herein is from a reinvestigation by the authors [6,10,11]. The uranium-

247

rich part of the diagram up to 4 at.% Si is taken from [12,13]. Crystallographic data of the boundary phases can be found in [13,14] and are listed in Table 1. 3.2. Phase relations at 850 and 1000 ◦ C Experimental data (XPD, EMPA) on the phase relations at various temperatures are summarized in Table 2. Accordingly, phase equilibria are characterized by the formation of a considerable solid solution of U in binary Hf5 Si4 extending up to 14.6 at.% U at 1000 ◦ C. Microprobe analyses of as-cast as well as of alloys annealed at 1400 and 1200 ◦ C, respectively, revealed a solubility of uranium rising with temperature from x = 1.32 (1000 ◦ C) to x = 1.47 (1200 ◦ C) and to x = 1.62 in the as-cast state. A complete isothermal section was established at 1000 ◦ C (see Fig. 1), whereas a partial section covering the Si–poor region U–U3 Si2 –Hf3 Si2 –Hf was investigated at 850 ◦ C. Except for somewhat reduced mutual solid solubilities at 850 ◦ C as compared to 1000 ◦ C, and except for the tie-line, U3 Si–Hf3 Si2 phase relations in general are identical. At both temperatures, the solid solution of Ux Hf5−x Si4 is compatible with the uranium-rich W-type

Table 1 Crystallographic data of binary boundary phases of the system Hf–Si–U Phase

Pearson symbol

Space group

Proto type

Lattice parameters (nm)

␥U ␤U ␣U ␤Hf ␣Hf Si Hf2 Si Hf3 Si2 Hf5 Si3 Hf5 Si4 HfSi HfSi2 ␥U3 Si ␤U3 Si ␣U3 Si U3 Si2 U5 Si4 USi USia U3 Si5 U3 Si5 (o1)

cI2 tP30 oC4 cI2 hP2 cF8 tI12 tP10 hP16 tP36 oP8 oC12 cP4 tI16 oF32 tP10 hP36 tI38 oP8 hP3 oP6

Im3m P42 /mnm Cmcm Im3m P63 /mmc Fd3m I4/mcm P4/mbm P63 /mmc P41 21 2 Pnma Cmcm Pm3m I4/mcm Fmmm P4/mbm P6/mmm I4/mmm Pnma P6/mmm Pmmm

0.3524, 0.35335 1.0759, 1.07589 0.28537 0.3610, 0.3615 0.3198 0.543065 0.6553 0.6988 0.7844 0.7039 0.6889 0.3672 0.4346 0.60328 0.8654 0.73299 1.0467 1.058 0.7585 0.3843 0.3869

0.86907 0.8549 0.8523 0.39004 0.7835 2.431 0.3903 0.5663 0.4069 0.6660 0.4073

U3 Si5 (o2)

oP6

Pmmm

0.3893

USi2−z

oI2

Imma

0.3953

USi2−z

tI2

I41 /amd

USi2 USi3

tI2 cP4

I41 /amd Pm3m

W ␤U ␣U W Mg Cdiamond CuAl2 U3 Si2 Mn5 Si3 Zr5 Si4 FeB ZrSi2 Cu3 Au ␤U3 Si ␣U3 Si U3 Si2 U20 Si16 C3 USi FeB AlB2 dist. AlB2 dist. AlB2 def. GdSi2 def. ThSi2 ThSi2 Cu3 Au

a

Probably oxygen stabilized [6].

a

Remarks

References

1132.3–774.8 ◦ C, at 787 ◦ C 774.8–667.7 ◦ C at 682 ◦ C <667.7 ◦ C 2231–1743 ◦ C, at 1800 ◦ C <1743 ◦ C <1414 ◦ C <2083 ± 12 ◦ C <2480 ± 20 ◦ C 2360−1925 ◦ C <2320 ± 15 ◦ C <2142 ± 15 ◦ C <1543 ± 8 ◦ C 930–759 ◦ C 762 to −153 ◦ C <−153 ◦ C, at −193 ◦ C <1665 ◦ C

