Ag1 - X Mg1 - X R1 + x (MoO 4)3 Ag+-conducting NASICON-like phases, where R = Al or Sc and 0 ≤ x ≤ 0.5

Article abstract of DOI:10.1134/S0036023611080122

Ag_{1 - x} Mg_{1 - x} R_{1 + x} (MoO ₄)₃ NASICON-like solid solutions, where R = Al or Sc and 0 ≤ x ≤ 0.5, were prepared; their crystal lattice parameters and thermal stabilities were determined. Silver-ion co

Full text of DOI:10.1134/S0036023611080122

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 8, pp. 1189–1193. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © I.Yu. Kotova, D.A. Belov, S.Yu. Stefanovich, 2011, published in Zhurnal Neorganicheskoi Khimii, 2011, Vol. 56, No. 8, pp. 1259–1263.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Ag Mg
R (MoO ) Ag+ꢀConducting NASICONꢀLike
1 + x 4 3
1 –
x
1 –
x
Phases, where R = Al or Sc and
0
≤
x 0.5
≤
I. Yu. Kotovaa, D. A. Belovb, and S. Yu. Stefanovichb
a Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences, ul. Sakh"yanovoi 8, UlanꢀUde,
Buryat Republic, 670047 Russia
b Moscow State University, Moscow, Russia
Received September 4, 2009
Abstract
were prepared; their crystal lattice parameters and thermal stabilities were determined. Silverꢀion conductivꢀ
ity was measured, and conductivity activation energy values were calculated for various temperature ranges.
Above 400°C Ag1 – xMg1 – xR1 + x(MoO4)3 phases have ionic conductivities comparable to the conductivities
—
Ag1 – xMg1 – xR1 + x(MoO4)3 NASICONꢀlike solid solutions, where R = Al or Sc and
0
≤
x
≤
0.5,
,
of sodiumꢀion and lithiumꢀion NASICONꢀlike conductors. The conductivity increases as the tervalent catꢀ
ion radius increases or the amount of mobile silver ions increases.
DOI: 10.1134/S0036023611080122
Progress in many fields of science and engineering grade), Al(NO3)3
⋅
9H2O (chemically pure grade),
is controlled by the development of novel ionꢀconducꢀ Sc2O3 (chemically pure grade), and molybdenum(VI)
tive materials and expansion of their application in oxide (chemically pure grade) taken stoichiometric
solidꢀphase electrochemical devices [1–5].
proportions by stepped annealing at 350–450
(Ag2MoO4), 400–750 С (MgMoO4), 300–700
(Al2(MoO4)3), and 400–800 С (Sc2(MoO4)3). To
prepare samples of Ag1 – xMg1 – xR1 + x(MoO4)3 solid
solutions where R = Al or Sc, stoichiometric amounts
of the components appropriate for samples where x =
0.1, 0.2, 0.3, 0.4, 0.5, 0.6, or 0.7 were carefully stirred
in an agate mortar and then annealed in air starting
°
С
С
°
°
NASICONꢀlike compounds MnR2(MoО4)3, where
M = Li or Na; R stands for a III–V Group element,
and n = 1–4, are known to be efficient singlyꢀchargeꢀ
°
cation ionic conductors [6–9]. The high mobility of
moderateꢀsized alkali cations here is due to the geoꢀ
metrical opportunity of their transport through a
rhombohedral crystal framework built of cornerꢀsharꢀ
ing MoО6 octahedra and RО4 tetrahedra. The distribuꢀ
tion of singly charged cations to allowed positions
changes with rising temperature toward a more uniꢀ
form occupation of all extraframework positions; in
some NASICONꢀlike compounds, this is manifested
as phase transitions [10, 11]. The ability of
with 350
°
С
in 50 С steps with intermittent homogeniꢀ
°
zation every 20–30 h. Depending on the composition
and the nature of the R3+ cation, the limiting annealꢀ
ing temperature was within 550–600
°
С for R = Al and
750–800 for R = Sc. The annealing length at each
°
С
temperature was at least 80–100 h. Homogeneity
boundaries were determined by Xꢀray powder diffracꢀ
tion in airꢀquenched samples.
