Syntheses and crystal structures of the quaternary uranium lanthanide oxyselenides UYb2O2Se3 and U2Ln 2O4Se3 (Ln=Pr, Sm, Gd)

Article abstract of DOI:10.1016/j.jssc.2011.12.002

Single crystals of the new uranium lanthanide oxyselenide compounds UYb₂O₂Se₃ and U₂Ln₂O ₄Se₃ (Ln=Pr, Sm, Gd) have been synthesized from an Sb ₂Se₃ flux. The structures have been determined from single-crystal X-ray diffraction data. UYb₂O₂Se ₃ is isostructural to UYb₂O₂S₃. The structure comprises layers of edge-sharing YbSe₆ octahedra and double layers of disordered (U/Ln)O₄Se₄ square antiprisms. The U₂Ln₂O₄Se₃ (Ln=Pr, Sm, Gd) compounds are isostructural to U₂Ln₂O₄S₃ (Ln=LaGd) whose structure had been deduced previously from X-ray powder diffraction data. In the structure a dodecahedron of four O atoms and four Se atoms surrounds a site primarily occupied by U and a distorted bicapped octahedron of five Se atoms and three O atoms surrounds a site primarily occupied by the lanthanide. These compounds represent the first examples of quaternary uranium oxyselenides.

Full text of DOI:10.1016/j.jssc.2011.12.002

Journal of Solid State Chemistry 186 (2012) 177–181
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Syntheses and crystal structures of the quaternary uranium lanthanide
oxyselenides UYb O Se and U Ln O Se (Ln¼Pr, Sm, Gd)
2
2
3
2
2 4
3
n
Adam D. Raw, James A. Ibers
Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208-3113, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Single crystals of the new uranium lanthanide oxyselenide compounds UYb O Se and U Ln O Se
3
2
2
3
2
2
4
Received 27 October 2011
Accepted 2 December 2011
Available online 14 December 2011
(Ln¼Pr, Sm, Gd) have been synthesized from an Sb Se flux. The structures have been determined from
2
3
single-crystal X-ray diffraction data. UYb
layers of edge-sharing YbSe octahedra and double layers of disordered (U/Ln)O
The U Ln Se Ln
(Ln¼Pr, Sm, Gd) compounds are isostructural to U
2 2 3 2 2 3
O Se is isostructural to UYb O S . The structure comprises
6
4
Se
4
square antiprisms.
Keywords:
2
2
O
4
3
2
2
O
4
S
3
(Ln¼La–Gd) whose
Uranium lanthanide oxyselenides
X-ray crystal structure
Mixed-anion compounds
Actinides
structure had been deduced previously from X-ray powder diffraction data. In the structure a
dodecahedron of four O atoms and four Se atoms surrounds a site primarily occupied by U and a
distorted bicapped octahedron of five Se atoms and three O atoms surrounds a site primarily occupied
by the lanthanide. These compounds represent the first examples of quaternary uranium oxyselenides.
Flux synthesis
&
2011 Elsevier Inc. All rights reserved.
1
. Introduction
layering determined by electron microscopy studies [7,8]. The unit
cells of the lanthanide compounds, Ce–Gd, have also been reported
by X-ray powder diffraction measurements [7]. In contrast, the
Although considerable attention has been paid to the effect of
cation substitution on structure, mixed-anion compounds repre-
sent a relatively unexplored and exciting field of solid-state
chemistry. Of particular interest is the role of multiple anions in
quaternary or higher systems where the differences in cation–
anion interactions can influence the crystal structure. Large
differences in electronegativity or bonding affinity can lead to
layered structures and result in interesting electrical and optical
properties [1]. The mixed-anion compounds of the lanthanides
with transition metals have a number of desirable electronic and
magnetic properties [2,3]. Many of these properties are depen-
dent on the anion, and changing anions within a group can greatly
influence the physical properties of the compounds [4].
By comparison the mixed-anion compounds of the actinides
have received little attention. Although there has been some interest
in uranium oxysulfides, there has been little in mixed anion
compounds with anions heavier than S [5–8]. Uranium lanthanide
oxysulfides crystallize primarily in two structures. The larger
smaller lanthanides, Gd–Lu and Y, form the compounds ULn
[5,9,10]; these were originally reported as U LnO [5,9]. In this
structure there are layers of LnS octahedra separating double layers
of U/Ln sites bound to both O and S. Previously there had been no
reported examples of quaternary uranium oxyselenides.
