Crystal structures of Rb2U2O7 and Rb 8U9O31, a new layered rubidium uranate

Article abstract of DOI:10.1016/j.jssc.2005.07.017

Two alkali metal uranates Rb₂U₂O₇ and Rb₈U₉O₃₁ have been synthesized by solid state reaction at high temperature and their crystal structures determined from single crystal X-ray diffraction data, collected with a three circles Brucker SMART diffractometer equipped by Mo(Kα) radiation and a charge-coupled device (CCD) detector. Their structures were solved using direct methods and Fourier difference techniques and refined by a least-square method on the basis of F² for all unique reflections, with R1=0.043 for 53 parameters and 746 independent reflections with I≥2σ(I) for Rb₂U ₂O₇, monoclinic symmetry, space group P2₁/c, a=7.323(2)A, b=8.004(3)A, c=6.950(2)A, β=108.81(1)°, ρmes=6.56(3)g/cm3, ρcal=6.54(2)g/cm3, Z=2 and R1=0.036 for 141 parameters and 2065 independent reflections with I≥2σ(I) for Rb ₈U₉O₃₁, orthorhombic, space group Pbna, a=6.9925(9)A, b=14.288(2)A, c=34.062(5)A, ρmes=6.47(3)g/ cm3, ρcal=6.48(2)g/cm3, Z=4. The Rb₂U₂O₇ structure presents a strong analogy with that of K₂U ₂O₇ and can be described by layers of distorted UO ₂(O₄) octahedra built from dimeric units of edge shared octahedra further linked together by opposite corners. In Rb₈U ₉O₃₁ puckered layers are formed by the association of two different uranium polyhedra, pentagonal bipyramids and distorted octahedra. The structure of Rb₈U₉O₃₁ is built from a regular succession of ∞1[U4O14]4- infinite ribbons similar to those observed in diuranates M₂U₂O₇ (MK, Rb) and infinite three polyhedra wide ribbons ∞1[U5O21]12-, to create an original undulated sheets ∞2[U9O31]8-. For both compounds Rb⁺ ions occupy the interlayer space and exhibit comparable mobility with conductivity measurements indicating an Arrhenius-type behavior.

Full text of DOI:10.1016/j.jssc.2005.07.017

ARTICLE IN PRESS
Journal of Solid State Chemistry 178 (2005) 3218–3232
www.elsevier.com/locate/jssc
Crystal structures of Rb U O and Rb U O , a new layered
2
2 7
8 9 31
rubidium uranate
Ã
S. Yagoubi, S. Obbade , C. Dion, F. Abraham
Laboratoire de Cristallochimie et Physicochimie du Solide, UMR CNRS 8012, ENSCL-USTL, B.P. 90108, 59652 Villeneuve d’Ascq Cedex, France
Received 30 May 2005; received in revised form 18 July 2005; accepted 19 July 2005
Available online 16 September 2005
Abstract
Two alkali metal uranates Rb U O and Rb U O have been synthesized by solid state reaction at high temperature and their
31
2
2
7
8
9
crystal structures determined from single crystal X-ray diffraction data, collected with a three circles Brucker SMART
diffractometer equipped by Mo(Ka) radiation and a charge-coupled device (CCD) detector. Their structures were solved using direct
methods and Fourier difference techniques and refined by a least-square method on the basis of F for all unique reflections, with
2
R1 ¼ 0:043 for 53 parameters and 746 independent reflections with IX2sðIÞ for Rb U O , monoclinic symmetry, space group P2 /c,
2
3
2
7
1
(
(
(
3
a ¼ 7:323ð2Þ A, b ¼ 8:004ð3Þ A, c ¼ 6:950ð2Þ A, b ¼ 108:81ð1Þ1, r ¼ 6:56ð3Þ g=cm , r ¼ 6:54ð2Þ g=cm , Z ¼ 2 and R1 ¼ 0:036 for
mes
cal
(
1
41 parameters and 2065 independent reflections with IX2sðIÞ for Rb
8
9
U O
31, orthorhombic, space group Pbna, a ¼ 6:9925ð9Þ A,
(
(
3
3
b ¼ 14:288ð2Þ A, c ¼ 34:062ð5Þ A, r ¼ 6:47ð3Þ g=cm , r ¼ 6:48ð2Þ g=cm , Z ¼ 4.
mes
cal
The Rb
2
U
2
O
7
structure presents a strong analogy with that of K
octahedra built from dimeric units of edge shared octahedra further linked together by opposite corners. In Rb
2
2
U O
7
and can be described by layers of distorted UO
2 4
(O )
8
9
U O
31 puckered
layers are formed by the association of two different uranium polyhedra, pentagonal bipyramids and distorted octahedra. The
31 is built from a regular succession of 1 ½U4O14Š infinite ribbons similar to those observed in diuranates
4À
structure of Rb
8
U
9
O
1
1
12À
8À
.
M
2
2
U O
7
(MQK, Rb) and infinite three polyhedra wide ribbons ½U5O21Š , to create an original undulated sheets 2 ½U9O31Š
1
1
+
For both compounds Rb ions occupy the interlayer space and exhibit comparable mobility with conductivity measurements
indicating an Arrhenius-type behavior.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Rubidium uranate; Solid state synthesis; Crystal structure refinement; Layered compound; Cationic conductivity
1
. Introduction
given by Griffiths and Volkovich in an extensive review
1], where the alkali metal uranates (VI) have been
[
The uranates in general and particularly those of alkali
examined in details and their structural, physical,
thermodynamic and spectroscopic properties were con-
sidered. However, an important first report of Van
Egmond [2], containing at the same time a critical
assessment of previous powder works and the results of
many structural studies carried out mainly by powder
X-ray diffraction data, give a good synthetic view of these
compounds. A particular attention was devoted to
cesium uranates in reason of their potential formation
in nuclear reactors by reaction of UO , resulting of UO
metals form a very important family of uranium
compounds. If one considers only the alkali metal
uranates containing uranium in its highest oxidation
6
+
state U , frequently observed, several compounds are
described in the literature and can be definite by the
molar ratio n ¼ UO /M O (M ¼ alkali metal), where n
3
2
presents numerous values in a large range from 0.33 to 15
(
Table 1). The majority of these compounds were recently
3
2
Ã
oxidation, with cesium obtained by fission reactions [2–5].
Following these studies, it was observed that the most
known uranates are the monouranates M UO , which
Corresponding author. Fax: +33 3 20 43 68 14.
E-mail addresses: said.obbade@ensc-lille.fr,
obbade@ensc-lille.fr (S. Obbade).
2
4
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.07.017

