Neutron diffraction study of the crystal structure of BaMoO4: A suitable precursor for metallic BaMoO3 perovskite

Article abstract of DOI:10.1006/jssc.1999.8352

BaMoO₃, metallic and Pauli paramagnetic, has been prepared by controlled reduction of BaMoO₄. This precursor, containing Mo(VI), is unusually stable against reduction, due to structural factors. The crystal structure of BaMoO₄ has been refined from neutron powder diffraction data: space group I4₁/a (no. 88), Z=4, a=5.5479(9), and c=12.743(2) A. A bond-valence study allowed us to detect the presence of slight tensile and compressive stresses in the crystal structure of BaMoO₄, in which Ba is overbonded and Mo is underbonded. However, this effect is less pronounced than in other AMO₄ oxides with a scheelite structure (A=Ca, Sr, Ba; M=Mo, W): BaMoO₄ contains the M cation exhibiting the closest valence to the nominal value of 6+, suggesting a large covalent contribution to the Mo-O bonds. This observation is coherent with the large thermal stability of this compound against reduction, taking place at temperatures above 920°C in H₂ flow.

Full text of DOI:10.1006/jssc.1999.8352

Journal of Solid State Chemistry 146, 266}270 (1999)
Article ID jssc.1999.8352, available online at http://www.idealibrary.com on
Neutron Diffraction Study of the Crystal Structure of BaMoO4:
A Suitable Precursor for Metallic BaMoO Perovskite
3
Vivian Nassif and RauH l E. Carbonio
Instituto de Investigaciones en Fisicoqun
H
mica de Co
H
rdoba (INFIQC), Departamento de Fn
Hs ico QunHmica, Facultad de Ciencias QunHmicas,
Universidad Nacional de Co
H
rdoba, Ciudad Universitaria, 5000 C o& rdobo, Argentina
and
Jose
H
A. Alonsoꢀ
Instituto de Ciencia de Materiales de Madrid, C.S.I.C., Cantoblanco, 28049 Madrid, Spain
Received February 1, 1999; accepted April 22, 1999
ature electronic conductivities of these compounds have
BaMoO
3
, metallic and Pauli paramagnetic, has been pre- been studied in the past: BaMoO and SrMoO are cubic
ꢁ
ꢁ
pared by controlled reduction of BaMoO
taining Mo(VI), is unusually stable against reduction, due to
structural factors. The crystal structure of BaMoO has been
re5ned from neutron powder di4raction data: space group I4 /a
no. 88), Z ؍ 4, a ؍ 5.5479(9), and c ؍ 12.743(2) A. A bond-
valence study allowed us to detect the presence of slight tensile
and compressive stresses in the crystal structure of BaMoO , in
which Ba is overbonded and Mo is underbonded. However, this
e4ect is less pronounced than in other AMO oxides with
a scheelite structure (A ؍ Ca, Sr, Ba; M ؍ Mo, W): BaMoO
4
. This precursor, con- perovskites showing metallic conductivity and Pauli para-
magnetism (3}5).
4
In our current research on the electronic properties of Vb
and VIb transition-metal oxides in intermediate oxidation
states (e.g., Nb(IV), Mo(IV)), we are interested in the prep-
aration and study of hole and electron-doped compounds,
1
(
s
4
Ba
A MoO (e.g., Aꢁ>"La; A>"K). As a preliminary
ꢀ
\V V
ꢁ
study, we prepared BaMoO as a reduction product of
BaMoO . The thermal stability under reducing conditions
4
ꢁ
4
ꢂ
contains the M cation exhibiting the closest valence to the nom- of this precursor containing Mo(IV) is surprisingly high (6):
inal value of 6؉, suggesting a large covalent contribution to the it only takes place at O pressures below log P(O )"
ꢃ
ꢃ
Mo+O bonds. This observation is coherent with the large thermal !13.7 at 12003C. The microscopic origin of the observed
stability of this compound against reduction, taking place at
temperatures above 9203C in H
high stability was uncertain, due to the lack of precise
structural information on the precursor material, barium
2
6ow. ( 1999 Academic Press
Key Words: neutron powder di4raction; crystal structure;
thermal analysis.
molibdate, BaMoO .
ꢂ
It is well known that BaMoO adopts a scheelite-like
ꢂ
structure (7). In the scheelite structure, AMO , which can be
ꢂ
considered as a superstructure of CaF #ourite, the two
ꢃ
INTRODUCTION
kinds of metal atoms are ordered along the c axis of the
tetragonal unit cell. The crystal structure is typi"ed by
CaWO and can be described in the space group I4 /a (no.