{R2(MoО4)3}3 rhombohedral frameworks to isovalent
∞
and heterovalent substitutions makes it possible to
control structural, thermodynamic, and electrophysiꢀ
cal properties of NASICONꢀlike materials. Specifiꢀ
cally, in cases of heterovalent substitutions of R atoms,
the change in framework charge can be balanced by a
change in the amount of extraframework univalent
cations.
Ceramics were prepared by sintering powders
ground with one or two ethanol drops and compacted
in tablets for 2 h at 500–550
°
С for aluminum comꢀ
pounds and at 650–700 for scandium compounds.
°
С
Xꢀray diffraction studies were carried out on a
Bruker AXS Advance D8 automated powder diffractoꢀ
This work is targeted at the synthesis of and study of
ion conductivity in phases of variable composition
Ag1 – xMg1 – xR1 + x(MoO4)3, where R = Al or Sc.
meter (Cu
K radiation; graphite monochromator;
α
maximal
2
θ
angle: 90
°
; scan step: 0.01
°
–0.02 ; expoꢀ
°
sure time per datapoint: 1 s). To determine unit cell
parameters, HUBER Guinier G670 and Termo ARL
automated diffractometers were employed (Cu
K
EXPERIMENTAL
α
radiation; reflection geometry;
scan step: 0.02 ).
2
θ
range: 10
°
–50 ;
°
The starting components used were silver, sodium,
magnesium, aluminum, and scandium molybdates
presynthesized by solidꢀphase reactions from AgNO3
°
A thermoanalytical study was carried out on a
(chemically pure grade), MgO (chemically pure Netzsch STA 449 Jupiter simultaneous (TG–
1189

1190
KOTOVA et al.
Table 1. Xꢀray diffraction parameters for Ag1 – xMg1 –
RESULTS AND DISCUSSION
x
R1 + x(MoO4)3
By Xꢀray powder diffraction data, the onset temꢀ
perature of the reaction between neutral molybdates is
Compound
a
, Å
c
, Å
V
, Å3
500
°
С
for scandium compounds and 400 С for alumiꢀ
°
AgMgSc(MoO4)3
9.4890(3) 23.221(2) 1810.7
num compounds. At these temperatures the reaction is
slow and does not go to end; so, the annealing temperꢀ
ature was consecutively raised to 650–700 and 550–
Ag0.8Mg0.8Sc1.2(MoO4)3 9.4954(4) 23.262(2) 1816.4
Ag0.5Mg0.5Sc1.5(MoO4)3 9.5119(8) 23.491(3) 1840.6
600
Ag1 – xMg1 – xR1 + x(MoO4)3 where
pared by annealing at 650–700
Ag1 – xMg1 – xR1 + x(MoO4)3 molybdates where
0.4 were annealed by annealing at 500–550
150–200 h. sample of composition
Ag0.5Mg0.5Al1.5(MoO4)3 quenched from 580–600 was
a single phase, which allows us to expand the homogeꢀ
neity range of the phase at these temperatures to
0.5. Phases of variable composition Ag1 – xMg1 –
°
С
, respectively. Singleꢀphase triple molybdates
0.5 were preꢀ
for 100–150 h, and
AgMgAl(MoO4)3
9.2320(16) 22.800(4) 1682.9
0
С
≤ ≤
x
Ag0.8Mg0.8Al1.2(MoO4)3 9.1870(13) 22.805(2) 1666.9
Ag0.6Mg0.6Al1.4(MoO4)3 9.1430(16) 22.817(3) 1651.8
°
0
≤ ≤
x
°
С for
A
DTA/DSC) thermal analyzer and an MOM ODꢀ103
derivatograph (the temperature rise rate was 10 K/min).