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3219
Table 1
The established alkali metal uranate compounds
UO
3
/M
2
O ratio
Formulae
Li
Li
Na
K
Rb
Cs
1
7
5
4
3
2
2
2
1
1
0
0
5
M
2
M
2
M
2
M
2
M
2
M
4
M
8
M
2
M
6
M
2
M
4
M
6
U
U
U
U
U
U
U
U
U
15
O
46
Cs
Cs
Cs
Cs
2
2
2
2
U
U
U
U
15
O
46
7
5
4
3
5
9
2
4
O
22
K
2
U
U
7
O
O
22
13
Rb
2
2
U
7
4
O
22
13
7
5
4
O
22
O
O
O
O
O
O
O
16
13
10
17
31
7
O
O
16
K
2
4
Rb
U
O
13
2
3 10
U O
,5
,25
Cs
Cs
Cs
4
2
2
U
5
2
O
17
7
a
Rb
Rb
8
U
U
9
O
O
31
a
Na
2
U
2
O
7
K
2
K
9
K
2
K
4
U
U
2
O
O
7
2
2
7
U
O
,33
15
6
22.5
UO
UO
UO
4
5
6
Li
2
Li
4
Li
6
Li
4
UO
UO
UO
4
5
6
Na
Na
2
UO
UO
4
UO
UO
4
5
Rb
2
UO
4
UO
4
,5
4
5
,33
4 2 6
Li K UO
2 6
K UO
a
This study.
exist for all the alkali metals, and the diuranates
M U O [2–20], and that the increase of alkali metal
character of these structures confers them remarkable
properties in the domains of insertion, exchanges and
electrical ionic conductivity, in relation with the cationic
mobility of the alkali ions located in the space between
layers. Thus, in the formed sheets, the linear uranyl ion
2
2
7
atoms size supports the formation of uranates with
higher n values. Thus, M U O , M U O and
M U O are obtained only for heavy alkali metals
2
4
12
2
4
13
2
7
22
2
+
M ¼ K, Rb, Cs [3,5–8,11,21,22] and only the cesium
compound Cs U O [3,5] illustrates the highest n value
found in a polyuranate after invalidation of Cs U O
[OQUQO] is surrounded in the equatorial plane by
four, five or more rarely six oxygen atoms to form
octahedra (UO )O , pentagonal bipyramids (UO )O or
2
15 46
2
16 49
2
4
2
5
[
are observed such as Cs U O [3,5–7] and Cs U O [3]
3]. Moreover, for cesium element, specific compounds
hexagonal bipyramids (UO )O , respectively. The ur-
2 6
anyl polyhedra are associated in equatorial planes by
sharing corners and/or edges of oxygen atoms to form
varied types of sheets. The crystal structure stability is
assured by interlayer alkali metal ions. For alkali metal
monouranate families M2m+2UO4+m with m ¼ 0, 1 and
2 corresponding to M UO , M UO and M UO ,
4
5
17
2
5
16
which do not exist for the other alkali metals. Inversely,
small alkali metals generate basic uranates with no1,
for example n ¼ 0.50 is illustrated by M UO with
4
5
M ¼ Li, Na, K and n ¼ 0.33 is represented by Li UO
6
6
and Li K UO [23–26].
4
2
6
2
4
4
5
6
6
Concerning rubidium uranates, only the five com-
pounds Rb UO , Rb U O , Rb U O , Rb U O , and
Rb U O have been found to exist and the indexing of
powder X-ray diffraction data evidenced that these
compounds are probably isostructural to the corre-
sponding potassium uranates [9,11,12,17,22].
respectively, different structural types depending on
the uranium polyhedra polymerisation degree have been
reported [1,5,8,11–14,25,26]. Thus, for the monouranate
family M UO , the crystal structure is obtained by
2
4
2
2
7
2
4
12
2
4
13
2
7
22
2
4
association of U(O )O octahedra sharing all equatorial
2
4
corners to build infinite sheets 2 ½ðUO2ÞO2Š , except in
2À
1
Alkali metal uranate compounds are generally ob-
tained by high temperature synthesis and for each
uranates family the thermal stability decreases with
alkali metal size [3–5,9,10,22,27,28]. Thus, the diura-
nates are generally considered to present the highest
thermal stability and remain stable beyond 1300 1C.
On the other hand, if lithium monouranate Li UO
a-Na UO where the octahedra are associated by two
2
4
opposite equatorial edges to form infinite chains
2À
1
½ðUO2ÞO2Š , however, in M UO compounds, a
1
4
5
decondensation of uranyl polyhedra is observed and
the U(O )O octahedra share only two opposites corners
2
4
1
4À
to form infinite chains ½ðUO ÞO Š . Finally indepen-
1
2
3
6
À
dent octahedra (UO6) are found in ultimate term of
decondensation in the structure of Li UO . When the
2
4
does not change at 1300 1C, the corresponding
sodium and potassium compounds decompose in air at
6
6
molar ratio n is higher than 1, as in polyuranates, an
increase of uranyl polyhedra condensation is observed
to give denser sheets. For example, in diuranates
M U O (n ¼ 2), the U(O )O octahedra are sharing
1
200–1300 1C to give a mixture of diuranate and alkali
metal oxide. However, the generally high melting points
explain the difficulties to obtain single crystals, thus,
numerous structural studies have been undertaken by
Rietveld method using powder X-ray diffraction data
and more recently neutron diffraction.
2
2
7
2
4
an equatorial edge to create dimeric units U O linked
2
10
together by different ways to form various layers. For
higher n values, the condensation is still more accen-
tuated and uranium atoms adopt two different environ-
ments, (UO )O pentagonal bipyramids and (UO )O
4
Most of the alkali metal uranates structures are built
from uranium–oxygen layers. Thus, the bi-dimensional
2
5
2

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3220
distorted octahedra, which are associated in complex
layered structures.
at different temperatures between 600 and 1200 1C with
intermediate grindings. For both alkali metals no
M U O was observed in the 600–1200 1C temperature
Owing to the difficulty to obtain single crystals of
alkali metal uranates, for most compounds only X-ray
powder diffraction studies were carried out until now.
Thus, for Rb U O , only one crystal structure determi-
8
9
31
range. For the potassium preparation, the X-ray powder
analysis of the sample at different temperatures evi-
denced the formation of two known phases K U O and
2
2
7
2
2
7
nation using powder X-ray diffraction data and Rietveld
method refinement has been reported by Van Egmond
K U O , which are stable until 1200 1C. For the cesium
2
4
13
sample, the formation of Cs U O accompanied with
cesium oxide Cs O was confirmed by powder X-ray
4
5
17
[
2]. In the same way, different unit cell parameters have
2
been reported for monoclinic [3,9], rhombohedral [12]
and orthorhombic symmetries [29].
diffraction.
The densities were measured with an automated
Micromeritics Accupy 1330 helium pycnometer using a
This work is dedicated to the crystal structure,
thermal, electrical and spectroscopic investigations of
two rubidium uranates, the new phase Rb U O and
3
1-cm cell.
For both compounds, differential thermal analysis
(DTA) was carried out in air on a Setaram 92-1600
thermal analyzer in the temperature range 20–1300 1C
with heating and cooling rate 1.5 1C/min using platinum
crucibles.
8
9
31
the diuranate Rb U O . Both compounds were synthe-
2
2
7
sized by solid-state reaction at high temperature and
their structures were determined using single crystal
X-ray diffraction data. The results concerning the
Rb U O structure are compared with the potassium
For conductivity measurements, powder samples were
palletized at room temperature using a conventional
cold press and then sintered at 800 1C for 15 h. For each
sample, gold electrodes were sputtered on both flat faces
and measurements were done by impedance spectro-
2
2
7
and cesium diuranates, previously established from
powder X-ray diffraction data. Conductivity and infra-
red measurements are reported for both compounds.
6
metry in the frequency range 1–10 Hz using a Schlum-
2
. Experimental
berger 1170 frequency response analyzer. Each set of
values was recorded in temperature range 200–600 1C
with a step of 20 1C and for every measurement the
temperature was stabilized during 1 h.
To characterize the vibration modes of uranium
atoms in both compounds, the infrared spectra were
recorded using the KBr dispersion technique (1 mg of
sample in 120 mg KBr) and a Bruker Vector 22 Fourier
2
.1. Powder samples preparation and characterization
Polycrystalline samples of Rb U O and Rb U O
31
2
2
7
8
9
were prepared by conventional solid-state reactions at
high temperature, using pure initial materials RbNO3
(
Adrich) and U O (Prolabo) as reagents. Mixture of
3 8
starting materials in the appropriate stoichiometries
were heated at 750 1C in air for 2 days with intermediate
grindings for Rb U O . For Rb U O , a thermal
transform infrared spectrometer, which covers the range
À1
400–4000 cm
.
2
2
7
8
9
31
treatment at 900 1C during 3 weeks has been necessary
to obtain a pure sample.
2.2. Crystal growth
The end of the synthesis process for each sample
was controlled by powder X-ray diffraction using a
Guinier–De Wolff focusing camera and CuKa radiation.
Powder X-ray diffraction data used for cell para-
meters refinement were recorded on a Siemens D5000 y/
The single crystals of Rb U O and Rb U O were
31
2
2
7
8
9
obtained during studies relating to the ternary system
Rb O–U O –MoO , starting from a molar ratio Mo/
2
3
8
3
U ¼ 2/1, in an excess of rubidium carbonate Rb CO
2
3
(10 mole for 2 mole of MoO ). In a first experiment, the
3
2
y diffractometer, at room temperature, using Brag-
mixture was heated in a platinum crucible at 940 1C for
3 h and then slowly cooled to room temperature at
0.2 1C/min. The product of the reaction was washed
with water to remove Rb CO excess and filtered, giving
g–Brentano geometry, with a back-monochromatized
CuKa radiation. The diffraction pattern was scanned
over the angular range 05–801(2y) in step of 0.031(2y)
and a counting time of 15 s per step. The unit cell
parameters were refined by a least-squares procedure
from the indexed powder diffraction pattern, using
Rietveld method.
isotypic com-
pounds with other alkali metals by solid state reactions
have been investigated from mixtures of MNO3
2
3
a two-phases sample containing a small quantity of
orange single crystals of Rb U O and a majority of
8
9
31
twinned yellow crystals corresponding to Rb U MoO
10
2
2
which is isotypic to the recently characterized
K U MoO [30]. The second crystallization experiment
Synthesis attempts of Rb U O
8
9
31
2
2
10
carried out on a mixture with the same molar ratio 2/1
heated at 1200 1C, then slowly cooled (0.3 1C/min),
allowed the growth of orange crystals of rubidium
diuranate Rb U O , once again accompanied by yellow
(MQK, Cs) and U O in molar ratio 8/3, correspond-
ing to M U O . The mixtures were slowly heated
3
8
8
9
31
2
2
7
(
11/min), in platinum crucibles, and maintained 3 days
crystals of Rb U MoO .
2 2 10