<1770 ◦ C at 63 at.% Si

[8,13,14] [8,13,14] [8,13] [8,14] [8] [8] [8,14] [8,14] [9] [8,14] [8,14] [8,14] [8,15] [12,14] [12,14] [8,14] [11] [10] [16] [8] [6]

0.6717 0.4042

at ∼63 at.% Si

[6]

0.3929 1.3656

at 64 at.% Si

[6]

b

c

0.56560.56531 0.58695 0.49548 0.5061 0.5186 0.3675 0.5492 1.2826 0.3772 0.5223 1.457 0.3641

<1580 ◦ C

0.39423

1.3712

<1710 ◦ C at 65 at.% Si

[6,8]

0.3922 0.4060

1.4154

<450 ◦ C <1510 ◦ C

[8,14] [8]

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F. Weitzer et al. / Journal of Alloys and Compounds 387 (2005) 246–250

Table 2 Crystallographic data of ternary alloys Hf–Si–U, annealed at 1000 or 850 ◦ C, respectively Nominal composition U–Hf–Si (at.%)

X-ray phase analysis

Pearson symbol

Space group

Prototype

Lattice parameters (nm)

USi3 Si HfSi2

cP4 cF8 oC12

Pm3m Fd3m Cmcm

Cu3 Au Cdiamond ZrSi2

0.40407 (3) 0.54317 (6) 0.36745 (3)

USi3 HfSi2 HfSi

cP4 oC12 oP8

Pm3m Cmcm Pnma

Cu3 Au ZrSi2 FeB

0.40449 (6) 0.36750 (6) 0.68759 (6)

USi3 USi1.88 HfSi

cP4 tl12 oP8

Pm3m I41 /amd Pnma

Cu3 Au def.ThSi2 FeB

0.40560 (5) 0.39319 (4) 0.68793 (4)

HfSi (Hf,U)5 Si4 USi1.88

oP8 tP36 tl12

Pnma P41 21 2 I41 /amd

FeB Zr5 Si4 def.ThSi2

0.68944 (5) 0.70652 (4) 0.39374 (5)

20.0–30.0–50.0, 1000 ◦ C

(Hf,U)5 Si4 U3 Si5

tP36 hP3

P41 21 2 P6/mmm

Zr5 Si4 AlB2

0.71499 (7) 0.38653 (5)

33.3–16.7–50.0, 1000 ◦ C

U3 Si5 U3 Si5 USi (Hf,U)5 Si4

hP3 oP6 tl38 tP36

P6/mmm Pmmm I4/mmm P41 21 2

AlB2 dist.AlB2 USi Zr5 Si4

0.38472 (4) 0.38406 (5) 1.06094 (7) small amounts

5.0–50.0–45.0, 1000 ◦ C

(Hf,U)5 Si4 Hf3 Si2

tP36 tP10

P41 21 2 P4/mbm

Zr5 Si4 U3 Si2

11.1–44.5–44.4, 1000 ◦ C

(Hf,U)5 Si4 Hf3 Si2

tP36 tP10

P41 21 2 P4/mbm

15.0–41.0–44.0, 1000 ◦ C

(Hf,U)5 Si4 U3 Si2

tP36 tP10

42.9–14.3–42.8, 1000 ◦ C

U3 Si2 USi (Hf,U)5 Si4

50.0–10.0–40.0, 1000 ◦ C

c

U

Hf

Si

1.45637 (9)

0.36441 (3)

24.5 0.0 0.4

0.5 0.1 32.9

75.0 99.9 66.7

1.4579 (2) 0.37751 (5)

0.36446 (3) 0.52263 (6)

24.3 0.2 0.0

0.4 33.1 49.9

75.3 66.8 50.1

0.37717 (2)

1.3674 (3) 0.52291 (3)

24.4 34.2 0.0

0.6 0.7 50.0

75.0 65.1 50.0

0.40695 (7) 0.40622 (7) 2.4281 (3)

0.3 0.0 17.4

35.2 36.0 38.0

64.5 64.0 44.6

0.71209 (7) 0.69993 (3)

1.2958 (2) 0.36763 (2)