°
С
x
=
Electrophysical properties were studied on ceramic
tablets 9–10 mm in diameter and 1–2 mm high; silver
and platinum electrodes were applied to the end surꢀ
faces of tablets. Direct current conductivity tests were
x
R1 + x(MoO4)3 represent AgMgR(MoO4)3ꢀbased subꢀ
traction solid solutions.
The highest thermal stability was found in scanꢀ
performed using a V7ꢀ38 tester. Ion transport was dium solid solutions, whose incongruent melting temꢀ
perature range was 1120–1140
solutions were stable up to 790–820
°
С; aluminum solid
studied by a twoꢀpoint dc method at frequencies from
1 Hz to 1 MHz using an automated impedance meter
designed at the Institute of Control Sciences, Russian
Academy of Sciences, at temperatures from 25 to
°
С.
Xꢀray powder diffraction showed that, judging from
the reflection positions in the patterns and the intenꢀ
sity ratios, Ag1–xMg1–xR1+x(MoO4)3 phases are isosꢀ
tructural to sodium zinc scandium molybdate [12].
Powder patterns for all of the materials synthesized
were satisfactorily indexed on the assumption of
600 С with linear temperature variation at a rate of
°
4 K/min in heating and cooling modes. Some meaꢀ
surements were performed on a Novocontrol BetaꢀN
impedance analyzer at frequencies from 10 mHz to
3 MHz. The conductivity activation energy was
derived from the slopes of straightꢀline plots correꢀ
sponding to the Arrhenius log conductivity versus
reciprocal temperature plots.
rhombohedral space group
R3c. Table 1 lists unit cell
parameters for some samples of Ag1 – xMg1 –
R1 + x(MoO4)3 solid solutions. The unit cell size
decreases in the progress of substitution a smaller aluꢀ
minum cation (rAl(VI) = 0.53 Å) for scandium (rSc(VI) =
x
0.745 Å [13]).
In both systems, a systematic increase is noticed for
the parameter of the solid solutions in the range of
290
259
235
°
°
°
C
C
C
Z
'',
Ω
с
3 MHz
lower silver percentages, which is in agreement with
the idea of weakening bonding strength in silverꢀionꢀ
filled positions lying on the triple axis of the crystal
structure. The other positions (which lie beyond the
triple axis and relate in number to the former as 3 : 1)
at room temperature are likely to remain dominantly
vacant.
Direct current conductivity measurements on a
V7ꢀ38 instrument showed that the roomꢀtemperature
conductivity of samples with platinum electrodes did
not exceed 10–7 to 10–8 S/cm and allowed us to classify
these materials as insulators. The relationships
between the resistive and reactive impedance (Fig. 1)
obtained for a ceramic AgMgAl(MoO4)3 sample with
the use of a Novocontrol BetaꢀN impedance analyzer
are representative for ionic conductors equipped with
blocking electrodes [11].
–5000
–2500
0.1Hz
50 kHz
10 mHz
0
5000 10000 15000 20000 25000
3 MHz
–5000
–2500
50 mHz
100 kHz
10 mHz
0
5000 10000 15000 20000 25000
–5000
–2500
3 MHz
10 mHz
0
5000 10000 15000 20000 25000
',
Z
Ω
It is known [11] that the impedance values correꢀ
sponding to the minimal reactive component of the
locus in its highꢀohmic portion at the termination
Fig. 1. Loci of impedance for AgMgAl(MoO ) at various
4 3
temperatures indicated in the figure.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 8 2011

Ag1 – XMg1 – R1 + X(MoO4)3 Ag+ꢀCONDUCTING
1191
X
(а)
–1
–2
–3
–4
–5
–6
–7
–8
–2
–3
–4
–5
–6
–7
–8
–9
1
2
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
–1
1000/T, K
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
–1
1000/T, K
(b)
–1
–2
–3
–4
–5
Fig. 3. Conductivity versus reciprocal temperature at a freꢀ
quency of 100 Hz in the heating mode for
AgMgSc(MoO ) and ( AgMgAl(MoO )
(1)
2)
.