2 2 3
O S
2
2 3
S
8
Inorganic structures with multiple cations are common; how-
ever, structures with multiple anions are often more difficult to
synthesize. Whereas it is possible to combine most binary com-
pounds of a single anion, the same bonding preferences that lead to
interesting properties in mixed-anion compounds can also limit
reactivity. If the preference of a cation for its starting anion is too
high, then it is unlikely for a reaction to occur. Likewise, if the anions
are introduced separately, then preferred coordination can lead to a
mixture of binaries rather than to a quaternary product. Such
challenges create the need for new synthetic approaches based on
the compound being synthesized and the reactivity of the starting
materials. We present here three methods for the syntheses of
quaternary uranium lanthanide oxyselenides: the direct combina-
tion of ternary and binary starting materials, a pseudo-metathesis
synthesis, and introduction of an external oxygen source.
lanthanides, La–Gd, form the compounds U
of the Ce structure type where the lanthanide and actinide
polyhedra are connected throughout the structure [7]. The structure
is the n¼2 variant of a family of compounds U
2 2 4 3
Ln O S , an example
4 4 3
O S
2 2n nþ1
Ln2nꢀ2O S
(
Ln¼La–Gd), where n ranges from 2 to 6 [7]. The cell parameters of
the lanthanum analogues of these extended structures have been
reported by X-ray powder diffraction measurements and their
2. Experimental
2.1. Syntheses
2
38
n
U turnings (Oak Ridge National Laboratory) were powdered
according to a modification [11] of a previous literature procedure
Corresponding author. Fax: þ1 847 491 2976.
E-mail address: ibers@chem.northwestern.edu (J.A. Ibers).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.12.002

1
78
A.D. Raw, J.A. Ibers / Journal of Solid State Chemistry 186 (2012) 177–181
[
9
12]. UO
9.8%) under flowing H
conducted in carbon-coated fused-silica tubes evacuated to
2
was prepared from the reduction of UO
3
(Alfa Aesar,
were mechanically separated from the unreacted starting materi-
2
at 1173 K. All other reactions were
als and the Sb
2
Se
3
flux.
ꢀ
4
1
9
9
0
Torr. Yb (Alfa, 99.9%), Sm (Aldrich, 99.9%), Sb (Aldrich,
9.5%), Se (Cerac, 99.999%), SeO (Aldrich 99.8%), Gd (Aldrich
9.9%), and Pr 11 (Aldrich 99.999%) were used as received.
Se was synthesized from the reaction of Sb and Se in a 2:3
ratio at 1273 K for 24 h. USe was prepared by the reaction of
stoichiometric amounts of the elements at 1273 K for 72 h.
2
2
O
3
2.1.3. U
Sm
oxygen source. Sm
stoichiometric amounts of the elements at 1273 K over 1 week.
0.12 mmol (0.03 g) U, 0.09 mmol (0.048 g) Sm Se , 0.06 mmol
0.0069 g) SeO , and 0.25 mmol (0.02 g) Se were heated to 1223 K
in 100 h, held there for 168 h, then cooled to 923 K at a rate of
K/h, and then air quenched. The resultant polycrystalline
2
Sm
2
O
4
Se
was formed with the use of SeO
Se was synthesized by the reaction of
3
6
O
U
2
2
O
4
Se
3
2
as external
Sb
2
3
2
3
2
2
3
(
2
2
.1.1. UYb
UYb
starting binary and ternary compounds. UOSe was formed by
heating a mixture of equal molar amounts of UO and USe at
073 K for 1 week and then cooling the reaction to 298 K over two
2
O
2
Se
3
3
2 2 3
O Se was formed by the direct combination of the
material comprised U, Sm, Se, and O, as determined by EDX
analysis. In order to form single crystals the powder was mixed
2
2
with 0.05 g Sb
for 168 h, cooled to 973 K at 3 K/h, and then air quenched. The
resultant U Sm Se black square plates, approximately 20 wt%
yield based on U, were mechanically separated from the
unreacted starting materials and the Sb Se flux.
2 3
Se and heated to 1273 K in 24 h, held at 1273 K
1
weeks. 1.2 mmol (0.2 g) Yb and 1.7 mmol (0.137 g) Se were
reacted at 1073 K for seven days to give a polycrystalline product
that consisted mostly of YbSe with a small amount of unreacted
Se, as determined by X-ray powder diffraction measurements.
2
2
O
4
3
2
3
0
.09 mmol (0.03 g) UOSe and 0.026 g of the YbSe/Se mixture were
ground along with 0.02 mmol (0.014 g) Se and the mixture was
heated to 673 K in 4 h, then to 1323 K in 11 h, left at 1323 K for
2
.1.4. U Se
2
Gd
2
O
4
3
U
2
2 4 3
Gd O Se was formed through a pseudo-metathesis synth-
1
68 h, and then cooled 2 K/h to 1123 K before being air quenched.
esis involving the transition from O and Se coordinated exclu-
sively to a single cation to a mixed anion coordination for both
The resultant powder was shown by diffraction measurements
and the use of JADE-8 software [13] to comprise a structure
cations.
