Silver magnesium molybdate and silver cobalt molybdate: Synthesis and ionic conductivity

Article abstract of DOI:10.1134/S0036023606060076

Impedance spectroscopy, X-ray powder diffraction, and electron microscopy are used to study silver magnesium and silver cobalt molybdates of composition Ag₂A ₂ ^II (MoO₄)₃ (A = Mg, Co) and the products of their aliovalent doping by scandium(III) and vanadium(V). The double molybdates have high ionic conductivities at temperatures above 600 K. A partial aliovalent substitution of scandium for magnesium or vanadium for molybdenum increases the ionic conductivity of the molybdates below 473 K. The defect mobility and the enthalpy of defect formation in the Ag₂Mg₂(MoO₄)₃ structure are estimated proceeding from the experimental data. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0036023606060076

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2006, Vol. 51, No. 6, pp. 890–894. © Pleiades Publishing, Inc., 2006.
Original Russian Text © A.A. Il’ina, I.A. Stenina, G.V. Lysanova, A.G. Veresov, A.B. Yaroslavtsev, 2006, published in Zhurnal Neorganicheskoi Khimii, 2006, Vol. 51, No. 6,
pp. 960–965.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Silver Magnesium Molybdate and Silver Cobalt Molybdate:
Synthesis and Ionic Conductivity
a
a
a
b
a
A. A. Il’ina , I. A. Stenina , G. V. Lysanova , A. G. Veresov , and A. B. Yaroslavtsev
a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
b
Chemistry Faculty, Moscow State University, Vorob’evy gory, Moscow, 119992 Russia
Received June 27, 2005
Abstract—Impedance spectroscopy, X-ray powder diffraction, and electron microscopy are used to study sil-
ver magnesium and silver cobalt molybdates of composition Ag2AII (MoO4)3 (A = Mg, Co) and the products of
2
their aliovalent doping by scandium(III) and vanadium(V). The double molybdates have high ionic conductiv-
ities at temperatures above 600 K. A partial aliovalent substitution of scandium for magnesium or vanadium for
molybdenum increases the ionic conductivity of the molybdates below 473 K. The defect mobility and the
enthalpy of defect formation in the Ag2Mg2(MoO4)3 structure are estimated proceeding from the experimental
data.
DOI: 10.1134/S0036023606060076
The demand for alternative energy sources gives an
The goal of this work was to synthesize and investi-
impetus to the search for new functional materials with gate the ionic conductivity of Ag2Ä2(MoO4)3 double
high ionic conductivities. NASICON compounds (e.g.,
LiZr2(PO4)3) are important in this respect [1, 2]. The
NASICON structure is distinguished by large inter-
stices, which are partially populated by low-charge cat-
ions and form two types of channels [3].
molybdates withA = Mg or Co and the products of their
aliovalent doping by scandium and vanadium ions.
EXPERIMENTAL
MI2AI2I (MoO4)3 molybdates of uni- and divalent
Two routes were used to synthesize the double
molybdates. The first route was as follows. Neutral sil-
ver and divalent metal molybdates were synthesized
from molybdenum(VI) oxide (Acros, 99%), reagent
grade silver nitrate, high-purity grade magnesium car-
bonate, and pure grade basic cobalt carbonate. Blends
of the reagents were stirred in an agate mortar and
annealed at 523–723 K for Ag2MoO4 and 523–973 K
for MgMoO4 and CoMoO4, with the temperature ele-
vated in 50-K steps. Annealing at each intermediate
temperature lasted 10–15 h and was followed by stir-
ring. In addition, Ag2MoO4 was prepared from solution
by the exchange reaction
metals may be classified with the NASICON family
[4]. The low charge, high polarizability of the electron
shell, and cation radius approaching 1 Å allow us to
classify silver as a “magic ion,” whose compounds typ-
ically have high ionic conductivities [2].
Silver zinc molybdate Ag2Zn2(MoO4)3, prepared in
[5], was found to have the Na2Mg5(MoO4)6 type struc-
ture [6]. Na2Mg5(MoO4)6 crystallizes in triclinic space
group P1. The Ag2A2(MoO4)3 (A = Mg, Mn, Co) dou-
ble molybdates are isostructural to zinc silver molyb-
date [7]. Their structure is a three-dimensional frame-
work, formed by MoO4 tetrahedra and distorted AO6
octahedra with average A–O distances of 2.081–
2.085 Å for the magnesium compound and 1.986–
2.136 Å for the cobalt compound [7]. The silver cations
reside in interstices which form channels in the frame-
work (Fig. 1). The channel size increases with the
radius of the divalent element. Site splitting is observed
2AgNO3 + Na2MoO4
Ag2MoO4↓ + 2NaNO3. (1)
Both compounds were dissolved in hot water and com-
bined under thorough stirring. The resulting precipitate
was repeatedly washed with water by decantation until
sodium cations were fully removed and then centri-
fuged.
for Ag(I)
Ag(1A) + Ag(1B) [7], which is intrinsic
The Ag2A2(MoO4)3 (A = Mg, Co) molybdates were
prepared according to [7] from silver molybdate and
divalent metal molybdate. A stoichiometric blend of the
components was stirred in an agate mortar and
annealed at 623–773 K with temperature elevation in
50-K steps, exposures at each temperature for 10–15 h,
to NASICON compounds [3]. It was also reported
in [7] that Ag2Mg2(MoO4)3 experiences a reversible
polymorphic transition at 773–803 K and that it melts
incongruently at 1013 K; Ag2Co2(MoO4)3 melts incon-
gruently at 900 K.
890