Twenty years ago, Goodenough established a scheme and
gave a criterion for localized vs collective d-electron behav-
ꢂ
ꢀ
8
8). Both metal atoms are placed at special positions, where-
iors in transition-metal oxides, ABO , with a perovskite
ꢁ
as O anions are at general positions, 16 f (x, y, z). Most of
structure (1, 2). According to this criterion, AMoO
A"Ca, Sr, Ba) have so su$cient electron-transfer energies
as to screen and cancel the electrostatic energy accompanied
ꢁ
the isostructural AMO ternary oxides (A"Ca, Sr, Ba;
(
ꢂ
M"Mo, W), except BaMoO , have been structurally re-
ꢂ
"
ned from neutron di!raction data, allowing the precise
by the electron transfer. Thus, AMoO can be classi"ed into
ꢁ
determination of oxygen positions [8}10]. The most recent
the superconducting possible materials. The low-temper-
determination (11) of BaMoO , from 1970, is basically in-
ꢂ
correct.
The lack of good structural data for BaMoO led us to
undertake a neutron di!raction study of this compound and
ꢀ
To whom correspondence should be addressed. Fax: 34 91 372 0623.
ꢂ
E-mail: ja.alonso@icmm.csic.esI.
PACS numbers: 61.12Ld, 61.66.Fn, 74.10#V.
to establish a comparison with other related molibdates and
2
66
0
022-4596/99 $30.00
Copyright ( 1999 by Academic Press
All rights of reproduction in any form reserved.

CRYSTAL STRUCTURE OF BaMoO FROM NEUTRON DIFFRACTION DATA
267
ꢂ
tungstates. The reduction process to give metallic BaMoO , factors for Ba and anisotropic for Mo and O, and unit-cell
ꢁ
studied by TG techniques, is also reported.
parameters.
RESULTS AND DISCUSSION
EXPERIMENTAL
Thermal Analysis under Reducing Conditions of BaMoO
ꢂ
White polycrystalline BaMoO was prepared according
to the following reaction:
ꢂ
The thermal behavior of BaMoO in a reducing H #ow
ꢂ
ꢃ
is illustrated in Fig. 1. The sample is stable up to 9203C;
beyond this temperature, the TG curve shows a reduction
process, which can be completed by isothermal heatings
(
NH ) MoO (aq)#BaCl (aq)
ꢂ ꢃ
ꢂ
ꢃ
above 10003C. After this process, BaMoO can be identi"ed
ꢁ
NH OH(aq,pH"10)
ꢂ
as a major reduction product by XRD. The observed weight
&
& && &&&" BaMoO (s)#2(NH )Cl(aq).
[1]
ꢂ
ꢂ
loss for this reduction agrees with the calculated weight loss
for the equation
Aqueous solutions of analytical grade BaCl and
ꢃ
(
NH ) MoO , 1 M each, were prepared separately.
BaMoO #H PBaMoO #H O.
[2]
ꢂ ꢃ
ꢂ
ꢂ
ꢃ
ꢁ
ꢃ
NH OH was added to the molibdate solution, to reach
pH"10. BaCl solution was poured slowly with constant
stirring into the (NH ) MoO solution. The precipitate was
digested for 48 h at 603C in the mother solution, to improve
its crystallinity. The resultant precipitate was "ltered,
washed thoroughly with distilled water, and dried in a vac-
uum oven at 1103C and then in an oven at 6003C in air for
4 h. BaMoO was prepared as a dark red colored powder
by heating BaMoO at 11503C in an H /N (5%:95%)
ꢂ
ꢃ
Structural Study
BaMoO and BaMoO were obtained as well-crystal-
ꢂ ꢃ
ꢂ
ꢂ
ꢁ
lized powders, whose XRD diagrams are characteristic of
sheelite and perovskite structures, respectively. Figure 2
shows the indexed XRD diagrams for both compounds. The
2
structural re"nement from NPD data of BaMoO was
performed by the Rietveld method, taking as the starting
ꢁ
ꢂ
ꢂ
ꢃ
ꢃ
#
ow, for 12 h. The reduction process was followed by ther-
parameters those of BaWO (9). The "nal atomic coordi-
ꢂ
mal analysis using a Mettler TA3000 system equipped with
a TC10 processor unit. Thermogravimetric (TG) curves
were obtained in a TG50 unit, working at a heating rate of
nates, unit-cell parameters, and discrepancy factors after the
re"nement are included in Table 1. Figure 3 shows the good
agreement between observed and calculated NPD pro"les
5
5
3C min\ꢀ, in a reducing H #ow of 0.3 l min\ꢀ. About
for BaMoO .