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3221
2
.3. Crystal structures determination
precise faces indexing was then applied using XPREP
program of the SHELXTL package [32] followed by
SADABS [33] for additional corrections. The crystal
structures were solved by a combination of direct
methods and difference Fourier syntheses, and refined
Appropriate single crystals of Rb U O and
2
2
7
Rb U O with dimensions 0.180 Â 0.080 Â 0.054 mm
8
9
31
and 0.070 Â 0.020 Â 0.030 mm, respectively, were se-
lected for X-ray diffraction analyses. Data collec-
tions were performed at 293(2) K on a Bruker X-ray
diffractometer with graphite monochromated MoKa
2
by full matrix least squares against F , using SHELXS
program [34]. The heavy atoms, Uand Rb, were
localized from the direct methods. The positions of
oxygen atoms were deduced from subsequent refine-
ments and difference Fourier syntheses using the
SHELXL option of the SHELXTL software [32]. For
both compounds, the summaries of the crystal data and
details concerning the intensity data collections and
structure refinements are given in Table 2.
(
radiation (l ¼ 0:71073 A) and equipped with a SMART
charge-coupled device (CCD) detector, with a crystal
to detector distance of 5.0 cm. For both compounds,
SMART was also used to determine the preliminary cell
parameters from 120 frames obtained with an exposure
time of 20 s per frame. For each crystal structure
determination, a total of 1800 individual frames were
collected using a o-scan technique with an o rotation of
For both compounds, the bond valence sums of the
atoms have been calculated using parameters given by
+
(
0
.31 and an acquisition time of 40 s per frame. Integral
Brese and O’Keeffe’s for Rb –O (Rij ¼ 2:260 A and
(
intensities were extracted from the collected frames with
the Bruker Saint Plus 6.02 software package [31] using a
narrow-frame integration algorithm. For each com-
pound, a Gaussian-type absorption correction based on
b ¼ 0:37 A) [35] and coordination-independent para-
6
+
(
meters given by Burns et al. for U –O (Rij ¼ 2:051 A
(
and b ¼ 0:519 A) [36].
Table 2
Crystal data, intensity collection and structure refinement parameters
3. Results and discussion
for Rb
2
U
2
O
7
and Rb
8
U
O
9 31
3.1. Structure solution
Rb
2 2
U O
7
8 9 31
Rb U O
Systematic absences of reflections were consistent
with P21=c and Pbna centrosymmetric space groups for
Rb U O and Rb U O , respectively. The indexed
Crystal data
Crystal symmetry
Space group
Monoclinic
P2 /c
Orthorhombic
Pbna
2
2
7
8
9
31
1
Unit cell (from single
crystal)
a ¼ 7:323ð2Þ A(
a ¼ 6:9925ð9Þ A(
powder patterns and refined cell parameters are reported
in the Tables 3 and 4 for Rb U O and Rb U O ,
2
2
7
8
9
31
(
(
b ¼ 8:004ð3Þ A
b ¼ 14:2884ð19Þ A
respectively. The reliability of the unit cell refinement
and indexing reflections is indicated by the conventional
figures of merit F , defined by Smith and Snyder [37],
(
(
c ¼ 6:950ð2Þ A
c ¼ 34:062ð5Þ A
b ¼ 108:8ð1Þ1
˚
3403.2(8) A
3
(
3
20
V ¼ 385:6ð2Þ A
Z
2
4
r
r
F20 ¼ 36 (0.0155; 36) and 44 (0.0115; 46) for Rb U O
2
2
7
À3
À3
À3
Calculated density
Measured density
Color
r
r
cal ¼ 6.54(2) g cm
cal ¼ 6.48(2) g cm
À3
and Rb U O , respectively. A good agreement is
8
9
31
mes ¼ 6.56(3) g cm
mes ¼ 6.47(3) g cm
observed between calculated and measured densities
for both compounds, with two formulas per unit
Orange
Orange
Data collection
Temperature (K)
Radiation Mo(Ka)
Scan mode
293(2)
˚
0.71073 A
293(2)
˚
0.71073 A
À3
À3
cell (rmes ¼ 6.56(3) g cm , rcal ¼ 6.54(2) g cm ) for
Rb U O and four formulas per unit cell (r
o
o
2
2
7
mes
À3
À3
Recording angular range (1)
Recording reciprocal space
4.01/29.15
À9php9
À10pkp10
À9php9
2.85/30.01
À9php9
À19pkp19
À46p1p47
¼ 6.47(3) g cm , rcal ¼ 6.48(2) g cm ) for Rb U O .
8
9
31
Refinement of atomic positional parameters and aniso-
tropic displacement parameters for all atoms yielded to
final R1 ¼ 0:0432 and wR2 ¼ 0:0982 for 746 indepen-
dent reflections for Rb U O and R1 ¼ 0:0365, wR2 ¼
Number of reflections
Measured/independent
À1
2211/746
544.70
12134/2065
541.26
2
2
7
Absorption, m (cm
Refinement
)
0
:0702 for 2065 independent reflections for Rb U O .
8 9 31
Refined Parameters/
restraints
53/0
141/0
The atomic positions with equivalent isotropic displace-
ment parameters are reported in Tables 5 and 6,
anisotropic displacement parameters are given in Tables
7 and 8 for Rb U O and Rb U O , respectively.
2
Goodness of fit on F
R1½I42sðIފ
1.117
1.041
0.0432
0.0365
wR2½I42sðIފ
0.0982
0.0702
2
2
7
8
9
31
Largest diff. Peak and hole
3.011/À3.810
2.155/À2.925
˚
À3
)
(
e A
Note: R1 ¼ ðj Fo j À j Fc jÞ= j Fo j
2
2 1=2
P
P
3.2. Description of the structure of Rb U O
2 2 7
P
P
2
2
c
2
wR2 ¼ ½ wðF 2F Þ = wðF Þ Š
:
o
o
2
2
2
The selected interatomic distances and O–U–O angles
are reported in Table 9 for Rb U O .
w ¼ 1=½s ðF Þ þ ðaPÞ þ bPŠ where a and b are refinable parameters
0
2
2
c
and P ¼ ðF þ 2F Þ=3.
o
2
2
7