SILVER MAGNESIUM MOLYBDATE AND SILVER COBALT MOLYBDATE
891
b
c
a
Fig. 1. Structure of Ag A (MoO ) (A = Mg, Mn, Co) double molybdates.
2
2
4 3
and intermittent stirring. The individual double molyb- heating rate was 10 K/min. The sample sizes were 0.3–
dates were obtained as a result.
0.5 g for the derivatograph and 10–20 mg for the ther-
mobalance.
The conductivity was measured on cylindrical com-
pacted pellets (R = 2.5 mm, h = 3 mm) with platinum
leads from 298 to 773 K in 10-K steps using an IPU-
p.62 ac bridge. The conductivity was found by extrapo-
lating a semicircle to the ohmic resistance axis.
Figures 2a and 2b are micrographs of samples pre-
pared using the second route. These samples are homo-
geneous crystalline masses with grain sizes of 1–2 mm
for Ag2Mg2(MoO4)3 and 2–6 mm for Ag2Co2(MoO4)3.
The electron probe shows the uniform distributions of
molybdenum and the divalent metals; the cation ratio is
as in the batch.
The above-described synthesis route requires a long
time. Therefore, we used an alternative route to prepare
the double molybdates, directly from molybdenum(VI)
oxide, divalent metal carbonates, and silver(I) nitrate. A
thoroughly stirred stoichiometric blend of the reagents
was annealed using a similar scheme at 473–773 K with
intermittent stirring, with the intermediate exposures
shortened to 2–3 h.
The same scheme was used to synthesize silver
magnesium molybdate samples in which magnesium
ions were partially replaced by scandium or molybde-
num ions were partially replaced by vanadium:
(Ag2 − xMg2 – xScx(MoO4)3 and Ag2 + 3xMg2(Mo1 – xVxO4)3
(x = 0.01). The dopants used were reagent grade scan-
dium oxide and high-purity grade vanadium oxide.
X-ray powder diffraction shows thatAg2MoO4 crys-
tallizes in the cubic system (a = 9.305(3) Å). However,
X-ray powder diffraction analysis was carried out the compound prepared by the solid-phase reaction
using a Guinier FR-552 camera (CuKα1 radiation, a ger- contained some impurities. Therefore, we used the
manium standard). X-ray diffraction patterns were
scanned and digitized using specially designed soft- thesis of the double molybdates. Magnesium and cobalt
Ag2MoO4 sample deposited from solution in the syn-
ware.
Microstructures were observed with a LEO Supra
50 VP scanning electron microscope. The distribution
of the chemical elements was studied on a Jeol JSM-
840A scanning electron microscope equipped with a
PGT IMIX attachment for electron probe micro-
analysis.
Thermal analysis was performed on a Paulik-Pau-
lik-Erdey Q-1500 D derivatograph and a Netzsch TG
209 F1 thermobalance using platinum crucibles. The
molybdates are monoclinic; their refined unit cell
parameters are a = 10.27(1) Å, b = 9.280(5) Å, c =
7.019(5) Å, β = 106.88(8)° and a = 4.655(8) Å, b =
5.680(6) Å, c = 4.911(4) Å, β = 89.511(9)°, respec-
tively.
Silver magnesium molybdate and silver cobalt
molybdate are triclinic. Their X-ray diffraction patterns
were indexed using the unit cell parameters from [7]
(space group P1). The products of aliovalent doping of
Ag2Mg2(MoO4)3 by scandium and vanadium ions have
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 6 2006