ꢃ
ꢂ
0 mg of the sample was used in the experiment.
The XRD pattern of BaMoO was re"ned in a simple
ꢁ
The products (BaMoO , BaMoO ) were characterized by
cubic perovskite unit cell, with edge a"4.0448(2) A
s
and all
ꢂ
ꢁ
X-ray di!raction (XRD) using CuKa radiation, in a Siemens
D-501 goniometer controlled by a DACO-MP computer,
by step scanning from 10 to 1003 in 2h, in increments of
the atoms placed at special positions. Since small amounts
of Mo Metal and unreacted BaMoO were detected as
ꢂ
impurity phases, they were included in a "nal multiphase
0
.053 and a counting time of 4 s each step.
The neutron powder di!raction (NPD) pattern of
BaMoO was collected at room temperature in the multi-
ꢂ
detector DN5 di!ractometer at the Siloe
Centre d'Etudes Nucleaires, Grenoble. A wavelength of
.345 A was selected from a Cu monochromator. The 800
H reactor of the
H
1
s
detectors covered a 2h range of 803, from 2h "123. The
counting time was about 4 h, using 10 g of sample contained
in a vanadium can.
G
NPD and XRD patterns were analyzed by the Rietveld
(12) method, using the FULLPROF program (13). The
lineshape of the di!raction peaks was generated by a
pseudo-Voigt function and the background re"ned to a
"
fth-degree polynomial. In the neutron re"nement, the co-
herent scattering lengths for Ba, Mo, and O were, respec-
tively, 5.07, 6.72, and 5.805 fm. In the "nal run, the following
parameters were re"ned: background coe$cients, zero-
point, half-width, pseudo-Voigt, and asymmetry parameters
FIG. 1. TG curve for BaMoO , obtained in a reducing H #ow. The
ꢂ
ꢃ
complete reduction to give BaMoO corresponds to a weight loss of
ꢁ
for the peak shape; scale factor, positional, thermal isotropic 5.38%.

2
68
NASSIF, CARBONIO, AND ALONSO
FIG. 2. XDR patterns of (a) BaMoO , indexed on the basis of a tetra-
ꢂ
gonal unit cell with a"5.5479(9) and c"12.743(2) A
s
, and (b) BaMoO ,
FIG. 3. Observed (crosses), calculated (solid line), and di!erence (at the
ꢁ
cubic with a"4.0448(2) A
s
.
bottom) neutron di!raction pro"les for BaMoO at 295 K. The tick marks
ꢂ
indicate the positions of the allowed Bragg re#ections.
re"nement. No additional superstructure re#ections or
splitting of the peaks were detected, excluding the departure valences around this cation (anion). This rule is satis"ed
of the Pm3m symmetry. Figure 4 shows the goodness of the only if the stress introduced by the coexistence of di!erent
t for BaMoO XRD pro"les.
structural units can be relieved by the existence of enough
Table 2 contains a selected list of distances and angles for degrees of freedom in the crystallographic structure. The
M
"
ꢁ
BaMoO , and a view of the structure is shown in Fig. 5. departure of the BVS rule is a measure of the existing stress
ꢂ
Baꢃ> cations are coordinated to eight oxygens placed in the in the bonds of the structure.
corners of distorted cubes, better described as scalenohedra.
Table 3 lists the valences calculated for Ba, Mo, and
The oxygen coordination polyhedra of Moꢄ> cations are O from the individual Ba}O and Mo}O distances of
slightly distorted tetrahedra, showing O}Mo}O angles of Table 2 for BaMoO . The valence of the Ba cation is slightly
ꢂ
1
08.3 and 111.83.
higher than the expected value of #2; in compensation, the
The valence of the cations and anions present in the solid valence of Mo atom is slightly lower than #6. This result
can be estimated by means of the Brown's bond valence suggests that Ba atoms are overbonded while Mo are under-
model (14, 15). This model gives a phenomenological rela- bonded in this structure; in other words, Ba}O bonds are,
tionship between the formal valence of a bond and the
corresponding bond length. In perfect nonstrained struc-
tures, the bond valence sum (BVS) rule states that the formal
charge of the cation (anion) is equal to the sum of the bond
TABLE 1
Crystallographic Parameters for BaMoO
Re5nement of NPD Data at 295 K (Space Group I4
Z ؍ 4, a ؍ 5.5479(9), and c ؍ 12.743(2) A
4
, after the Pro5le
1
/a (No. 88),
s
)
Atom
Site
x
y
z
s
B%ꢀ ( A)
Ba
Mo
O
4b
4a
16 f
0
0
ꢀ
ꢅ
ꢆ
ꢀ
ꢆ
0.27(9)
0.90(9)?
