Synthesis, crystal structure and TEM study of the new hollandite-type Ba～8/7Mo8O16

Article abstract of DOI:10.1016/S0925-8388(00)01325-6

Investigation of the Cs-Ba-Mo-O system by fused salt electrolysis at 960°C has led to the new related-hollandite compound Ba_～8/7Mo₈O₁₆. Single-crystals of Ba_～8/7Mo₈O₁₆ were examined by electron diffraction and high-resolution electron microscopy. Ba_～8/7Mo₈O₁₆ was found to present a tetragonal I basic unit-cell (a=10.21 A?, c=2.89 A?) with a one-dimensional modulation of wavevector q close to 4/7 along c* at room temperature. Refinement of the crystal structure was made on X-ray single crystal data in the seven-fold supercell in the three-dimensional space group I4 (a=10.214(2) A?, c=20.255(5) A?, R_w(F²)=0.0844 for all 7767 independent reflections and 149 parameters). In the latter supercell approach, the Mo atoms in the double edge-sharing rutile-type chains form Mo₃ triangles and planar rhomboidal Mo₄ clusters that coexist in equal proportions. The Mo-Mo distances vary from 2.5341(8) to 2.696(2) A? and from 2.5341(8) to 2.894(4) A? in the Mo₃ and Mo₄ clusters, respectively. The shortest intercluster distance is 3.047(4) A?. The Mo-O distances range between 1.81(1) to 2.418(8) A? as usually observed in reduced molybdenum oxides. The Ba²⁺ cations occupy the large square channels in square prismatic environment of oxygen atoms with the distribution sequence ...Ba-□-Ba-Ba-□Ba-□-....

Full text of DOI:10.1016/S0925-8388(00)01325-6

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Journal of Alloys and Compounds 317–318 (2001) 120–126
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Synthesis, crystal structure and TEM study of the new hollandite-type
Ba|8 / 7Mo8O16
a
b
N. Barriera , , P. Gougeon , R. Retoux
*
a
´
´
´ ´
Laboratoire de Chimie du Solide et Inorganique Moleculaire, U.R.A. C.N.R.S. n8 1495, Universite de Rennes I, Avenue du General Leclerc,
35042 Rennes Cedex, France
b
´
Laboratoire des Fluorures, UPRES-A CNRS 6010, Universite du Maine, 72085 Le Mans Cedex 9, France
Abstract
Investigation of the Cs–Ba–Mo–O system by fused salt electrolysis at 9608C has led to the new related-hollandite compound
Ba|8 / 7Mo8O16. Single-crystals of Ba|8 / 7Mo8O16 were examined by electron diffraction and high-resolution electron microscopy.
˚
˚
Ba|8 / 7Mo8O16 was found to present a tetragonal I basic unit-cell (a510.21 A, c52.89 A) with a one-dimensional modulation of
wavevector q close to 4/7 along c* at room temperature. Refinement of the crystal structure was made on X-ray single crystal data in the
2
˚
˚
seven-fold supercell in the three-dimensional space group I4 (a510.214(2) A, c520.255(5) A, Rw(F )50.0844 for all 7767 independent
reflections and 149 parameters). In the latter supercell approach, the Mo atoms in the double edge-sharing rutile-type chains form Mo3
˚
triangles and planar rhomboidal Mo4 clusters that coexist in equal proportions. The Mo–Mo distances vary from 2.5341(8) to 2.696(2) A
˚
˚
and from 2.5341(8) to 2.894(4) A in the Mo3 and Mo4 clusters, respectively. The shortest intercluster distance is 3.047(4) A. The Mo–O
21
˚
distances range between 1.81(1) to 2.418(8) A as usually observed in reduced molybdenum oxides. The Ba cations occupy the large
square channels in square prismatic environment of oxygen atoms with the distribution sequence . . . Ba–h–Ba–Ba–hBa–h– . . . .
2001 Elsevier Science B.V. All rights reserved.
Keywords: Hollandite; Barium; Molybdenum; Oxide; Clusters; TEM studies
1. Introduction
tunnels having 131 octahedron in cross section (T(1,1)
tunnels). Reduced molybdenum oxides which crystallize
with a hollandite-type structure constitute an interesting
subgroup. Indeed, the Mo atoms form Mo–Mo bonds
which manifest as tetrameric rhomboidal Mo4 clusters in
K2Mo8O16 [4] and Bal.14Mo8O16 [5] synthesized by solid
state reaction, and Mo3 triangles either isolated or forming
infinite chains in Lal.16Mo8O16 [6] and NdMo6O12 (i.e.
Ndl.33Mo8O16) [7], respectively, obtained by fused-salt
electrolysis. We report here the synthesis, the electron
microscopy study and the single-crystal structure determi-
nation of a new reduced ternary oxide of molybdenum
which adopts the hollandite-type structure: Ba|8 / 7Mo8O16.
Since the report of the crystal structure of the mineral
¨
¨
hollandite BaxMn8O16 (x |0.96) by Bystrom and Bystrom
in 1950 [1], a large number of ternary transition-metal
oxides AxM8O16 (A5K, Rb, Cs, Ba, Pb . . . .; M5Ti,V, Cr,
Mn, Fe, Mo, Ru . . . ) which crystallize in the hollandite-
type structure have been synthesized by solid state reaction
or fused-salt electrolysis. Most of these studies were
motivated by the possible use of the hollandite-type
compounds as nuclear waste immobilizers, solid state
electrolytes or fast ionic conductors [2]. Most importantly,
these phases frequently exhibit complex commensurate or
incommensurate superstructures due to partial occupation
and ordering of the A cation within the four-sided tunnels
[3]. Indeed, the hollandite structure consists of double
rutile chains sharing edges and corners of the MO6
octahedra to form one-dimensional four-sided tunnels of
232 octahedra in cross section, often noted T(2,2), in
which the A cations reside as well as empty smaller
2. Experimental section
2.1. Synthesis
The starting reagents used in this synthesis were
Cs2MoO4, BaMoO4, MoO3 (Strem, 99.9%). Caesium and
barium molybdates were prepared by reacting Cs2CO3 and
*Corresponding author.
0925-8388/01/$ – see front matter
PII: S0925-8388(00)01325-6
2001 Elsevier Science B.V. All rights reserved.