A
mixture of 0.08 mmol (0.03 g) USe
0.0137 g) Gd , and 0.28 mmol (0.022 g) Se was heated to
273 K in 24 h, held there for 168 h, and then cooled to 297 K
2
, 0.04 mmol
similar to that of UYb
Crystals of UYb Se
powder with 0.01 g Sb
over 100 h, holding it at 1223 K for 1 week, and then cooling it to
23 K at 3 K/h. The resultant black plates, approximately 30 wt%
yield based on U, could then be separated from the needles of
Sb Se and plates of UOSe.
2
O
2
S
3
along with some unreacted UOSe [10].
were formed by combining the resultant
Se and heating the mixture to 1223 K
(
1
2 3
O
2
O
2
3
2
3
in 12 h. The resultant black micrometer-sized crystals showed the
presence of U, Gd, Se, and O by EDX analysis. The crystals were
9
mixed with 0.01 g Sb
273 K for 168 h, cooled to 1073 K at 2 K/h, and then air
2 4
quenched. The resultant black square plates of U Gd O Se3,
2 3
Se and heated to 1273 K in 12 h, held at
1
2
3
2
approximate yield 20 wt% based on U, were mechanically sepa-
rated from the starting materials and Sb Se flux.
2
2.1.2. U Pr
2
O
4
Se
3
2
3
U
2
Pr
2
O
4
Se
3
was formed through a pseudo-metathesis synth-
esis involving the transition from O and Se coordinated exclu-
sively to a single cation to a mixed anion coordination for both
2.2. Structure determinations
cations.
A
mixture of 0.08 mmol (0.03 g) USe
11, and 0.15 mmol (0.012 g Se) was heated at
273 K for 168 h and then air quenched. Examination of the
2
, 0.01 mmol
(0.011 g) Pr
6
O
The structures of the above compounds were determined from
single-crystal X-ray diffraction data collected on a Bruker SMART
APEX CCD diffractometer [14,15]. Face-indexed absorption, inci-
dent beam, and decay corrections were performed with the use of
the program SADABS [16]. Each structure was solved with the
direct-methods program SHELXS and refined with the least-
squares program SHELXL [17]. The program STRUCTURE TIDY
was used to standardize the positional parameters [18].
1
resultant microcrystals on an EDX-equipped Hitachi 3400 SEM
showed the presence of U, Pr, Se, and O. The powder was mixed
with 0.02 g Sb
for 100 h, cooled to 1073 K at 2 K/h, held there for 72 h, cooled to
73 K in 36 h before cooling to 298 K in 12 h. The resulting black
2 3
Se and heated to 1273 K in 24 h, held at 1273 K
9
rectangular block crystals, approximately 5 wt% yield based on U,
Table 1
a
Crystallographic details of the uranium lanthanide oxyselenide compounds .
U
2
Pr
2
O
4
Se
3
U
2
2
Sm O
4
Se
3
U
2 2
Gd O
4
Se
3
2 2 3
UYb O Se
Space group
Pbam
Pbam
Pbam
I4/mmm
a ( A˚ )
7
1
4
.0466(3)
6.9873(2)
14.7923(4)
3.9854(1)
6.9724(4)
14.8052(7)
3.9678(2)
409.59(4)
3.8534(9)
b ( A˚ )
4.9241(5)
.0235(1)
3.8534(9)
21.610(5)
c ( A˚ )
V ( A˚ 3
423.13(3)
411.924(19)
320.88(13)
)
ꢀ
3
r
m
%
%
(g/cm
)
8.310
8.688
8.850
8.828
ꢀ
1
(cm
)
62.33
66.45
68.68
70.989
occupancy of U on 4h site
93.5(1)
93.5(1)
0.0253
0.0633
0.0232
81.5(1)
81.5(1)
0.0182
0.0436
0.0172
80.5(1)
80.5(1)
0.0222
0.0578
0.0196
occupancy of Ln on 4g site
b
R(F)
0.0317
0.0945
0.0323
2
c
w
R (F )
c
q
a
b
c
For all compounds T¼100 K, Z¼2.
2
2
R(F)¼
S
:F
o
:ꢀ:F
c o o o
2 2 2 4 1/2
:/S9F 9 for F 42s(F ).
