Low-temperature synthesis of uranium tetraboride by solid-state metathesis reactions

Article abstract of DOI:10.1021/ic015607a

A novel synthesis of uranium tetraboride (UB₄) by solid-state metathesis reaction is demonstrated. This approach significantly lowers the temperature required to synthesize this material to ≤850 °C. When UCl₄ is reacted with 2 equiv of MgB₂ at 850 °C, crystalline UB4 is formed. Powder X-ray diffraction and ICP-AES data support the reduction of UCl₄ to UCl₃ as the initial step in the reaction. The UB4 product is purified by washing with water.

Full text of DOI:10.1021/ic015607a

Inorg. Chem. 2002, 41, 2316−2318
Low-Temperature Synthesis of Uranium Tetraboride by Solid-State
Metathesis Reactions
,†
Anthony J. Lupinetti,* Julie L. Fife,‡ Eduardo Garcia,† Peter K. Dorhout,§ and Kent D. Abney†
Nuclear Materials Technology DiVision (NMT-2), Mail Stop E511, and Chemistry DiVision,
Isotope and Nuclear Chemistry Group (C-INC), Mail Stop J514, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545, and Department of Chemistry, Colorado State UniVersity,
Fort Collins, Colorado 80523
Received October 25, 2001
A novel synthesis of uranium tetraboride (UB ) by solid-state
metathesis reaction is demonstrated. This approach significantly
lowers the temperature required to synthesize this material to e850
been synthesized at lower temperatures but require the use
4
of molten plutonium (800 °C), which is extremely corrosive,4
or the use of PuH3 (900 °C).5 Plutonium borides are known
to be refractory, but other properties such as chemical
behavior and stability have not been evaluated. In contrast,
°C. When UCl4 is reacted with 2 equiv of MgB at 850 °C,
2
crystalline UB is formed. Powder X-ray diffraction and ICP-AES
4
6,7
many transition metal and lanthanide borides, such as ZrB2
data support the reduction of UCl4 to UCl3 as the initial step in the
and LaB6,8 have been extensively studied and have been used
as refractory materials and corrosion-resistant coatings. It is
therefore expected that some actinide-boride phases will also
be corrosion resistant.
reaction. The UB product is purified by washing with water.
4
The Department of Energy strategy for plutonium has
shifted focus within the past decade from production and
recycling to stabilization and disposal. This change results
from the reduction in the nuclear stockpile and the ac-
companying need for plutonium disposition. Today’s strategy
uses plutonium oxide as the optimum intermediate (i.e., <50-
year) storage form, even though Haschke reported that PuO2
slowly reacts with moisture to form hydrogen, causing
numerous safety and storage concerns.1 Our work focuses
on the preparation of actinide borides as an alternative
intermediate storage form for actinide elements. These
materials would offer the prospect of highly refractory boride
phases of plutonium and other actinides that are stable to
moisture, are highly compact, and will have many of the