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N. Barrier et al. / Journal of Alloys and Compounds 317–318 (2001) 120–126
121
BaCO3 (Strem, 99.9%), respectively, with a stoichiometric
amount of MoO3 at 6008C and 8008C overnight. MoO3
was annealed in air at 4008C before use. Single crystals of
Ba|8 / 7Mo8O16 were obtained by the electrolysis of a melt
formed at 9608C from a mixture of Cs2MoO4, MoO3 and
BaMoO4 having the molar ratio 5.2:1.5:2. The electrolysis
was carried out in air using an high-purity alumina
crucible. The anode was a Pt foil with a surface area of
about 1 cm2 and the cathode a 10 turn Pt spiral of about 1
cm long made from a 0.4 mm-diameter wire. At the end of
the process, the electrodes were raised above the melt to
allow them to cool rapidly to room temperature. Crystals
which grow perpendicularly to the cathode in the form of
black needle shaped square prisms with the needle axis
parallel to the tetragonal c-axis (max. dimensions 0.83
0.835 mm3) were obtained by applying a constant current
of 36 mA for 12 h. Single crystals were separated from the
matrix and the cathode by repeated and alternate washings
in hot dilute solutions of potassium carbonate and hydro-
chloric acid.
2.2. Structure determination
A needle-like crystal of approximate dimensions 0.283
0.02830.027 mm3 was selected for data collection. In-
tensity data were collected on a Nonius Kappa CCD
diffractometer using a graphite-monochromatized MoKa
˚
radiation (l50.71073 A) at room temperature. The frames
were recorded using Dv528 rotation scans with an X-ray
exposure time of 60 s. Reflection indexing, Lorentz-polari-
zation correction, peak integration and background de-
termination were performed using the program DENZO of
the Kappa CCD software package [8]. An empirical
absorption correction was applied with the SORTAV
program [9]. Examination of the data set did not show any
supplementary systematic extinctions other than that corre-
sponding to the I-type lattice and revealed that the Laue
class was 4/m leading to space groups I4, I4 or I4/m. The
structure was solved by direct methods using SHELXS
[10] and subsequent difference Fourier syntheses in space
group I4. All structure refinements and Fourier syntheses
]
Table 1
Crystallographic and experimental data for Ba|8 / 7Mo8O16
Formula
Ba|8 / 7Mo8O16
1180.48
tetragonal
I4
10.2160(5)
20.2449(8)
2112.9(2)
7
Molecular mass
Crystal system
Space group
˚
a(A)
˚
c(A)
˚
V(A)
Z
Density (calc., g.cm23
Temperature (K)
Diffractometer
Radiation
)
6.494
293
Nonius Kappa CCD
MoKa radiation (l50.71073A)
˚
Crystal color
Morphology
black
needle
0.2830.02830.027
11.78
Crystal size (mm3)
Linear absorption coeff. (mm21
Monochromator
)
Oriented graphite
Scan mode
18 Dv scan
Recording range 20 (8)
hkl range
4–90.4
220/17, 218/20, 240/27
No. of measured reflections
No. of independent reflections
28 718
7767
4591
No. of observed reflections with F2.2
s(F2)
Rint
0.052
Absorption correction
Transmission (min.–max.)
Refinement
Sortav[9]
0.260–0.747
F2
Calculated weights
w 5 l/[s2(Fo2) 1 (0.0571P)2 1 0.7462P]
where P 5 (Fo2 1 2Fc2)/3
0.00026(3)
Extinction coefficient
R1, wR2 [F2 . 2 s(F2)]
R1, wR2 on all data
S
0.0444, 0.1153
0.0844, 0.1401
1.079
No. of refined parameters
149
23
˚
Dr min, max. (e.A
)
23.478, 3.620