1.12(6)?
ꢂ
ꢀ
ꢂ
0.2362(5)
0.1331(4)
0.0473(2)
Note. R factors: R "1.60, Rꢂꢁ"2.15, R%ꢃꢁ"1.04%, Xꢃ"4.24, and
ꢁ
R "3.50%.
'
?
Anisotropic thermal factors: for Mo,
b
"b "0.0099(6), and
ꢀ ꢃꢃ
FIG. 4. Observed, calculated, and di!erence XRD pro"les for
ꢀ
b
b
"0.0015(2); for O, bꢀꢀ"0.0163(6), b "0.0076(5), bꢁꢁ"0.0013(1), BaMoO . The second and third series of tick marks correspond to minor
ꢁ
ꢁ
ꢃ
ꢃꢃ
ꢁ
"0.012(1), bꢀꢁ"0.0006(3), and bꢃꢁ"0.0013(4).
BaMoO and Mo impurity phases.
ꢀ
ꢂ

CRYSTAL STRUCTURE OF BaMoO FROM NEUTRON DIFFRACTION DATA
269
ꢂ
TABLE 2
Selected Interatomic Distances (A) and Angles ( 3 ) in BaMoO
TABLE 3
) for Ba+O and Mo+O Bonds, Multiplicity
of the Bonds [m], and Valences (+s ) for Ba, Mo, and O Atoms
s
4
Bond Valences (s
i
i
BaO scalenohedron
2.764(3)
2.718(3)
within the Respective Coordination Polyhedra in the BaMoO
Structure
4
ꢆ
Ba}O
Ba}O
;4
;4
O}Ba}O
O}Ba}O
O}Ba}O
O}Ba}O
O}Ba}O
O}Ba}O
O}Ba}O
O}Ba}O
75.5(1)
72.2(1)
67.0(1)
79.2(1)
146.1(2)
130.8(2)
138.0(2)
97.4(1)
Atom
s , [m]
+s
G
G
Ba
Mo
O
0.278(2), [4] 0.315(2), [4]
1.47(1), [4]
2.372(6)
5.88(2)
2.06(1)
0.278(2), [1] 0.315(2), [1] 1.47(1), [1]
Note. The valence is the sum of the individual bond valences (s ) for
G
MoO tetrahedron
1.765(3)
Ba}O and Mo}O bonds. Bond valences are calculated as s "
ꢂ
G
Mo}O
;4
O}Mo}O
O}Mo}O
111.8(2)
108.3(2)
exp[(r !r )/B]; B"0.37; r (Ba)"2.29, r (Mo)"1.907 A
s
for the
ꢇ
G
ꢇ
ꢇ
Baꢃ>}Oꢃ\ and Moꢄ>}Oꢃ\ pairs, respectively (from Ref. (15)). Individual
Ba}O and Mo}O distances are taken from Table 2.
transition-metal cation exhibits the closest valence to the
on average, under compressive stress and Mo}O bonds are
under tensile stress, giving rise to a structure with a slight
metastable character. The large thermal factor of Mo
expected value of 6#.
In the AMoO series (A"Ca, Sr, Ba) this fact can be
ꢂ
understood, taking into account the contribution of
(
B "0.90(9) Aꢃ) compared with that of Ba (0.27(9) Aꢃ)
s
s
%ꢀ
covalent bonding to the strength of Mo}O bonds: A and
Mo cations compete against each other for the electron
cloud of Oꢃ\ through their Coulombic potential Zeꢃ/r (Z,
valence; r, ionic radius); Aꢃ> cations with a lower Coulom-
bic potential enables an increase the overlap of electron
clouds between Mo and O ions, giving rise to a stronger
covalent bonding: the highest valence is expected in the
ternary oxide with the largest Aꢃ> cation, in this case Baꢃ>.
For a given Aꢃ> cation, Mo}O distances are systemati-
cally shorter than W}O bond lengths, corresponding to the
smaller ionic size of Mo cations in the nominal oxidation
seems to be related to the underbonded character of the
former.
It is interesting to compare BaMoO with other related
ꢂ
oxides with a scheelite structure, AMO , where A"Ca, Sr,
ꢂ
Ba and M"W, Mo, from the point of view of the cation
valences in the corresponding oxygen coordination poly-
hedra. In Table 4, the common feature for all of the studied
oxides is the underbonded character of M cations. However,
it is striking to note that BaMoO is the oxide in which the
ꢂ
state of 6# (Shannon's (16) ionic radii: Moꢄ>, 0.41 A
s
;
Wꢄ>, 0.42 A
s
, in tetrahedral coordination). The higher val-
ence of Mo compounds is also related to the stronger
Coulombic potential of Moꢄ> cations with respect to larger
Wꢄ> ions, giving rise in the former case to stronger covalent
bondings to oxygen.
