Solid State Ionics 263 (2014) 119–124
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Solid State Ionics
journal homepage: www.elsevier.com/locate/ssi
Effect of conditional glass former variation on electrical transport in
Li2O–P2O5 glassy and glass-ceramic ionic system
Munesh Rathore, Anshuman Dalvi ⁎
Physics Department, Birla Institute of Technology and Science, Pilani, RJ 333031, India
a r t i c l e
i n f o
Article history:
Received 11 March 2014
Received in revised form 21 May 2014
Accepted 23 May 2014
Available online 11 June 2014
Keywords:
Ionic conductivity
Crystallization
Li+ ion glasses
Glass-ceramics
a b s t r a c t
Giving emphasis to electrical transport in the thermally unstable region, a conditional glass former based system 50Li2O–(50-x)P2O5–xMoO3 is investigated. Though glass forming region is narrow, the electrical conductivity exhibits significant rise up to x ≤ 15 mol%. Scanning electron microscopy investigations suggest existence of
tiny crystallites well separated by glass tissues for higher MoO3 content samples. It is therefore revealed that addition of MoO3 improves the thermal stability. Electronic conductivity in this system is found to be fairly low and
suggests phonon assisted polaron hopping. Electrical conductivity is found to be comparable to glass and glassceramic samples.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Lithium ion sulfide (Li2S based) and oxide (Li2O based) glasses have
drawn considerable attention in the last three decades due to their potential candidature as solid electrolytes [1–7]. Lithium ion oxide glasses
though exhibit better thermal stability, their ionic conductivity at room
temperature is generally poor. On the other hand, Li+ ion sulfide glasses
exhibit high ionic conductivity near room temperature, however, their
thermal stability is poor and applicability to low dimensional ionic
devices is always questionable [3–6].
In view of possible applications to thin film as well as high temperature solid ionic devices, Li2O based glasses are promising, provided that
ionic conductivity could be significantly improved. Therefore, mainly
two approaches have been used to increase the ionic transport, viz.,
(i) addition of salt to provide mobile ions and (ii) use of mixed glass
formers. Salt addition (Li2SO4, LiCl etc.) [7–9] leads to an increase in
the number of charge carries, whereas, use of mixed glass formers
(e.g. P2O5–B2O3) essentially increases the free volume of the glass matrix
[11–23]. Typical glass formers e.g. SiO2, P2O5, and B2O3form a glassy state
naturally, following the Zachariasen rules [10]. However, there is another type, viz., conditional glass formers (transition metal based oxide
e.g. V2O5, MoO3) which can form a glassy state only in the presence of
other compounds [10]. Use of one of these conditional glass formers
certainly enhances the ionic conductivity. Even though, their addition
also incorporates electronic conductivity surprisingly. This is essentially
⁎ Corresponding author at: Physics department, BITS Pilani, RJ 333031, India. Tel.: +91
1596 515640; fax: +91 1596 244183.
E-mail address: [email protected] (A. Dalvi).
http://dx.doi.org/10.1016/j.ssi.2014.05.018
0167-2738/© 2014 Elsevier B.V. All rights reserved.
due to the tendency of these oxides to loose oxygen during molten state
that results into lower valence state for some of the transition metal
ions. This further leads to availability of electrons in the glass matrix.
Therefore, in view of search of novel glassy and glass-ceramic cathode
materials, conditional glass former based systems have got importance
[11–14].
Numerous studies are available on the mixed glass former effect
using substitution of conditional glass formers with emphasis on electrical
transportand structural and thermal aspects. For example, Lee et al. reported the enhancement in the ionic conductivity with the addition of
V2O5 into the binary Li2O–B2O3 glassy system at constant Li2O content
[15]. It was demonstrated by Jozwiak et al. [16] that the electrical conductivity of Li2O–V2O5–P2O5 glasses changes from predominant electronic to
ionic with compositional variations. Recently, Gedam et al. observed a rise
in the electrical conductivity in Li2O–B2O3–V2O5 glassy system with addition of V2O5[17]. Further, Dabas et al. [18] observed that addition of
Nb2O5 on binary lithium phosphate glass increases the thermal stability.