4 3
4 3
Another pointer to the ion character of electrical
conductivity is the ascending branch of the locus in the
lowest frequency portion thereof; this corresponds to
the presence of ionꢀtransportꢀblocking platinum elecꢀ
trodes on the sample.
Above 420
°
С
for AgMgAl(MoO4)3 and above 410°С
for AgMgSc(MoO4)3, at all frequencies there is a conꢀ
siderable rise in conductivity in the form of a
smoothed kink; as a result, the conductivity at 550
reaches a value of 8.29
10–3 S/cm for AgMgAl(MoO4)3
and 2.58
10–2 S/cm for AgMgSc(MoO4)3 with an actiꢀ
°
С
×
–6
1.1
1.3
1.5
1.7
1.9
1000/
2.1
–1
, K
×
T
vation energy of 0.3–0.4 eV (Table 2). These values are
characteristic of superionic states and are close to the
conductivity parameters of the best stateꢀofꢀtheꢀart
sodiumꢀion conductors [10]. The rise in conductivity
with increasing tervalent octahedral cation radius (R =
Al or Sc) is on account of the structural features of
NASICONꢀlike phases, whose structure is a threeꢀ
dimensional framework built of cornerꢀsharing MoO4
tetrahedra and octahedra of doubly or triply charged
cations. The framework is penetrated by two types of
channels (type I and type II); normal to these chanꢀ
nels, there is a zigzag net of type III channels. The
positions of singly charged silver cations, which balꢀ
ance the excess charge of the framework, lie inside
voids formed by the intersection of type I and type II
channels with type III channels. Silver cations ranꢀ
domly occupy these positions and are responsible for
the electrical conductivity of the compounds under
study. The ion radii of the structureꢀforming cations
have a great effect on the ionꢀconductive properties of
compounds and solid solutions that have rhomboheꢀ
dral frameworks, because an octahedral edge and an
edge of one of the tetrahedra are at the same time
edges of the polyhedron under consideration and dicꢀ
Fig. 2. Conductivity versus reciprocal temperature at a freꢀ
quency of ( ) 10, ( ) 100, ( ) 1000, and ( ) 10 000 Hz in
the heating mode for (a) Ag Mg Al (MoO ) and (b)
᭡
ᮀ
᭛
×
0.6
0.6 1.4
4 3
Ag Mg Sc (MoO ) .
0.8
0.8 1.2
4 3
point of the characteristic semicircle in Fig. 1, are
equal to the direct current conductivity σDC of the
sample. From the data displayed in Fig. 1, it follows
that in our samples the impedance at 100 Hz over a
wide range of temperatures systematically appears
near the aboveꢀindicated point, and therefore may be
taken, with a satisfactory accuracy, to be the σDC value.
The applicability criterion of this choice is the weakest
frequency dependence of the logarithm of conductivꢀ
ity when the frequency deviates from the chosen value
of 100 Hz (Fig. 2). Another support for the lucky
choice of frequency for representing ionic conductivꢀ
ity for the entire range of the compounds prepared is
provided by the obedience of the quantities to the
Arrhenius law (Figs. 3, 4).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 8 2011

1192
–1
KOTOVA et al.
Before the superionic transition (before the kink)
in the Ag1 – xMg1 – xR1 + x(MoO4)3 solid solution series,
electrical conductivity increases as rises from 0 to 0.5
х
–2
–3
–4
–5
–6
–7
–8
(Fig. 4). This trend is associated with the fact that
additional vacancies that appear in the major silver
positions (these positions are populated at room temꢀ
perature) have a positive effect on the ability of silver
cations to move freely through voids along conductivꢀ
ity channels. In the highꢀtemperature range, there is a
more uniform distribution of charge carriers over all
positions. It is for this reason that a maximal conducꢀ
tivity in Ag1 – xMg1 – xR1 + x(MoO4)3 samples, where
Al or Sc, at highꢀtemperatures is observed for
compositions (Table 2), where there is the greatest
amount of silver ions.