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122
N. Barrier et al. / Journal of Alloys and Compounds 317–318 (2001) 120–126
Table 2
rate modulated hollandite-type structure [12]. Some data
collection and refinement parameters are given in Table 1.
The final values of the positional parameters, isotropic-
equivalent atomic displacement parameters and their stan-
Atomic coordinates (3104) and equivalent isotropic displacement param-
2
3
˚
eters (A 310 ) for Ba|8 / 7Mo8O16
X
Y
Z
U(eq)a
Ba(1)
Ba(2)
Ba(3)
Ba(4)
Ba(5)
Ba(6)
Ba(7)
Mo(1)
Mo(2)
Mo(3)
Mo(4)
Mo(49)
Mo(5)
Mo(6)
Mo(7)
O(1)
O(2)
O(3)
O(4)
O(5)
O(6)
O(7)
O(8)
O(9)
5000
5000
3619(1)
6852(3)
5890(1)
4090(2)
3172(2)
1353(1)
490(20)
5642(1)
5009(1)
4391(1)
8487(1)
8717(2)
7824(1)
7201(1)
6558(1)
6419(3)
6447(5)
5748(4)
5755(4)
5011(6)
4991(4)
4306(4)
4280(3)
8594(6)
8611(4)
7875(5)
7831(3)
7170(4)
7175(4)
16(1)
11(1)
20(1)
21(1)
15(1)
21(1)
60(14)
9(1)
12(1)
13(1)
10(1)
7(1)
0
0
0
0
0
0
0
0
0
0
0
0
1493(1)
3231(1)
1548(2)
1494(2)
1485(2)
3360(2)
1721(1)
3506(1)
1557(8)
1693(10)
3319(10)
3497(10)
1662(5)
1221(4)
2403(9)
3489(7)
3294(5)
3477(6)
1672(10)
1312(7)
3343(8)
3712(8)
3284(1)
1829(1)
3268(1)
3267(2)
3272(2)
17763(1)
3192(1)
1725(1)
1955(8)
4587(10)
425(9)
3129(10)
4736(5)
1710(4)
3327(9)
3117(7)
408(5)
13(1)
10(1)
9(1)
8(1)
13(2)
12(1)
20(2)
12(1)
11(1)
10(1)
11(1)
10(1)
11(1)
13(2)
15(1)
9(1)
O(10)
O(11)
O(12)
O(13)
O(14)
3039(6)
4657(9)
1696(7)
295(8)
3280(8)
15(1)
a Ueq is defined as one third of the trace of the orthogonalized Uij
tensor.
were carried out using SHELXL-97 [11]. Full-matrix least-
squares refinement of a model based on seven Ba, six Mo
and fourteen O atoms on F2 with anisotropic atomic
displacement parameters for the Ba and Mo atoms and
occupancy factors for the Ba atoms converged to a residual
R(F20)50.065 for the 4591 reflections having F02 . 2s(F20)
with a positive residual electron density Dr56.2 e.A23
.
However, the isotropic atomic displacement parameter of
Mo4 was about three time higher than those of the other
Mo atoms. Consequently, the Mo4 position was split in
two positions Mo4 and Mo49 and their occupancy factor
were allowed to be refined. Refinement of the latter model
lowered the R value to 0.0444 and the maximum electron
density residue to 3.62 e.A23. Refinements of the oc-
cupancy factors for the Mo sites yielded values of 0.71(1)
and 0.29(1) for the Mo4 and Mo49 sites respectively and
showed that the remaining Mo sites were fully occupied.
The total occupancy number of Ba atoms corresponded to
the formula Ba|8 / 7Mo8O16 within the e.s.d. equal to 0.01.
It is interesting to note that the stoichiometry in barium
corresponds to 2q as previously observed in incommensu-
Fig. 1. Electron diffraction pattern of Ba|8 / 7Mo8O16 along [001]* (a) and
[1210]* (b) of the tetragonal unit cell. In (b) the extra spots are indexed
with a four indices notation where the fourth number is related to the
order of satellite of an intense spot of the main cell.