2
2
o
o0, wꢀ1
2
2
o
2
o
Z0, wꢀ1¼
2
2
o
2 2
R
w
(F
o
)¼{
S
[w(F
o
ꢀF
c
) ]/
S
wF
o
}
. For F
¼
s
(F
); for F
s
(F
)þ(qF
o
) .

A.D. Raw, J.A. Ibers / Journal of Solid State Chemistry 186 (2012) 177–181
179
The structure of UYb
2
O
2
Se
3
was refined with 50% occupancy of
Yb1 at the U site and full Yb occupancy at the octahedral
lanthanide site, as was found for the S analogue UYb [10].
Although the U Ln Se compounds formally charge balance
, Ln , O , Se ), refinements with pure U and pure
2 2 3
O S
2
2
2ꢀ
O
4
3
4
þ
3þ
2ꢀ
(U
lanthanide positions led to unrealistic oxygen displacement
parameters and poor overall refinement quality. Far better refine-
ments were obtained by imposing charge balance but allowing
the U/Ln ratio at a given site to vary. Additional details of the
refinements can be found in Table 1 and in the Supporting
material.
3
. Results and discussion
3.1. Syntheses
The synthetic method used for UYb
2
O
2
Se
3
is similar to that
previously reported for the compound UYb O S [10]. UOSe
2 2 3
cannot be formed by the same method as UOS and this necessi-
tated an additional synthetic step [19]. The change in color from
2 2 3 2 2 3
red to black in going from UYb O S to UYb O Se is in keeping
with the decrease in band gap expected from replacement of S
with Se.
2 2 4 3
The syntheses of the U Ln O Se compounds were all con-
ducted at temperatures below that of 1673 K that was used in
syntheses of the analogous oxysulfides [7]. This was accom-
2 3
plished through the use of Sb Se as a low melting flux, which
also allowed the formation of diffraction-quality single crystals.
Several heating profiles and fluxes were tested for each composi-
tion in order to achieve the highest quality crystals, although the
most successful profiles remained similar. However, under all
synthetic conditions yields of diffraction-quality crystals were
less than 30 wt%, based on U.
3.2. Structures
UYb Se crystallizes in the space group I4/mmm with two
2
O
2
3
formula units in the unit cell. The structure consists of two cation
positions. In one, the disordered U/Yb1 cation (site symmetry
4mm) is coordinated in a distorted square antiprism with one face
consisting of four O atoms and the other of four Se atoms; in the
other the ordered Yb2 cation (site symmetry 4/mmm) is octahed-
rally coordinated by six Se atoms (Fig. 1). Selected interatomic
distances can be found in Table 2. The U–O interatomic distance
˚
˚
of 2.2966(7) A is in good agreement with that of 2.28(2) A in
˚
Fig. 1. The UYb
2
O
2
Se
3
structure: U/Yb sites in black, Yb sites in blue, O red, Se
2 2 3
UYb O S and the U–Se distance of 3.052(2) A agrees with that of
orange. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
3
.012(5) for the shorter U–Se distance in UOSe [10,19]. The
˚
˚
distances from the ordered Yb site of 2.7248(6) A and 2.777(3) A
are also in good agreement with those of other octahedrally
coordinated Yb
distances of 2.7109(7) A and four of 2.8258(8) A [20].
The U Ln Se structure shown in Fig. 2 also has two crystal-
3
þ
3
selenides, such as LaYbSe with two Yb–Se
Table 2
˚
˚
Selected interatomic distances (A) in UYb
˚
2
O
2
Se
3
.
2
2
O
4
3
(
U/Yb1)–O ꢁ 4
U/Yb1)–Se ꢁ 4
2.2966(7)
3.052(2)
2.7248(6)
2.777(3)
lographically independent cation positions. However, owing to
the similarities in coordination environment and oxophilicity of
Ln and U there is disorder at both sites. The site primarily
occupied by U (symmetry . . m) (Table 1) is surrounded by four
O and four Se atoms arranged in a dodecahedron. The site
primarily occupied by Ln (symmetry . . m) is surrounded by three
O atoms and five Se atoms in a distorted bicapped octahedron.
The relative occupancies (Table 1) conform to the expected trend
where the more oxophilic U should preferentially occupy the site
with greater oxygen coordination. The degree of disorder
increases going from Pr to Gd because the difference in ionic
(
Yb2–Se ꢁ 4
Yb2–Se ꢁ 2
Table 3 provides details on interatomic distances. Because of
the U/Ln disorder only general trends can be discussed. The U–O
interatomic distances with an average length of 2.24 A are in good
agreement with those found in the mixed anion compound
˚
3
þ
4þ
˚
˚
radii between Ln
and U
decreases.