R =
= 0
х
3
1
2
Proceeding from the occurrence of a temperature
hysteresis on
σ
(1/Т) curves in the heating–cooling
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
, K–1
cycle, the superionic transition in Ag1 – xMg1 –
x
1000/
T
R1 + x(MoO4)3, where R = Al or Sc, may be interpreted
as a diffuse firstꢀorder phase transition. We should note
that all structural alterations occur in the sublattice of
mobile silver cations. Ag+ ions at low temperatures are
located in positions lying on the triple axis, but as temꢀ
perature rises they are uniformly distributed over posiꢀ
tions in between, thereby shortening ionꢀhopping
length, increasing electrical conductivity, and reducꢀ
ing the conductivity activation energy.
Fig. 4. Conductivity versus reciprocal temperature at a freꢀ
quency of 100 Hz in the heating mode for
(
1)
Ag Mg Al (MoO ) , ( Ag Mg Al (MoO )
and (3) AgMgAl(MoO ) .
4 3
2
)
,
0.6
0.6 1.4
4 3
0.8
0.8 1.2
4 3
tate both the size of “bottleneck” of conductivity
channels and the volume of the polyhedron itself [1,
6–8, 10, 11]. Thus, to the increasing cation size there
corresponds a trend to rise in ion conductivity; that is,
To summarize, we have prepared NASICONꢀlike
solid solutions Ag1 – xMg1 – xR1 + x(MoO4)3, where R =
to the maximal ion radius (rSc(VI) = 0.745 Å), there corꢀ
Al or Sc and
parameters and thermal stabilities. Electrical conducꢀ
tivity has been measured, and log versus reciprocal
0
≤
х
≤
0.5, and determined their unit cell
responds the highest conductivity. In the framework
with R3+ having a smaller radius (rAl(VI) = 0.53 Å), silver
cations are more “jammed” with the correspondingly
lower conductivity of the aluminumꢀcontaining phase
of variable composition (Fig. 3).
σ
temperature plots have been constructed for samples
having various
х
values. The Arrhenius
σ
plots consist
Table 2. Electrical conductivity parameters for Ag1 – xMg1 – xR1 + x(MoO ) phases
4 3
Electrical conductivity, S/cm, at
Activation energy
temperature
range,
300
°
C
550
°C
E
a, eV
°
C
60–410
460–600
90–440
0.70
0.41
0.70
0.37
0.61
0.32
0.81
0.39
0.70
0.36
0.68
0.31
AgMgSc(MoO4)3
3.99
4.22
1.57
3.85
1.15
1.70
×
×
×
×
×
×
10–5
10–5
10–4
10–6
10–5
10–5
2.58
7.64
3.63
8.29
2.36
1.33
×
×
×
×
×
×
10–2
10–3
10–3
10–3
10–3
10–3
Ag0.8Mg0.8Sc1.2(MoO4)3
Ag0.5Mg0.5Sc1.5(MoO4)3
AgMgAl(MoO4)3
465–600
50–325
340–550
195–420
455–600
110–430
500–570
100–420
525–570
Ag0.8Mg0.8Al1.2(MoO4)3
Ag0.6Mg0.6Al1.4(MoO4)3
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 8 2011

Ag1 – XMg1 – R1 + X(MoO4)3 Ag+ꢀCONDUCTING
1193
X
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7. V. B. Kalinin, S. Yu. Stefanovich, and A. Nogai, Neorg.
of two segments. Conductivity activation energy valꢀ
ues have been calculated for various temperature
ranges. We have found that phases of variable compoꢀ
sition Ag1 – xMg1 – xR1 + x(MoO4)3 have dominantly
ionic conductivity, which rises as the tervalent cation
radius increases or the amount of mobile silver ions
increases. Highꢀtemperature conductivity reaches a
value of 10–3 to 10–2 S/cm, and we may regard the triꢀ
ple molybdates we have prepared as silverꢀionꢀconꢀ
ductive solid electrolytes.
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