47.4 59.4

8.2 0.6

44.4 40.1

Zr5 Si4 U3 Si2

0.71189 (5) 0.70011 (4)

1.2985 (1) 0.36787 (4)

P41 21 2 P4/mbm

Zr5 Si4 U3 Si2

0.71021 (6) 0.73162 (7)

1.3014 (1) 0.39068 (5)

tP10 oP8 tP36

P4/mbm Pnma P41 21 2

U3 Si2 FeB Zr5 Si4

0.73182 (7) 0.7601 (1) traces

U3 Si2 (Hf,U)5 Si4

tP10 tP36

P4/mbm P41 21 2

U3 Si2 Zr5 Si4

0.73144 (5) small amounts

0.39160 (4)

58.3 20.7

1.6 30.6

40.1 48.7

5.0–61.6–33.4, 1000 ◦ C

Hf3 Si2 Hf2 Si

tP10 tI12

P4/mbm I4/mcm

U3 Si2 CuAl2

0.70025 (9) 0.65534 (5)

0.36740 (8) 0.51908 (4)

60.0–15.0–25.0, 850 ◦ C

␥(U,Hf) U3 Si (Hf,U)5 Si4

tP36

P41 21 2

Zr5 Si4

99.6 74.9 15.1

0.0 0.0 42.6

0.4 25.1 42.3

␣(Hf,U) Hf2 Si

tI12

I4/mcm

CuAl2

0.0 0.5

99.9 66.3

0.1 33.2

Hf2 Si ␣(Hf,U) ␥(U,Hf)

tI12 hP2 traces

I4/mcm P63 /mmc

CuAl2 Mg

0.2 1.1 95.6

66.4 98.5 4.4

33.4 0.4 0.0

10.0–10.0–80.0,

5.0–28.3–66.7,

1000 ◦ C

1000 ◦ C

15.0–25.0–60.0, 1000 ◦ C

5.0–45.0–50.0,

1000 ◦ C

33.0–52.0–15.0,

850 ◦ C

60.0–30.0–10.0, 850 ◦ C

a

solid solution ␥ (U,Hf) and all the uranium silicides up to USi2 . Due to the rather high stability of Ux Hf5−x Si4 and Hf3 Si2 , tie-lines exist with the ␥ (U,Hf) solid solution and as a consequence, no compatibility exists for the join U3 Si2 –␣ (Hf,U) and U3 Si–␣ (Hf,U), respectively. Mutual solubilities of U-silicides and Hf-silicides in alloys annealed at 1000 ◦ C are found to be very small i.e. below about 0.6 at.% except for 1.6 at.% Hf in U3 Si2 . Thus, although isotypic, the phases U3 Si2 and Hf3 Si2 show only very limited mutual solid solubility.

b

EMPA (at.%)

0.37760 (3)

0.52404 (4) 1.2896 (4) 1.3718 (8) 1.3112 (2) 0.40566 (8)

0.6718 (1)

0.39009 (8)

0.65534 (7) 0.32116 (8)

0.39097 (7) 0.5637 (1)

0.51993 (6) 0.51000 (7)

3.3. Structural chemistry Alloys along the homogeneous region Ux Hf5−x Si4 , structurally revealed closely related X-ray powder data deriving from binary Hf5 Si4 (Zr5 Si4 -type), however, sample specimens did not yield single crystals of suitable quality for single crystal X-ray investigations even after high-temperature anneal at 1400 ◦ C for 10 h. Structural details were evaluated from an alloy with nominal composition U15Hf41Si45 (in at.%, x = 1.35) via room temperature X-ray diffraction on

F. Weitzer et al. / Journal of Alloys and Compounds 387 (2005) 246–250

249

Fig. 1. System U–Hf–Si; isothermal section at 1000 ◦ C.