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3222
Table 3
Observed and calculated powder X-ray diffraction pattern for Rb
2
2 7
U O
h
k
l
2yobs
2ycal
I (%)
h
k
l
2yobs
2ycal
I (%)
1
0
1
1
1
0
2
2
0
1
1
0
2
1
2
0
0
2
1
3
1
1
1
2
2
3
1
2
2
0
1
1
1
0
1
1
2
2
2
2
1
2
1
2
0
1
3
1
3
3
3
1
1
2
2
3
0
12.76
17.44
18.89
24.14
25.68
26.04
27.06
27.09
28.28
29.33
30.83
31.01
32.85
33.57
33.69
38.95
39.99
40.52
40.14
40.52
42.56
42.72
43.40
43.58
43.65
45.10
45.20
45.28
12.76
17.45
18.92
24.13
25.67
26.05
27.08
27.11
28.28
29.36
30.86
31.02
32.90
33.59
33.69
38.94
39.98
40.52
40.15
40.57
42.62
42.76
43.43
43.56
43.67
45.08
45.25
45.28
35
1
3
1
2
4
2
1
0
1
2
0
2
0
4
0
4
4
0
1
4
1
3
3
2
4
4
2
0
4
5
1
3
0
3
3
2
1
3
0
3
4
2
4
2
2
4
4
2
3
1
3
3
0
2
1
4
2
2
47.04
47.08
47.22
47.24
48.73
51.00
51.25
52.56
52.78
52.84
52.86
52.93
53.36
53.56
55.79
55.86
57.13
57.61
57.64
57.77
59.24
60.96
61.22
61.82
64.14
64.24
64.66
66.76
47.02
47.12
47.25
47.26
48.74
51.06
51.27
52.61
52.78
52.85
52.92
52.96
53.42
53.58
55.83
55.90
57.18
57.62
57.63
57.76
59.25
61.02
61.23
61.87
64.11
64.30
64.65
66.74
2
7
0
À2
1
À1
1
1
7
2
0
9
2
50
26
100
45
5
1
4
0
À4
À4
À3
4
15
28
22
23
18
2
À1
0
À1
0
3
1
À2
1
29
58
1
À1
À1
À2
0
7
À2
À2
1
9
7
25
19
6
1
À2
0
3
8
2
6
0
3
À1
1
8
1
3
1
2
1
À1
À2
0
6
3
1
1
1
2
2
À4
3
6
À3
À3
0
38
16
2
5
À3
4
3
4
2
2
À4
2
4
À1
3
41
24
13
3
À2
a ¼ 7:3182ð3Þ, b ¼ 8:0027ð3Þ, c ¼ 6:9385ð2Þ, b ¼ 108:68ð1Þ1 F20 ¼ 36ð0:0155; 36Þ.
The crystal structure of Rb U O contains only one
2
U uranium cation strongly bonded to two oxygen
atoms O(1) and O(2) at shorter distances 1.848(10) and
one uranium atom is completed by a long U–O distance
˚
2
7
6
+
(dU–O ¼ 2.89 A) to give a pentagonal bipyramid. In the
present arrangement, each O(4) oxygen atom is common
to two UO (O ) deformed octahedra whereas each O(3)
˚
.851(11) A, respectively, giving nearly linear uranyl ion
1
(
2
4
2
+
UO2)
[O(1)–U–O(2) ¼ 178.7(5)1]. The uranyl ion is
oxygen atom is shared between three uranium octahe-
dra, to give empty triangles O(3)–O(3)–O(4). Similar
[U O ] dimers are also observed in Cs U O . In this
coordinated by four additional oxygen atoms (three
O(3) and one O(4)) located at the equatorial vertices of a
distorted octahedron. The U–O average bond lengths
8
À
2
10
2
2
7
compound the dimers are connected by sharing two
value 2.25(1) A corresponding to the equatorial oxygen
trans edges to create infinite chains 1 ½U2O8Š running
4
À
˚
1
along b-axis. These chains are themselves linked
˚
atoms, ranging from 2.150(10) to 2.284(12) A, is in good
agreement with the average bond lengths of 2.28(5) A
together by sharing corners to form 2 ½U2O7Š layers
4À
˚
1
determined by Burns et al. [36] from numerous well
refined structures containing uranyl ions in tetragonal
bipyramid coordination.
parallel to (100), Fig. 1c, with Autunite-type sheet anion
topology but with an original occupation of the squares
not found in mineral structures [38] (Fig. 1d).
+
The structure can be described starting from centro-
8
The Rb rubidium ions occupy the interlayer space
À
symmetric dimeric units [U O ]
2
created by two
octahedra sharing O(3)–O(3) edges. These dimeric units
are connected together by opposite O(4) corners to
and ensure the cohesion of the structure (Fig. 2a).
Thus, the unique independent rubidium atom Rb is
surrounded by six oxygen atoms in the range
2.774(11)–2.911(12) that form a distorted octahedral
environment (Fig. 2b). All the Rb–O distances lower
10
develop parallel infinite chains 1 ½U2O9Š running along
6À
1
the ~c -axis. The neighbouring 1 ½U2O9Š infinite chains
6À
1
are associated by sharing all O(3) corners to create
˚
than 3.0 A involve only oxygen atoms of uranyl ions.
infinite 2 ½U O Š sheets parallel to (100) plane, Fig. 1a,
2À
Each rubidium octahedron is edge shared to six
neighboring octahedra to form a very dense RbO2
layers in (100) plane similar to those found, for example,
1
comparable to those observed in K U O described
2
7
2
2
7
in the non-centrosymmetric space group P21 [20],
however in this compound the uranium coordination
in the CdI -type structure (Fig. 2c). Thus the structure
2