892
IL’INA et al.
(a)
(b)
Mag = 50.00 K X
1 µm EHT = 5.00 kV Signal A = InLens MSU HSMS
WD = 2 mm Photo No. = 8824 25 May 2005
Mag = 5.00 K X
10 µm EHT = 5.00 kV Signal A = InLens MSU HSMS
WD = 2 mm
Photo No. = 8777 24 May 2005
(c)
(d)
Mag = 50.00 K X
1 µm EHT = 5.00 kV Signal A = InLens MSU HSMS
WD = 2 mm Photo No. = 8827 25 May 2005
Mag = 5.00 K X
10 µm EHT = 5.00 kV Signal A = InLens MSU HSMS
WD = 2 mm
Photo No. = 8785 24 May 2005
Fig. 2. Micrographs of (a, b) Ag Mg (MoO ) and (c, d) Ag Co (MoO ) : (a, c) intact samples and (b, d) discs annealed at 773 K.
2
2
4 3
2
2
4 3
similar sets of reflections with slightly different unit conductivity is most likely due to improved grain con-
cell parameters; therefore, we may say that solid solu- tacts.
tions are formed.
Thus, pellets of the double molybdates are sintered
at high temperatures. Accordingly, the ionic conductiv-
ity was then measured on samples presintered at 773 K.
Compacted materials noticeably change their con-
ductivity upon sintering. The conductivities of the sam-
ples in the range 350–550 K during the second heating
are 1.5–2 orders of magnitude higher than the values
measured during the first heating. Subsequent heat-
ing/cooling cycles almost do not change the conductiv-
ity. The micrographs of the fracture of pellets obtained
after preheating to 773 K show that the microstructure
has noticeably changed during the annealing: single
crystals are aggregated, and their edges are smoothed
(Figs. 2b, 2d). Most likely, incipient (surface) melting
occurs, although melting per se occurs at higher tem-
peratures as shown by DTA: for Ag2Mg2(MoO4)3 at
~950 K (the half-width of the DTA peak is 80 K). The
enhanced growth in conductivity of Ag2Mg2(MoO4)3
Figure 4 displays the ionic conductivity σ versus
reciprocal
temperature
for
preannealed
Ag2Mg2(MoO4)3 and Ag2Co2(MoO4)3 samples in the
coordinates of the Frenkel equation:
σT = Aexp (–Ea/RT).
(2)
Here A is the frequency factor, Ea is the activation
energy of conductivity, T is temperature in Kelvin
degrees, and R is the universal gas constant. The calcu-
lated values of the activation energy and frequency fac-
tor for these samples are listed in the table. The magne-
crystals at high temperatures (Fig. 4, curve a) also pro- sium compound has a high activation energy of conduc-
vides an argument in favor of the possibility of incipi- tivity, likely because of the smaller sizes of the
ent, surface melting. The density of the substance dur- interstices in which silver ions reside and the bordering
ing the annealing changes only insignificantly, increas- faces of coordination polyhedra that silver(I) ions have
ing from 92
1 by 0.5–0.7%. The change in the to overcome when migrating.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 6 2006