desired nonproliferation physical characteristics (e.g., chemi-
cal inertness and stability to radiolytic decay).
During the past two decades significant advances have
been made in the low-temperature synthesis of highly
refractory materials.9,10 New methods, such as molecular
precursors, preceramic polymers, chemical vapor deposition,
sol-gel and hydrothermal syntheses, low-temperature molten
salts, self-propagating high-temperature synthesis (SHS), and
solid-state metathesis reactions (SSM), virtually eliminate
the problems associated with slow solid-state diffusion by
mixing the constituents of the ceramic at a molecular level.
SHS and SSM methods have been used successfully to
synthesize transition metal borides, nitrides, and oxides and
actinide oxides and nitrides at low to moderate tempera-
tures.11,12,13 The key to low-temperature synthesis is identi-
fication of suitable precursors that lead to ceramic materials
having the desired physical characteristics described above.
Our approach focuses on the use of solid-state metathesis
reactions for the production of new actinide boride materials,
specifically uranium borides as proof of concept for pluto-
The area of actinide borides is underdeveloped, in part
due to the high temperatures required to produce these
materials.2 The total list of known binary thorium- and
uranium-boride phases includes only ThB4, ThB6, ThB66,
UB2, UB4, and UB12, with little information reported on their
chemical properties.3 In contrast, plutonium borides have
(3) Katz, J. J.; Seaborg, G. T.; Morss, L. R. The Chemistry of the Actinide
Elements; Chapman and Hall: New York, 1986; pp 56, 280, 317 for
Th, U, and Pu, respectively.
(4) Eick, H. A. Inorg. Chem. 1965, 4, 1237.
(5) Skavdahl, R. E.; Chikilla, T. D.; McNeilly, C. E. Trans. Am. Nucl.
Soc. 1964, 7, 403.
* Author to whom correspondence should be addressed. E-mail:
lupi@lanl.gov.
(6) Su, K.; Sneddon, L. G. Chem. Mater. 1993, 5, 1659.
(7) Rao, L.; Gillan, G.; Kaner, R. B. J. Mater. Res. 1995, 10, 353.
(8) Kher, S. S.; Spencer, J. T. J. Phys. Chem. Solids 1998, 59, 1343.
(9) Wynne, K. J.; Rice, R. W. Annu. ReV. Mater. Sci. 1984, 14, 297.
(10) Rice, R. W. Am. Ceram. Soc. Bull. 1983, 62, 889.
(11) Gillan, E. G.; Kaner, R. B. Chem. Mater. 1996, 8, 333.
(12) Parkin, I. P.; Fitzmaurice, J. C. J. Mater. Sci. Lett. 1994, 13, 1185.
(13) Parkin, I. P.; Fitzmaurice, J. C. New J. Chem. 1994, 18, 825.
† Nuclear Materials Technology Division (NMT-2), Mail Stop E511, Los
Alamos National Laboratory.
‡ Chemistry Division, Isotope and Nuclear Chemistry Group (C-INC),
Mail Stop J514, Los Alamos National Laboratory.
§ Colorado State University.
(1) Haschke, J. M.; Allen, T. H.; Morales, L. A. Science 2000, 287, 285.
(2) Eick, H. A.; Mulford, R. N. R. J. Inorg. Nucl. Chem. 1969, 31, 371.
2316 Inorganic Chemistry, Vol. 41, No. 9, 2002
10.1021/ic015607a CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/05/2002