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3223
Table 4
Observed and calculated X-ray powder diffraction pattern for Rb
8
9 31
U O
h
k
l
2yobs
2ycal
I (%)
h
k
l
2yobs
2ycal
I (%)
h
k
l
2yobs
2ycal
I (%)
0
0
0
1
1
0
1
1
1
0
0
1
2
0
1
2
1
0
2
2
0
2
1
0
2
1
1
1
2
2
2
2
2
1
1
1
1
1
2
1
2
2
1
1
2
2
1
2
1
1
1
2
2
2
1
1
2
3
3
3
4
4
1
0
4
2
0
3
4
1
1
2
1
0
4
1
1
2
3
0
1
2
2
2
3
2
1
3
4
3
2
3
3
1
4
3
0
4
2
2
1
4
0
1
2
5
6
7
2
3
4
0
1
8
0
2
7
2
5
3
0
1
9
2
9
4
3
9
8
6
4
4
0
1
2
7
9
10
8
5
0
10
1
2
11
6
3
8
7
7
11
12
8
12.38
12.65
13.43
19.20
21.08
22.09
23.17
23.91
24.90
24.91
25.04
25.25
25.46
25.46
25.53
25.99
26.12
26.13
26.22
26.35
26.63
26.75
26.76
27.05
27.39
27.49
27.50
27.55
27.56
28.27
28.40
28.52
28.89
29.16
29.58
29.81
30.91
31.00
31.72
31.77
31.83
32.17
32.20
32.23
32.71
33.13
33.63
33.95
34.02
34.65
35.19
12.36
12.66
13.44
19.21
21.09
21.10
23.18
23.92
24.91
24.92
25.06
25.26
25.46
25.47
25.54
26.00
26.14
26.15
26.23
26.36
26.65
26.76
26.78
27.06
27.40
27.51
27.51
27.57
27.57
27.28
28.41
28.53
28.90
29.17
29.60
29.83
30.93
31.02
31.74
31.78
31.85
32.18
32.22
32.25
32.73
33.14
33.65
33.96
34.05
34.67
35.21
17
66
15
3
2
2
1
1
0
0
1
0
0
0
1
2
2
1
3
2
1
0
2
2
1
1
2
2
3
1
1
3
3
0
3
1
1
1
3
2
3
2
3
1
2
1
1
0
1
3
2
1
1
0
2
4
4
4
1
6
6
4
2
6
6
4
5
5
6
2
5
5
2
4
5
6
4
2
5
0
6
5
1
2
2
1
4
5
6
0
1
1
6
3
2
6
5
7
2
0
3
6
0
7
0
6
0
35.91
36.31
36.88
37.15
37.84
38.60
38.70
39.09
39.25
40.09
40.63
40.89
41.11
41.40
41.49
41.55
41.62
41.70
41.89
42.16
42.19
42.65
42.91
42.94
42.99
43.14
43.27
43.48
43.82
44.36
44.74
44.78
45.04
45.49
45.68
45.84
46.14
46.16
46.18
46.32
46.81
46.91
47.02
47.07
47.15
47.22
47.37
47.40
47.58
48.02
48.08
35.93
36.33
36.90
37.17
37.86
38.62
38.72
39.10
39.27
40.10
40.65
40.90
41.13
41.42
41.50
41.57
41.64
41.72
41.91
42.18
42.22
42.68
42.93
42.96
43.01
43.17
43.29
43.50
43.84
44.39
44.76
44.80
45.06
45.52
45.69
45.86
46.16
46.18
46.20
46.34
46.83
46.83
47.05
47.09
47.17
47.24
47.39
47.43
47.61
48.07
48.11
2
1
1
3
1
2
2
3
0
0
3
2
0
3
0
1
2
3
1
0
0
3
3
2
3
4
0
4
3
0
2
2
3
4
1
1
2
0
0
2
4
1
3
3
2
3
2
2
4
1
0
2
7
6
3
5
3
2
3
2
4
4
1
8
3
8
2
2
5
6
6
8
2
5
7
0
0
2
1
3
8
7
1
5
0
8
6
7
8
6
3
2
8
4
5
2
3
7
4
2
8
4
0
5
48.29
48.38
48.41
48.89
49.45
49.49
49.72
49.81
49.82
50.32
50.56
51.10
51.17
51.40
51.61
51.65
51.69
51.74
51.84
52.29
52.34
52.36
52.49
52.50
52.59
52.59
52.71
52.74
52.96
53.01
53.02
53.17
53.47
53.52
53.59
53.66
53.76
53.82
53.93
53.96
54.03
54.05
54.12
54.26
54.41
54.46
54.56
54.60
54.68
54.86
55.14
48.32
48.41
48.43
48.91
49.47
49.52
49.75
49.84
49.85
50.35
50.59
51.13
51.20
51.43
51.63
51.68
51.72
51.77
51.87
52.32
52.36
52.39
52.52
52.53
52.60
52.62
52.73
52.76
52.92
53.04
53.05
53.20
53.49
53.55
53.62
53.69
53.76
53.86
53.96
53.98
54.06
54.08
54.14
54.29
54.44
54.49
54.59
54.63
54.71
54.89
55.17
8
2
2
10
8
8
9
4
8
13
1
2
13
14
15
9
4
3
4
2
2
3
7
8
2
10
14
4
2
2
2
2
18
16
7
15
10
13
5
1
7
7
5
4
9
11
1
19
13
2
16
0
5
6
7
2
10
0
16
5
61
22
3
4
3
3
2
18
12
4
5
3
10
3
5
2
9
6
30
43
20
3
15
8
6
12
13
4
11
5
3
4
8
7
5
12
7
12
5
5
10
100
6
12
12
5
12
3
2
3
3
13
2
4
1
7
7
3
46
74
25
3
6
16
3
19
0
10
14
12
12
6
10
7
10
5
11
5
6
3
16
8
13
28
2
4
13
58
15
23
26
8
17
6
5
13
11
8
13
6
3
4
9
3
5
9
5
13
5
5
14
9
3
7
9
46
10
2
6
10
9
2
1
14
16
0
2
6
3
39
19
7
16
3
3
4
14
14
5
4
10
4
12
3
10
7
2
5
13
7
17
17
7
12
3
17
12
6
10
5
2
4
4
23
2
4
8
15
3
4
17
4
3
12
5
7
8
5
2
18
5
17
3
18
18
3
14
2
(
(
a ¼ 6:990ð2Þ A, b ¼ 14:280ð3Þ, c ¼ 34:044ð5Þ A; F20 ¼ 44ð0:01154; 43).
can be considered as the stacking along [100] of two
alternating dense layers of UO and RbO octahedra
polytype of the molybdenite MoS compound [39] in
2
which the M atoms occupy the octahedral sites created
by two different successive oxygen layers to form a
AAcBBaCCb sequence where the minus letter denote M
layers. The interspace between two successive identical
oxygen layers, AA for example, is occupied by a UO1.5
6
6
(
Fig. 3a). In fact the structure of M U O , M ¼ K, Rb
2
2
7
compounds can be described as a stacking of hexagonal
type layers of oxygen atoms parallel to the (100) plane in
the sequence AABBCC found, for example, in the 3R