a Siemens D 5000 diffractometer (Cu K␣ radiation, scanning range: 13◦ ≤ 2 ≤ 120◦ ). The refinement using the Rietveld method clearly identified two coexisting phases: minor amounts of practically binary Hf3 Si2 with the U3 Si2 -type and a major phase Ux Hf5−x Si4 with the Zr5 Si4 -type. Within the error bars of the Rietveld refinement the structural formula derived was UHf4 Si4 (x = 1.0) revealing a high degree of random substitution of Hf by U in one of the hafnium sites (site 8b of space group P41 21 2). Table 3 documents the results of the Rieveld refinement for U15Hf41Si44. Interatomic distances reflect the tight atom bonding as typical for intermetallic clusters. U–U distances, 0.3596
Zr5 Si4 -type. Microprobe analyses on as cast alloys yield this phase with a uranium-content as high as U1.6 Hf3.4 Si4 almost reaching the ideal phase-composition “U2 Hf3 Si4 ”.

4. Summary Phase equilibria in the system Hf–Si–U were established in an isothermal section at 1000 and 850 ◦ C for the Si-poor region below 30 at.% Si. No ternary compound was observed; however, at 1000 ◦ C, a rather wide solid solution extends from binary Hf5 Si4 into the ternary system, Ux Hf5−x Si4 , 0 ≤ x ≤ 1.32. The crystal structure of the solid solution was characterised by X-ray powder data refinement and found to be isotypic with the Zr5 Si4 -type for the entire homogeneity region revealing a high degree of random substitution of

Table 3 Crystallographic data of the ternary solid solution Ux Hf5- x Si4 –Rietveld refinement of U15Hf41Si44a Atom

Site

x

y

z

B

Occupation

Hf1 Hf2 Hf3 U1 Si1 Si2

4a 8b 8b 8b 8b 8b

0.1682 (8) 0.1510 (6) 0.1508 (5) 0.1508 (5) 0.279 (4) 0.369 (4)

0.1682 (8) 0.0015 (6) 0.4982 (6) 0.4982 (6) 0.054 (4) 0.311 (4)

0 0.3682 (4) 0.4668 (2) 0.4668 (2) 0.196 (1) 0.314 (2)

0.37 (4) 0.22 (5) 0.38 (3) 0.38 (–) 0.36 (8) 0.37 (9)

1.0 (3) 1.0 (3) 0.5 (3) 0.5 (–) 1.0 (–) 1.0 (–)

Profile parameters: Rp = 0.118; Rexp = 0.049; Chi2 = 9.0. Major phase: Ux Hf5- x Si4 ; ordered Zr5 Si4 -type (Sc2 Re3 Si4 -type); P41 21 2 no. 92, Z = 4; origin at 21 2. Lattice parameters: a = 0.71257(5) nm, c = 1.30388(12) nm; residual values: RI = 0.057, RF = 0.031. Structural formula derived: U1 Hf4 Si4 . Minor phase: Hf3 Si2 , U3 Si2 -type, P4/mbm no. 127, Z = 2, origin at 4/m. Lattice parameters: a = 0.70015(4), c = 0.36794(2); residual values: RI = 0.068, RF = 0.042. a Crystal structure data were standardized using program structure tidy [17].

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F. Weitzer et al. / Journal of Alloys and Compounds 387 (2005) 246–250

Hf by U in one of the hafnium sites (site 8b of space group P41 21 2) consistent with a partially ordered version of the Zr5 Si4 -type in terms of the Sc2 Re3 Si4 -type. Mutual solubility of U-silicides and Hf-silicides was found to be below about 0.5 at.% except for about 1.5 at.% Hf in U3 Si2 in alloys annealed at 1000 ◦ C. The Hf–U-rich part of the diagram was also investigated at 850 ◦ C establishing the tie-lines to the low temperature compounds U3 Si and Hf2 Si. Due to the rather high stability of Ux Hf5−x Si4 and of Hf3 Si2 , tie-lines with the ␥(U,Hf) solid solution counteract compatibility for the phase pairs U3 Si2 –␣(Hf,U) and U3 Si–␣(Hf,U), respectively.

Acknowledgements This research is part of the INTAS project 234. The authors are grateful to the French–Austrian Bilateral Scientific Technical Exchange Programme “Amadee”, project 17/2003. Expertised assistance in the EMPA-XMA-measurements is due to M. Bohn from CNRS-URA 1278, IFREMER, Brest, France.

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