COMMUNICATION
Figure 1. Powder X-ray diffraction pattern for the water-washed solid
product from the reaction of UCl4 + 2MgB2 for 1 day at 850 °C (top) and
a powder X-ray diffraction pattern of UB4 calculated from single-crystal
data (bottom).
Figure 2. Powder X-ray diffraction patterns of the crude products of the
reaction of UCl4 with (a) 0.5 MgB2, (b) 1 MgB2, (c) 2 MgB2, and (d) 2.5
MgB2 at 850 °C for 1 day. The powder patterns are normalized to a constant
MgCl2 peak height. The x-axis corresponds to pattern a; patterns b-d are
offset by +0.5° in 2Θ from the pattern above for clarity. All powder patterns
were taken with the sample under a dry, inert atmosphere.
nium boride materials. Typically, 50-250 mg of UCl4 and
2 equiv of MgB2 were reacted in a vacuum-sealed 1.5 mL
quartz tube and heated to 850 °C for 1 day:14
UCl4 + 2MgB2 850 °C, 1 day8 UB4 + 2MgCl2
15
20 mL of 3.2 M HNO3. Any unreacted UCl4, as well as UCl3
and MgCl2 produced, will dissolve in the water; unreacted
MgB2 and UB4 produced are insoluble in water and will
remain in the solid. The resulting samples were analyzed
for uranium, magnesium, and boron content and compared
with the initial amounts present in the reaction. At 0.5 equiv
of MgB2, almost all of the uranium (96%) and magnesium
(83%) is present in the filtrate, which is consistent with the
production of soluble UCl3 and MgCl2 as proposed above.
This is also consistent with the lack of UB4 and MgB2 in
the pXRD pattern of this sample (Figure 2a). The lack of
significant quantities of uranium (3%) and magnesium (1%)
in the x ) 0.5 solid is consistent with the formation of
elemental boron as the only boron-containing product in the
reduction reaction proposed above (i.e., no MgB2 remains
and no UB4 has yet been formed).
With increasing amounts of MgB2 (i.e., x ) 0.5), the
amount of uranium in the filtrate is expected to decrease
and the amount in the solid increase due to the conversion
of soluble UCl3 into insoluble UB4. Indeed, at x ) 2.5, 90%
of the uranium is in the solid product. If all of the uranium
in the solid samples is in the form of UB4, the elemental
boron in x ) 0.5 must be converted to UB4 as x approaches
2.5 (Figure 2d). For all values of x, less than 4% of the
magnesium is in the solid product; thus, all of the magnesium
is dissolved in the filtrate (presumably as MgCl2). This is
consistent with the observation that no magnesium uranium
boride ternary phases are present in the pXRD patterns.
Experiments were also conducted to determine the effect
of time (1-5 days) and temperature (600-850 °C) on the
reaction of UCl4 with 2 equiv of MgB2. In all cases, the
products were the same: UB4, UCl3, and MgCl2. The major
difference in these reactions is the crystallinity of UB4; as
might be expected, longer reaction times and higher tem-
peratures yield a more crystalline product.
After reaction, the tubes were opened and the contents
washed in water to remove soluble salts. The sample was
then rinsed with ethanol and dried. In Figure 1, a powder
X-ray diffraction (pXRD) pattern of the washed product is
compared with a UB4 pattern calculated from single-crystal
data. From the similarity of the experimental pattern and the
calculated pattern, it is clear that UB4 is the only crystalline
phase readily identified in the remaining solid.
The reaction of UCl4 with substoichiometric amounts of
MgB2 produces a mixture of crystalline phases composed
of UB4, UCl3, and MgCl2. X-ray powder diffraction patterns
of the reactions of UCl4 + xMgB2 (x ) 0.5, 1, 2, 2.5) for
reactions conducted at 850 °C for 1 day are shown in Figure
2. As the amount of MgB2 in the reaction increases, the
amount of UCl3 remaining in the reaction decreases while
the amount of UB4 produced increases. The presence of UCl3
in the reaction mixture was also confirmed by single-crystal
X-ray diffraction. This evidence suggests that MgB2 acts as
a reducing agent as well as a boron source:
UCl4 + 0.5MgB2 f UCl3 + 0.5MgCl2 + B
To support this hypothesized reduction of UCl4 to UCl3
and in order to better understand the nature of the reaction
products, two sets of samples were taken for ICP-AES
analysis. First, the reaction products were washed with 40
mL of distilled water and the filtrate was collected. Sepa-
rately, the water-insoluble, solid products were dissolved in
(14) Note: Although depleted uranium (238U) was used in these studies,
care must still be taken in the handling and disposal of starting
materials and products as they are still R particle emitters. Sealed
ampules were run in furnaces in a fume hood to prevent contamination
issues in the case of a tube rupture.
(15) ∆Hrxn ) -58 kcal/mol.
Inorganic Chemistry, Vol. 41, No. 9, 2002 2317

COMMUNICATION
In this paper, we have shown that UCl4 can be converted
in high yield (90+% by ICP-AES based on total uranium)
to the refractory boride UB4 by solid-state metathesis
methods. No other uranium boride compounds were observed
in these studies. These methods allow for the formation of
crystalline boride materials at low temperatures (i.e., 600-
850 °C) using quartz reaction vessels. We plan to combine
metathesis reactions and low-temperature molten salt fluxes
in future studies to probe the reaction chemistry of other
actinide halides and boron-containing materials.
Oldham for useful discussions and assistance during the
course of these experiments and Dr. Brian Scott for the use
of his powder X-ray diffractometer. The authors would also
like to thank Ms. Brandy Duran for running the ICP-AES
samples. This work was performed under the auspices of
the Department of Energy, Los Alamos National Laboratory
Nuclear Weapons Program Office. Los Alamos National
Laboratory is operated by the University of California for
the U.S. Department of Energy under Contract
W-7405-ENG-36.
Acknowledgment. The authors thank Mr. Ryan Hess, Mr.
Antonio Maestas, Dr. Warren Oldham, and Dr. Susan
IC015607A
2318 Inorganic Chemistry, Vol. 41, No. 9, 2002