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3224
Table 5
Atomic coordinates and equivalent displacement parameters (A ) for
Table 7
Anisotropic displacement parameters (A ) for Rb
2
2
˚
˚
2
U
2 7
O
2 2 7
Rb U O
Atom U11
U
22
U
33
U
12
U
13
U
23
Atom
Site
x
y
z
Ueq
U0.0131(3) 0.0143(3) 0.0109(3) 0.0008(2) 0.0029(2)
À0.0023(3)
0.0241(9) 0.0205(9) 0.0203(7) À0.0024(6) 0.0071(6) 0.0029(6)
0.008(5) 0.027(6) 0.019(5) 0.000(5) 0.003(4) 0.002(4)
0.010(5) 0.034(7) 0.026(6) À0.008(6) 0.007(4) À0.008(5)
0.030(7) 0.012(6) 0.025(5) À0.001(5) 0.010(4) 0.009(4)
0.040(14) 0.14(2) 0.038(12) À0.002(15) 0.011(10) 0.057(14)
U
4e
4e
4e
4e
4e
2a
0.99385(7)
0.4929(2)
0.2602(14)
0.7268(15)
0.0145(16)
0
0.62403(12)
0.8827(3)
0.6343(14)
0.6190(14)
0.3615(12)
1/2
0.22357(7)
0.2280(2)
0.3096(15)
0.1377(16)
0.1074(16)
1/2
0.0130(2)
0.0216(4)
0.019(2)
0.023(2)
0.022(2)
0.033(8)
Rb
O1
O2
O3
O4
Rb
O1
O2
O3
O4
Note: The anisotropic displacement factor exponent takes the form
ꢀ2
P P
ꢀ
ꢀ
2
2
ꢀ2
2
2
ꢀ2
ꢀ ꢀ
Note: The Ueq values are defined by Ueq ¼ 1=3ð
Uija a aiaj).
À2p ½h a U11 þ k b U22 þ l c U33 þ ꢁ ꢁ ꢁ þ 2kla c U23Š.
i
j
i
j
Table 6
and built from squares and triangles that results from
2
˚
Atomic coordinates and equivalent displacement parameters (A ) for
Rb
0
0
the stacking of two arrowhead chains S, S , T and T
Fig. 1b). The uranium atoms occupy all the square sites
when the triangular sites are empty.
8
9 31
U O
(
Atom Site
x
y
z
Ueq
On the basis of the U–O and Rb–O bond length
values obtained from crystal X-ray structure determina-
tion, the bond valence sums were calculated to be 5.75
and 1.31 vu for Uand Rb, respectively. Usual values,
2.17, 2.08 and 1.98 vu are obtained for O(1), O(2) and
O(3), respectively, on the contrary for O(4) that belongs
only to two Uatoms with a large atomic displacement
parameter the value is significantly lower, 1.71 vu.
U
1 c
2 d
3 d
4 d
5 d
8d
8d
8d
8d
8d
8d
8d
4c
40.04867(13) 0.25000
80.58563(10) 0.21247(5)
80.56331(10) 0.13223(6)
0.00000
0.0085(2)
U
0.05647(2) 0.0080(2)
0.16955(2) 0.0131(2)
0.11001(2) 0.0091(2)
0.21559(2) 0.0106(2)
U
U
80.11966(9) 0.17285(5)
80.01624(10) 0.11660(6)
U
Rb1
Rb2
Rb3
Rb4
O1
0.7400(3)
0.2526(3)
0.7832(3)
0.3034(3)
0.1130(17)
0.0509(17)
0.5883(17)
0.711(2)
0.48597(16) 0.02708(7) 0.0225(5)
0.44846(16) 0.08736(7) 0.0236(5)
0.39441(15) 0.14707(7) 0.0230(5)
0.37347(17) 0.21998(7) 0.0264(5)
0.0496(9)
0.3723(10)
0.0881(10)
0.25000
0.0969(4)
0.0145(4)
0.0416(5)
0.00000
0.0144(3)
O2
O3
0.0164(32)
0.0228(40)
0.0107(44)
0.0203(37)
0.0179(32)
0.0253(40)
0.0166(32)
0.0269(39)
0.0189(35)
0.0126(33)
0.0297(41)
0.0178(37)
0.0186(36)
0.0279(40)
0.0171(35)
3.3. Description of the structure of Rb U O
31
8
9
O4
O5
The selected interatomic distances and O–U–O angles
are reported in Table 10 for Rb U O .
8d
8d
8d
8d
8d
8d
8d
8d
8d
8d
8d
8d
0.1273(18)
0.4545(17)
0.5623(19)
0.8737(18)
0.587(2)
0.2911(9)
0.1644(9)
0.0075(10)
0.1371(9)
0.3351(10)
0.2054(10)
0.2167(9)
0.2559(10)
0.1005(9)
0.9883(9)
0.1307(11)
0.2414(10)
0.1265(4)
0.1111(4)
0.1595(5)
0.1554(4)
0.0734(5)
0.0663(4)
0.0453(4)
0.1812(5)
0.2292(4)
0.2113(5)
0.1744(4)
0.2248(4)
8
9
31
O6
O7
The structure of Rb U O contains five symmetri-
8 9 31
O8
O9
cally distinct Uatoms. The composition deduced from
the structure refinement indicates that all Uoccurs as
O10
O11
O12
O13
O14
O15
O16
0.9000(18)
0.2700(18)
0.566(2)
6+
U
, and as is typical in oxides, each Uis bonded to two
oxygen atoms at short distances forming a nearly linear
2
þ
uranyl ion UO , with U–O bond lengths in the range
2
0.6995(17)
0.0252(18)
0.2474(18)
0.0025(18)
˚
1.781(13)–1.848(15) A and O–U–O bond angles in the
range 174.5(6)–179.0(6). The coordination of U(2),
U(3) and U(5) is completed in the equatorial plane
by four oxygen atoms at intermediate distances
P P
i
n
n
Note. The Ueq values are defined by Ueq ¼ 1=3ð
Uija a aiajÞ.
j
i
j
(
2.150(13)–2.298(13)) forming a distorted octahedron
and
2
2
þ
whereas the two other uranyl ions Uð1ÞO
2
þ
Uð4ÞO are coordinated in equatorial planes by five
2
layer built from Uatoms in a positions to form linear
uranyl ions and by O(3) atoms ordered on half the
trigonal sites, the O(4) atom is localized between two
unoccupied trigonal sites and logically its atomic
displacement is elongated towards the empty sites
oxygen atoms at distances varying in a largest range
(2.189(13)–2.566(14)) to form pentagonal bipyramids.
In this compound, the equatorial mean bond lengths,
2
þ
2þ
˚
2.39(2) and 2.37(1) A for Uð1ÞO
and Uð4ÞO2
2
in pentagonal bipyramid coordinations, and 2.21(1),
2
2
þ
2þ
˚
(
Fig. 3b). This description explains the first structural
2.23(1) and 2.25(1) A for Uð2ÞO , Uð3ÞO2 and
2
þ
approaches concerning M U O (MQK, Rb) com-
Uð5ÞO , respectively, in distorted tetragonal bipyramid
2
2
7
2
environments are in excellent agreement with the
pounds reported in R3m trigonal symmetry [11,12,19]
where the hexagonal c parameter represents the width of
the layers sequence (3asin b). The UO1.5 layer constitute
an anion sheet topology not described for mineral
materials in the completeness review by Burns et al. [38]
˚
average bond lengths of 2.37(9) and 2.28(5) A deter-
mined from numerous well refined structures [36], for
uranyl ions in pentagonal and tetragonal bipyramid
+
coordination, respectively. The U6 valence of uranium

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3225
Table 8
Anisotropic displacement parameters (A ) of (U, Rb) for Rb
2
˚
8
U
9
O
31
Atom
U
11
U
22
U
33
U
12
U
13
U
23
U1
U2
0.0070(5)
0.0081(3)
0.0115(4)
0.0083(3)
0.0119(4)
0.0220(11)
0.0202(11)
0.0221(11)
0.0183(11)
0.012(7)
0.016(7)
0.008(6)
0.005(9)
0.019(8)
0.023(7)
0.028(9)
0.021(7)
0.032(9)
0.010(7)
0.013(7)
0.045(10)
0.009(7)
0.013(7)
0.014(7)
0.021(7)
0.0102(6)
0.0101(4)
0.0168(4)
0.0110(4)
0.0137(4)
0.0254(13)
0.0243(13)
0.0234(13)
0.0224(13)
0.020(8)
0.015(8)
0.031(10)
0.027(12)
0.010(8)
0.015(7)
0.020(9)
0.020(8)
0.029(9)
0.030(9)
0.020(8)
0.012(9)
0.015(9)
0.016(8)
0.038(10)
0.020(9)
0.0084(6)
0.0057(4)
0.0109(4)
0.0080(4)
0.0062(3)
0.0201(14)
0.0261(14)
0.0234(14)
0.0384(16)
0.011(9)
0.00000
0.00000
0.0007(3)
0.0005(5)
0.0015(3)
0.0030(4)
0.0012(3)
À0.0004(3)
0.0009(3)
0.0004(3)
À0.0019(3)
0.0013(9)
0.0002(9)
À0.0017(9)
0.0027(10)
0.001(6)
U3
U4
À0.0016(3)
À0.0005(3)
À0.0001(3)
À0.0016(10)
0.0032(10)
0.0042(10)
0.004(1)
U5
À0.0001(4)
À0.0022(10)
0.0076(11)
À0.0012(10)
0.0050(12)
À0.001(7)
À0.005(7)
À0.010(8)
0.007(9)
Rb1
Rb2
Rb3
Rb4
O1
0.002(6)
0.004(6)
O2
O3
0.019(8)
À0.003(7)
0.008(6)
0.00000
0.037(11)
0.009(11)
0.031(11)
0.016(9)
0.009(7)
0.00000
O4
O5
0.001(6)
0.007(7)
0.004(7)
À0.002(7)
0.002(6)
0.006(7)
O6
O7
À0.001(6)
À0.003(7)
0.006(7)
0.028(11)
0.009(8)
0.02(1)
À0.007(8)
0.007(6)
0.007(7)
O8
O9
0.010(7)
0.004(8)
0.008(6)
0.003(6)
À0.009(7)
0.000(7)
0.012(6)
O10
O11
O12
O13
O14
O15
O16
0.016(9)
0.012(8)
À0.005(6)
À0.005(6)
0.002(8)
0.032(11)
0.034(11)
0.027(11)
0.031(10)
0.010(9)
À0.005(8)
0.010(6)
0.004(6)
0.003(7)
À0.002(6)
0.000(7)
0.008(7)
À0.006(7)
0.001(7)
0.017(9)
À0.001(7)
0.003(7)
À0.004(6)
0.001(6)
2
2
ꢀ2
2
ꢀ2
2
ꢀ2
ꢀ ꢀ
Note: The anisotropic displacement factor exponent takes the form À2p ½h a U11 þ k b U22 þ l c U33 þ ꢁ ꢁ ꢁ þ 2kla c U23Š.
Table 9
octahedra share the O(8)–O(13) equatorial edge to
8À
˚
Interatomic distances (A), valence bond and uranium–oxygen angles
1) in Rb
create dimeric units [U O ] . These dimers are
2
10
(
2 2 7
U O
associated by sharing opposite corners O(15) to form
6À
1
d
U–O
sij
Uranium–oxygen angles
an infinite chain ½U2O9Š running along the ~a -axis.
1
1
1
6À
Environment of U
U–O1
U–O2
Two parallel
by O(13) oxygen atoms creating an infinite ribbon
½U2O9Š
chains are linked together
1.848(10) 1.479
1.852(11) 1.467 O1–U–O2
2.150(10) 0.826 O1–U–O4
2.246(10) 0.687 O1–U–O3
2.274(10) 0.651 O1–U–O3
178.69(49)
90.89(32)
88.39(46)
89.83(46)
90.36(43)
4À
1
½
U2O8Š
(Fig. 4a). Each U(1)O pentagonal bipyr-
7
1
amid is sharing two opposite equatorial edges
U–O4
i
U–O3
U–O3
i
O(11)–O(10) with two U(4)O pentagonal bipyramids
7
ii
ii
16À
U–O3
2.284(12) 0.638 O1–U–O3
5.75
to form a [U O ]
trimer, which is connected to two
octahedra U(2)O by sharing a two O(6)–O(11) edge to
P
3
17
sij
6
1
8À
create a pentameric unit [U O ]
24
(Fig. 4b). These
uranyl pentameric groups are connected together by
sharing edges to form an infinite three polyhedra wide
5
Environment of Rb
iv
Rb–O1
Rb–O1
2.774(11) 0.251
2.793(12) 0.239
2.810(13) 0.228
2.829(12) 0.217
2.861(12) 0.199
1
12À
ribbon ½U5O21Š
(Fig. 4b). Finally a puckered layers
v
1
31
Rb–O2
2
8À
i
½U O Š parallel to the (010) plane is obtained by
1
sharing equatorial edges O(6)–O(15) and O(8)–O(15)
9
Rb–O2
Rb–O1
Rb–O2
iii
vi
1
12À
2.911(12) 0.173
1.31
between U(4)O pentagonal bipyramids of ½U O Š
7
1
ribbons on one side and distorted octahedra U(3)O and
5
21
P
sij
6
1
1
4À
U(5)O of ½U2O8Š ribbons on other side (Fig. 4c).
6
Symmetry codes: (i) Àx; 0:5 þ y; 0:5 À z; (ii) Àx; 1 À y; Àz; (iii)
x; 1:5Ày;À0:5þz; (iv) 1Àx; 0:5þy; 0:5Àz; (v) 1 þ x; 1:5 À y; 0:5 þ z;
The 1 ½U2O8Š ribbon is part of the M U O (MQK,
4À
1
2
2
12À
7
1
(vi) 1 þ x; y; z.
Rb) diuranate sheets whereas the ½U5O21Š
ribbon is
part of sheet found, for example, in USbO [40] with
1
5
commonly adopted uranophane sheet anion topology.
Thus, the sheet in Rb U O can be described as an
atoms is confirmed by the sum of the valences bonds
(
8
9
31
Table 10).
As previously observed in M U O diuranate struc-
ordered intergrowth of parts of the two types of sheets.
The 1 ½U5O21Š
12À
ribbons are also observed in Sr U O
3 11 36
2
2
7
1
tures with M ¼ K, Rb, the U(3)O6 and U(5)O6
[41], where layers result from the combination of