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N. Barrier et al. / Journal of Alloys and Compounds 317–318 (2001) 120–126
123
Table 3
a
Selected interatomic distances for Ba|8 / 7Mo8O16
Ba(1)–O(12)[l
Ba(1)–O(12)[2
Ba(1)–O(12)[3
Ba(1)–O(12)[4
Ba(1)–O(8)[5
Ba(1)–O(8)
2.710(7)
2.710(7)
2.710(7)
2.710(7)
2.807(7)
2.807(7)
2.807(7)
2.807(7)
Ba(1)–O(8)[6
Ba(1)–O(8)[7
Ba(2)–O(1)[8
Ba(2)–O(1)[9
Ba(2)–O(1)
Ba(2)–O(1)[10
Ba(2)–O(12)
Ba(2)–O(l 2)[9
Ba(2)–O(12)[10
Ba(2)–O(12)[8
2.700(8)
2.700(8)
2.700(8)
2.700(8)
2.954(8)
2.954(8)
2.954(8)
2.954(8)
Ba(2)–Ba(1)[11
3.577(6)
Ba(3)–O(1)
2.769(8)
2.769(8)
2.769(8)
2.769(8)
2.815(7)
2.815(7)
2.815(7)
2.815(7)
Ba(3)–O(1)[9
Ba(3)–O(1)[10
Ba(3)–O(1)[8
Ba(3)–O(6)[8
Ba(3)–O(6)[9
Ba(3)–O(6)[10
Ba(3)–O(6)
Ba(3)–Ba(4)
3.644(2)
Ba(4)–O(10)[1
Ba(4)–O(10)[12
Ba(4)–O(10)[13
Ba(4)–O(10)[14
Ba(4)–O(6)
Ba(4)–O(6)[9
Ba(4)–O(6)[10
Ba(4)–O(6)[8
2.715(6)
2.715(6)
2.715(6)
2.715(6)
2.817(7)
2.817(7)
2.817(7)
2.817(7)
Fig. 2. HREM image along the [001] zone showing the absence of planar
defects.
dard uncertainties are reported in Table 2. Selected inter-
atomic distances and anisotropic displacement parameters
are presented in Tables 3 and 4, respectively.
Ba(5)–O(10)[1
Ba(5)–O(10)[13
Ba(5)–O(10)[14
Ba(5)–O(10)[12
Ba(5)–O(14)[12
Ba(5)–O(14)[1
Ba(5)–O(14)[13
Ba(5)–O(14)[14
2.687(6)
2.687(6)
2.687(6)
2.687(6)
2.981(8)
2.981(8)
2.981(8)
2.981(8)
Ba(5)–Ba(6)
3.683(4)
Ba(6)–O(4)[12
Ba(6)–O(4)[1
Ba(6)–O(4)[13
Ba(6)–O(4)[14
Ba(6)–O(14)[1
Ba(6)–O(14)[13
Ba(6)–O(14)[12
Ba(6)–O(14)[14
2.734(10)
2.734(10)
2.734(10)
2.734(10)
2.755(8)
2.755(8)
2.755(8)
2.755(8)
Ba(7)–O(4)[1
Ba(7)–O(4)[13
Ba(7)–O(4)[12
Ba(7)–O(4)[14
2.509(13)
2.509(13)
2.509(13)
2.509(13)
Ba(7)–Ba(1)[12
Mo(1)–O(5)
Mo(1)–O(3)[10
Mo(1)–O(4)
3.79(4)
1.966(8)
1.972(9)
2.066(10)
Fig. 3. View of the Ba|8 / 7Mo8O16 structure along the tetragonal c-axis.
Displacement ellipsoids are drawn at the 97% of probability level.