TABLE 4
Valences (+s
A, M, and O within the AO
AMO Oxides with a Scheelite Structure (A ؍ Ca, Sr, Ba;
M؍ Mo, W)
i
) Determined from the Bond Valence Model for
8
and MO Coordination Polyhedra in
4
4
AMO
CaWO CaMoO SrWO SrMoO BaWO BaMoO
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
Ref.
(9)
(9)
(8)
(8)
(8)
This work
A}O (;4) 2.478(5) 2.465(3) 2.610(3) 2.610(4) 2.778(3) 2.764(3)
A}O (;4) 2.441(5) 2.451(3) 2.580(3) 2.583(4) 2.738(3) 2.717(3)
M}O (;4) 1.782(5) 1.771(3) 1.779(3) 1.767(4) 1.782(3) 1.765(3)
+
+
+
s (A)
2.12(1)
5.77(4)
1.97(2)
2.122(7) 2.208(5) 2.194(9) 2.261(6) 2.372(6)
G
s (M)
5.78(3)
1.98(1)
5.81(2)
2.01(1)
5.84(3)
2.01(2)
5.76(2)
2.01(1)
5.88(2)
2.06(1)
G
s (O)
G
FIG. 5. View of the BaMoO structure, along the [100] direction, with
ꢂ
c axis vertical. Tetrahedra correspond to MoO units. Large spheres are
Note. r values for Caꢃ>, Srꢃ>, and Wꢄ> are, respectively, 1.967, 2.118,
ꢂ
ꢇ
Baꢃ> cations. The coordination of peripheral Baꢃ> cations is incomplete.
and 1.921 from Ref. (15).

2
70
NASSIF, CARBONIO, AND ALONSO
funds to a Proyecto de Cooperacio
the MDN group at the CEN}Grenoble for their hospitality and
for the facilities at the Siloe reactor. R.E.C. thanks Fundacio
Antorchas, SECYT-UNC, CONICOR, ANPCYT, and CONICET for
research grants.
H n Iberoamericana. They also thank
This picture, in which the observed valence of M cations
leads to a relative estimate of the strength of the covalent
H
H n
bondings in the MO polyhedra, is complementary to the
ꢄ
results reported by Kamata et al. (6) about the thermal
stability of the series of alkaline-earth molibdates, AMoO :
ꢂ
the high valence state of Mo in these ternary oxides becomes
REFERENCES
more durable against reduction as the Aꢃ> radius is larger.
In the present work, a precise determination of the oxygen
1
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ꢂ
us to understand the microscopic origin of this behavior.
3
. L. H. Brixner, J. Inorg. Nucl. Chem. 14, 225 (1960).
CONCLUSIONS
4. G. H. Bouchard and M. J. Sienko, Inorg. Chem. 7, 441 (1968).
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6
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(
Neutron di!raction data reveal that although Mo(VI)
cations are slightly underbonded in the BaMoO crystal
(
ꢂ
structure, they form stronger covalent bondings to oxygen
than any of the isostructural AMO oxides (A"Ca, Sr, Ba;
ꢂ
8
9
. E. Guermen, E. Daniels, and J. S. King, J. Chem. Phys. 55, 1093 (1971).
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ꢂ
other AMoO oxides with a scheelite structure. Thermal
1
0. G. Wandal and A. Norlund-Christensen, Acta Chem. Scand. 41, 358
1987).
ꢂ
ꢃ
analysis in H #ow shows that the reduction process to give
(
the cubic perovskite BaMoO , containing Mo(IV), takes 11. T. I. Bylichkina, L. I. Soleva, E. A. Pobedimskaya, M. A. Porai-
ꢁ
Koshits, and N. V. Belov, Kristallogra,ya 15, 165 (1970).
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1
1
1
2. H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969).
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H
ACKNOWLEDGMENTS
A. Navrotsky, Eds.), Vol. 2, p. 1. Academic Press, New York, 1981.
The authors acknowledge the "nancial support of the DGICYT to the 15. N. E. Brese and M. O'Keefe, Acta Crystallogr., Sect. B 47, 192 (1991).
Project PB97-1181 and the spanish Ministerio of Educacion y Cultura for 16. R. D. Shannon, Acta Crystallogr., Sect. A 32, 751 (1976).