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3226
Fig. 1. Comparison of the 2 ½U2O7Š layers built from edges and corners—sharing UO
2À
octahedra, (a) in M
U
O
7
, (MQK, Rb) and (c) in
1
. The sheet anion topology that contains squares and triangles can be described from arrowed chains S, S , T and T in (K–Rb)
6
2
0
2
0
2
M U
O
2 7
2
U
O
2 7
(b),
2 2 7
and from Autunite-type sheet in Cs U O (d).
1
12À
ribbons with 1 ½Sr U O Š ribbons where
4À
½
U O Š
deduced one after the other by an inversion center to
give a unit cell with two infinite layers. There are four
distinct rubidium atoms coordinated by six oxygen
atoms forming distorted octahedra for Rb(1), Rb(2) and
Rb(3) and a distorted trigonal prism for Rb(4) involving
1 1
the pentagonal Uatoms are replaced Sr connected
5
21
3
2
11
through chains similar to those observed in a-U O [42].
3
8
In terms of chains the stacking sequence of nine chains is
0
TT RDRURDR where the chains denoted by R, U, D
were defined by Miller et al. [43]. Note that Cs U O
2
+
2+
oxygen atoms that belong to the U(3)O and U(5)O2
4À
4
5
17
2
[
(
5–7] can be described by the five-chains sequence
0
uranyl groups of the 1 ½U2O8Š
ribbon. The RbO6
polyhedra are edge shared to form a dense layer which is
1
TT RDR) .
2
The 2 ½U O Š corrugated layers are stacked along
8À
built from CdI -type octahedra ribbons, three octahedra
2
1
9
31
the (010) direction and the rubidium ions are located
between the layers and hold them together (Fig. 5a). In
width, connected through RbO trigonal prisms. Within
6
this layer some trigonal sites are empty and the formula
of the layer is Rb &O (Fig. 5b).
this structure, two consecutive layers 2 ½U9O31Š8À are
1
8
9

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3227
Fig. 2. (a) Stacking of 2 ½U2O7Š layers in Rb
2À
+
2 2 7
U O and occupation of the interlayer space by Rb ions, (b) Octahedral coordination of the Rb
+
1
ions by uranyl oxygen atoms and (c) the dense RbO
2
layer of edge-shared TbO
6
octahedra.
Fig. 3. (a) The crystal structure of Rb
to form an octahedral shared RbO layer. (b) The interspace between two identical hexagonal type oxygen layers are occupied by UO1.5 layers
formed by U, O(3) and O(4) atoms.
2 2 7
U O described as a stacking of AABBCC of hexagonal-type oxygen layers, with octahedral abc sites occupied
2

ARTICLE IN PRESS
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3228
Table 10
˚
8 9 31
Interatomic distances (A), valence bond and uranium–oxygen angles (1) in Rb U O
U Environment
sij
i
U1–O2
U1–O2
U1–O11
U1–O11
1.816(14)
1.816(14)
2.237(13)
2.237(13)
2.361(14)
2.566(14)
2.566(14)
1.555
1.555
0.689
0.689
0.553
0.37
Uranium– oxygen angles
i
O2–U1–O2
O2–U1–O11
179.02(64)
90.64(52)
88.68(52)
90.49(38)
90.22(53)
90.18(53)
i
i
O2–U1–O11
ii
ii
U1–O4
O2–U1–O4
O2–U1–O10
iii
ii
iii
U1–O10
ii
U1–O10
0.37
O2–U1–O10
P
sij
5.781
sij
U2–O9
U2–O3
U2–O4
U2–O6
U2–O10
1.845(15)
1.848(15)
2.181(6)
1.484
1.484
0.777
0.781
0.707
0.704
5.937
sij
Uranium– oxygen angles
O9–U2–O3
O9–U2–O4
177.53(64)
92.30(51)
91.98(60)
89.48(57)
91.87(55)
2.185(13)
2.226(13)
2.240(13)
O9–U2–O6
O9–U2–O10
O9–U2–O11
U2–O11
P
sij
U3–O12
U3–O7
U3–O6
U3–O15
U3–O8
1.811(14)
1.815(14)
2.180(13)
2.215(13)
2.224(13)
2.289(13)
1.576
1.573
0.775
0.732
0.711
0.63
Uranium– oxygen angles
O12–U3–O7
O12–U3–O6
178.18(64)
89.89(62)
90.21(60)
90.39(56)
89.68(61)
O12–U3–O15
O12–U3–O8
O12–U3–O13
U3–O13
P
sij
5.997
sij
U4–O5
U4–O1
U4–O10
U4–O6
1.781(13)
1.817(13)
2.189(13)
2.345(12)
2.368(13)
2.444(14)
2.521(13)
1.669
1.573
0.774
0.563
0.548
0.464
0.404
5.995
sij
Uranium– oxygen angles
O1–U4–O5
O1–U4–O6
175.83(58)
88.83(49)
86.15(51)
89.49(56)
92.10(51)
91.19(55)
ii
ii
O1–U4–O8
O1–U4–O15
O1–U4–O11
ii
U4–O8
U4–O15
U4–O11
ii
O1–U4–O10
P
sij
U5–O16
U5–O14
U5–O15
U5–O13
U5–O13
1.813(14)
1.840(13)
2.150(13)
2.274(12)
2.287(13)
2.298(13)
1.576
1.525
0.831
0.644
0.643
0.615
5.834
Uranium– oxygen angles
vii
vii
O16–U5–O14
O16–U5–O15
174.48(61)
93.47(58)
90.72(52)
89.22(53)
90.34(53)
ii
ii
O16–U5–O13
viii
O16–U5–O13
viii
ii
U5–O8
P
ii
O16–U5–O8
sij
Rb environment
sij
sij
x
Rb1–O1
2.746(14)
2.747(13)
2.765(13)
2.778(16)
2.878(15)
2.871(14)
0.272
0.276
0.255
0.247
0.189
0.193
1.432
sij
Rb2–O5
2.757(13)
2.766(15)
2.884(14)
2.903(16)
2.955(12)
3.055(14)
0.260
0.254
0.186
0.177
0.151
0.115
1.143
sij
iv
x
Rb1–O2
Rb2–O3
Rb2–O9
x
Rb1–O3
i
x
Rb1–O3
Rb2–O7
xii
Rb2–O1
Rb1–O9
xi
Rb1–O2
P
Rb2–O2
sij
Rb3–O12
x
2.752(15)
2.892(13)
2.895(16)
2.908(13)
2.937(14)
2.983(16)
0.265
0.181
0.179
0.177
0.164
0.141
1.107
Rb4–O12
Rb4–O16
2.818(15)
2.831(14)
2.839(13)
2.965(16)
3.006(14)
3.100(16)
0.224
0.217
0.205
0.144
0.134
0.102
1.026
Rb3–O1
Rb3–O14
v
ix
Rb4–O14
x
iv
Rb3–O5
Rb4–O7
Rb4–O16
xiii
Rb4–O14
x
vi
Rb3–O7
Rb3–O9
P
sij
Symmetry codes: (i) x; 0:5 À y; Àz; (ii) À1 þ x; y; z; (iii) À1 þ x; 0:5 À y; Àz; (iv) 1 þ x; y; z; (v) 1 À x; À0:5 þ y; z; (vi) 0:5 þ x; y; 0:5 À z;
(
vii) x;À1þy; z; (viii) À0:5þx; y; 0:5Àz; (ix) Àx;À0:5 þ y; z; (x) 1Àx; 0:5þy; z; (xi) 1Àx; 1Ày;Àz; (xii) Àx; 0:5þy; z; (xiii) 0:5Àx;À0:5 þ y; 0:5 À z;
xiv) x; 1 þ y; z; (xv) 0:5 À x; 0:5 þ y; 0:5 À z.
(