SILVER MAGNESIUM MOLYBDATE AND SILVER COBALT MOLYBDATE
893
I
a
b
10
20
30
40
50
60
2θ, deg
Fig. 3. X-ray diffraction patterns for (a) Ag Mg (MoO ) and (b) Ag Co (MoO ) .
2
2
4 3
2
2
4 3
Aliovalent doping appreciably increases the ionic tive charge of the particle relative to its position in the
conductivity at low temperatures (below 473 K). At
higher temperatures, the conductivities of doped mate-
rials nearly coincide with the conductivity of undoped
Ag2Mg2(MoO4)3 (Fig. 5). This is because of doping-
induced defect generation. Dopant scandium ions enter
lattice. Accordingly, dopant vanadium ions enter
'
molybdenum positions (VMo), and extra silver ions
*
enter interstices (Agi ). At low dopant levels, the defect
concentration may roughly be equated to the dopant ion
concentration and the activation energy of conductivity
equals the activation energy of mobility for defects
(Em). From the table, it follows that the activation
energy of migration through vacancies for silver ions is
slightly higher than the activation energy of migration
through interstices. The frequency factor is dramati-
*
magnesium positions (ScMg), with silver vacancies
'
(VAg ) generated in order for the electroneutrality to
hold. Here, in accordance with the Kröger and Wink
symbolism [8], subscripts indicate the structural posi-
tion in which the particle is situated and superscripts
“*” and “ ' ”, respectively, denote the positive or nega- cally reduced since the transport is controlled by the
Activation energies of conductivity for the Ag2A2(MoO4)3 (A = Mg, Co) double molybdates
Compound
Ag2Mg2(MoO4)3
Temperature range, K
Ea, kJ/mol
79
62.4 0.6
77
18.5 0.9
A, S K/cm
773–300
773–300
773–473
473–297
773–473
473–295
1
(1.2 0.3) × 105
(1.1 0.1) × 104
(4.7 0.9) × 104
(1.3 0.3) × 10–2
(3.6 0.8) × 104
(2.1 0.8) × 10–3
Ag2Co2(MoO4)3
Ag1.99Mg1.99Sc0.01(MoO4)3
1
Ag2.03Mg2(Mo0.99V0.01O4)3
74
13
1
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 6 2006

894
logσT [S K/cm]
IL’INA et al.
At high temperatures, intrinsic disorder dominates:
0(ideal Ag2Mg2(MoO4)3 crystal)
0
–1
–2
–3
–4
–5
(3)
'
*
⇔ VAg + Agi .
The activation energy of conductivity equals Em
+
1/2∆H, where ∆H is the enthalpy of defect formation by
reaction (2); for Ag2Mg2(MoO4)3, ∆H is calculated to
be 120 8 kJ/mol.
In summary, the Ag2AI2I (MoO4)3 (A = Mg, Co)
b
compounds have relatively high ionic conductivities
above 600 K; they are distinguished by high silver ion
mobilities in migration through both vacancies and
interstices. The high enthalpy of defect formation
(120 8 kJ/mol) is the major factor inhibiting a high
ionic conductivity at low temperatures. We have shown
that the directed defect generation in the structure
through aliovalent doping is a way to solve the
problem.
a
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
1000/T, ä–1
Fig. 4. Ionic conductivity vs. reciprocal temperature plots in
the rectifying coordinates of the Frenkel equation for
(a) Ag Mg (MoO ) and (b) Ag Co (MoO ) .
2
2
4 3
2
2
4 3
ACKNOWLEDGMENTS
logσT [S K/cm]
This work was in part supported by the Russian Foun-
dation for Basic Research (project no. 03-03-32439) and
the Complex Program of the RAS.
0
–1
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–2
–3
–4
–5
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Fig. 5. Ionic conductivity vs. reciprocal temperature plot in
the rectifying coordinates of the Frenkel equation for
Ag Mg (Mo
V
O ) .
2.03
2
0.99 0.01 4 3
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concentration of inserted defects [2], which is far lower
than the overall silver ion concentration. The calculated
conductivity values for silver interstitials and vacancies
at 300 K are 3.5 × 10–4 and 6 × 10–4 cm2/(V s), res-
pectively.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 6 2006