4 3 5
UY O S , which range from 2.196 (3) A to 2.303 (6) A [21]. The

1
80
A.D. Raw, J.A. Ibers / Journal of Solid State Chemistry 186 (2012) 177–181
4. Conclusions
We present the first examples of quaternary uranium oxyse-
lenides. Unlike most previous syntheses of uranium oxychalco-
genides, the reported compounds were formed at low
temperatures, o1373 K, with the use of a low melting flux to
aid crystal growth. The three synthetic routes presented, namely,
direct combination of binaries and ternaries, a pseudo-metathesis
reaction between the binaries, and the introduction of a con-
trolled amount of oxygen from an external source, show a
flexibility that should prove useful in the syntheses of other
mixed-anion compounds.
The compound UYb
0% Yb1/U occupancy on the square antiprismatic site [10]. The
compounds U Ln Se
(Ln¼Pr, Sm, Gd) are isostructural and
possess the structure deduced earlier from X-ray powder diffrac-
tion data for U La [7]. Disorder of the U/Ln sites in
Ln Se follows the trends expected for the relative oxophili-
2 2 3 2 2 3
O Se is isostructural with UYb O S with
5
2
2
O
4
3
2
2 4 3
O S
U
2
2
O
4
3
cities of U and Ln and for the relative ionic radii.
Supporting material
The crystallographic data for UYb
Sm Se , and U Gd Se have been deposited with FIZ
Karlsruhe as CSD numbers 423731, 423730, 423729, and
23732, respectively. These data may be obtained free of charge
2
O
2
Se
3
,
2 2 4 3
U Pr O Se ,
U
2
2
O
4
3
2
2
O
4
3
4
by contacting FIZ Karlsruhe at þ497247808666 (fax) or crysda-
ta@fiz-karlsruhe.de (email).
Acknowledgments
Funding for this research was kindly provided by the U.S.
Department of Energy, Basic Energy Sciences, Chemical Sciences,
Biosciences, and Geosciences Division and Division of Materials
Science and Engineering Grant ER-15522. SEM analyses were
conducted in the Electron Probe Instrumentation Center (EPIC)
at the Northwestern University Atomic and Nanoscale Character-
ization Experimental (NUANCE) Center, supported by NSF-NSEC,
NSF-MRSEC, Keck Foundation, the State of Illinois, and North-
western University. Single-crystal data were collected at the
IMSERC X-ray Facility at Northwestern University, supported by
the International Institute of Nanotechnology (IIN). Powder X-ray
diffraction data were collected at the J. B. Cohen X-Ray Diffraction
Facility supported by the MRSEC program of the NSF (DMR-
Fig. 2. The U
2
Ln
2
O
4
Se
3
(Ln¼Pr, Sm, Gd) structure: primarily U sites are in black,
primarily Ln sites are in blue, O red, Se orange. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
Table 3
Selected interatomic distances ( A˚ ) in U
2 2 4 3
Ln O Se
(Ln¼Pr, Sm, Gd).
a
Distance
U
2
2
Pr O
4
Se
3
U
2
Sm Se U Gd O Se
2
O
4
3
2 2 4 3
(
(
(
(
(
(
(
(
(
(
(
Ln/U)–O1
2.367(4)
2.425(3)
3.0486(3)
3.0694(5)
3.1104(5)
2.162(2)
2.245(4)
2.420(5)
2.9871(7)
3.1259(2)
3.2898(7)
2.324(4)
2.392(2)
3.0161(3)
3.0392(5)
3.0925(5)
2.144(1)
2.231(4)
2.398(4)
2.9604(6)
3.1053(2)
3.2541(7)
2.298(5)
2.367(3)
3.0143(4)
3.0412(6)
3.0912(7)
2.141(2)
2.246(6)
2.417(6)
2.9547(8)
3.0873(3)
3.2562(8)
0520513) at the Materials Research Center of Northwestern
Ln/U)–O2 ꢁ 2
Ln/U)–Se1
University.
Ln/U)–Se2 ꢁ 2
Ln/U)–Se2 ꢁ 2
U/Ln)–O1 ꢁ 2
U/Ln)–O2
Appendix A. Supplementary Information
U/Ln)–O2
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.12.002.
U/Ln)–Se2
U/Ln)–Se1 ꢁ 2
U/Ln)–Se2
a
Sites are denoted with the element primarily occupying the site followed by
the less prevalent element. Ln/U corresponds to the 4g site, U/Ln to the 4h site.
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[
[
[
[
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˚
˚
from 3.2562(8) A in U Gd O Se to 3.2898(7) A in U Pr O Se .
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A.D. Raw, J.A. Ibers / Journal of Solid State Chemistry 186 (2012) 177–181
181
[
[
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