ARTICLE IN PRESS
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3229
Fig. 4. The 2 ½U9O31Š infinite layer in Rb
8À
31 (c) which can be described as the intergrowth of 1 ½U5O21Š12À ribbons of uranophane sheet anion
8
U
9
O
1
1
topology (b) and ½U2O8Š4À ribbons similar to that found in Rb
1
1
2 2 7
U O (a).
3
.4. Electrical properties
The layered structure of Rb U O and Rb U O
31
analyses of residues after each DTA measurement
showing that both phases are stable until 1300 1C. These
observations confirm the high thermal stability of
uranates.
2
2
7
8
9
compounds allows us to consider some mobility of the
alkali metal cations localized in interlayer spaces. Thus,
Fig. 6 indicates the temperature dependence of the
conductivity for the two compounds. The electrical
measurements show that the conductivity obeys to the
Arrhenius law in the studied temperature domain, with a
linear evolution of log s according to 1=T. Both studied
compounds exhibit low and comparable conductivity
with activation energies about 0.81 and 0.85 eV for
3.5. Spectroscopic properties
For both compounds the infrared spectrum
À1
(400–1000 cm ) (Fig. 7), is characterized by two
vibration domains, corresponding to [OQUQO]
uranyl ions and U–O equatorial vibrations. Thus, to
interpret the IR spectrum two following building units
2
þ
have been considered: UO
2
Rb U O and Rb U O , respectively. The low con-
9
groups and equatorial
(secondary) U–O bonds. For Rb U O , the spectrum
2
2
7
8
31
ductivity and high activation energy observed in both
compounds may be explained by the fact that there is a
direct relation between structural aspect and electric
2
2
7
À1
shows two strong bands at about 807 and 745 cm
which can be attributed to asymmetrical and symme-
trical UO2 uranyl stretching vibrations n3 and n1,
,
+
2þ
conductivity. Indeed, in these two structures the Rb
ions are not very mobile because of their size, and
particularly of the rigidity of their octahedral environ-
ment where the rubidium atoms in interlayer spaces are
strongly linked to the uranyl layers with almost Rb–O
respectively, for uranyl ion in octahedral coordination,
in good agreement with Volkovich et al. infrared
investigation [44]. However, in Rb U O , two asymme-
8
9
31
trical and symmetrical vibrations couples (805, 740) and
À1
˚
distances lower than 2.90 A and finally the lack of
partially occupied rubidium sites.
(835, 760) cm have been observed, corresponding to
(n n ) vibration bands of uranyl ions in octahedral and
3
, 1
No transition or melting point has been observed on
the DTA experiments. The X-ray powder diffraction
pentagonal bipyramid environments, respectively. For
both spectra, the (n3, n1) vibration bands values are in

ARTICLE IN PRESS
S. Yagoubi et al. / Journal of Solid State Chemistry 178 (2005) 3218–3232
3230
good agreement with the mathematical model suggested
by Bagnall and Wakerley [45] to determine the value of
stretching vibration n for uranyl groups, dU–O
¼
3
À2/3
81.2n3 +0.895, leads to the predicted uranyl bond
length, in good agreement with the average value
obtained from X-ray structure results (Table 11). The
À1
n from that of n , n ¼ 0.912n –1.04 (cm ).
1
3
1
3
The application of Veal et al.’s empirical equation
46], relating bond length (d) to the asymmetric
À1
[
lower bands observed in the range 440–650 cm may be
assigned to the U–Oeq vibrations between uranium and
equatorial oxygen atoms, these bands were also
observed in other mineral and inorganic compounds
[47]. A summary of the assignment of absorption bands
is also given for both compounds in Table 11.
4. Conclusion
The structures of the diuranate Rb U O and
2
2
7
the new uranate in Rb U O contains bi-dimensional
9
8
31
layers of uranyl polyhedra connected by edges and
corners. In Rb U O the layer is built from UO6
2
2
7
distorted octahedra and is planar, the rubidium poly-
hedra forming an hexagonal sheet of edge-shared
octahedra of CdI -type. The addition in the formula
2
of supplementary UO units from Rb U O (Rb U O )
8
3
2
2
7
8
28
to Rb U O (Rb U O +UO ) and the previously
3
8
9
31
8
8
28
reported Cs U O
4
(Cs U O +2UO ) introduce
8 8 28 3
5
17
UO7 pentagonal bipyramids in the U–O sheet and
induce the replacement of some octahedra by trigonal
prism in the Rb–O sheet which can be considered
as lacunar Rb O , Rb &O , and Cs & O , in
8
16
8
18
8
2
20
Rb U O , Rb U O
9
and Cs U O
5
This
17, respectively.
2
2
7
8
31,
4
structural filiation encourages us to take again the
investigation of the M–U–O diagrams with modern
characterization methods such as electronic microscopy
to evidence new phases and to rule on certain still
discussed results.
Fig. 5. Stacking of 2 ½U9O31Š
8À
+
8 9
layers Rb U O31 with Rb ions in
1
interlayer spaces (a), forming with uranyl oxygen a puckered layer
8 9
Rb &O (b).
2 2 7 8 9
Fig. 6. Comparison between Rb U O and Rb U O31 cationic conductivity vs. temperature.

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3231
2 2 7 8 9 31
Fig. 7. Infrared spectra of Rb U O and Rb U O .
Table 11
Vibration Bands in infrared spectrum, and calculated uranyl distances
[
12] L.M. Kovba, E.A. Ippolitova, Yu.P. Smanov, V.I. Spitsyn, Dokl.
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[
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n
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805(m)
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745(m)
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