Effect of MoO3 addition in Li2O–P2O5–MoO3 glassy system was also
studied by Chowadri et al. [19], but mainly in the thermally stable region (T ≪ Tg). Such a substitution leads to enhancement in electronic
as well as ionic conductivity. In another investigation, Montani et al.
studied Li2O–V2O5–MoO3–TeO2 glassy system and revealed separate regions for predominant electronic and ionic transports[20]. It was found
that addition of Bi2O3 in Li2O–P2O5 glassy matrix leads to an enhancement in the electrical conductivity and ionic nature of the system due
to the presence of Li+ ions and BiO6 octahedra in the glass matrix
[21]. Similarly, it was also found that incorporation of SeO2 into binary
Li2O–B2O3 glassy matrix leads to significant enhancement in electrical
conductivity [22].
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M. Rathore, A. Dalvi / Solid State Ionics 263 (2014) 119–124
In spite of several important studies on conditional modifier based
systems, there are certain aspects which are either missing in the literature or have not gotten attention. For example, (i) crystallization and
its effect on electrical transport, (ii) mechanism of glass-ceramic formation and (iii) estimation of comparative electronic transport in glasses
and glass-ceramics with compositional alterations. These studies are
important in view of development of solid electrolytes for new generation solid ionic devices. Therefore, the present study emphasizes the
effect of MoO3 substitution in Li2O–P2O5 matrix on thermal, ionic and
electronic transports in Li2O–P2O5–MoO3. Thus the glass and glassceramic composites in this system are investigated.
2. Experimental
The glassy samples of the composition 50Li2O–(50-x)P2O5–xMoO3
(x = 5–20 mol%) were prepared by conventional melt quenching. A
sample with x = 0 i.e. 50Li2O–50P2O5 is abbreviated as LP, whereas,
compositions x = 5–20 as 5–20MLP, respectively. The glassy/
amorphous nature of the samples was examined by X-ray diffraction
(Rigaku, MiniFlex II). Further, to investigate the thermal events, differential scanning calorimetry (DSC, Simadzu DSC-60) measurements were
performed on the as prepared glassy samples. Field emission type scanning electron microscopy (FEI-Nova NanoSEM 450) was performed on
the annealed flakes of glasses to visualize the microstructural changes
during crystallization.
The melt-quenched glassy samples were thoroughly ground and
pelletized. The cylindrical shaped pellets, with conductive graphite
paint pasted on both sides, were used for conductivity measurements.
Impedance spectroscopy was carried out in the frequency range
42 Hz–5 MHz using HIOKI 3532-50 LCR meter. Further to avoid the expected water absorption by the samples which may lead to the proton
conduction, samples were annealed prior to each measurement. To
study electronic transport dc polarization technique was used. To determine the electrochemical stability of these glassy samples, cyclic
voltammetry (CV) was performed using Princeton 263A potentiostat/
galvanostat on cells of type C|glass|C.
3. Results and discussion
3.1. X-ray diffraction (XRD)
The XRD patterns of as prepared glassy and glass-ceramic samples
for x = 5 (5MLP) and 15 (15MLP) are shown in Fig. 1. The absence of
significant peaks in case of both the samples (Fig. 1a and c) confirms
Fig. 1. XRD patterns for glass and glass-ceramic samples: (a) 5MLP as prepared glass;
(b) 5MLP glass-ceramic; (c) 15MLP as prepared glass; and (d) 15MLP glass-ceramic sample.
their glassy nature. For x N 15 mol% the samples are found to be partially
crystalline (Fig. 1a) in nature. Thus the glass forming region is found to
be narrow.
The samples were further annealed above their respective crystallization temperatures (as later revealed by DSC) and slowly cooled. The
XRD patterns for these glass-ceramic samples are shown in Fig. 1b
and d. Appearance of tiny peaks strongly suggests crystalline domains
of LiPO3 and possibly another compound Li(MoP2O5) embedded in the
glass matrix. The crystallite size for 5MLP and 15MLP is found to be in
the range of 30–70 nm and 20–40 nm, respectively. Estimation of area
under the peaks suggests that in case of 5MLP, precipitation of LiPO3 is
in appreciable amount, whereas, in case of 15MLP, crystallization of
Li(MoP2O8) is facilitated and that of LiPO3 is suppressed. Therefore, it
may be suggested that (i) the addition of MoO3 suppresses the crystallization of LiPO3 and (ii) samples are complex in nature.
Further to understand the sequence of precipitation of compounds
during crystallization, the samples were annealed at two distinct temperatures of 400 °C and 500 °C and slowly cooled to room temperature.
The XRD results for these samples are shown in Fig. 2. On annealing at
400 °C, significant peaks corresponding to Li(MoP2O8) grow as shown
in Fig. 2a, hence the first major precipitation confirmed is most likely of
Li(MoP2O8) compound. On further heating, at 500 °C, area under the
peaks corresponding to LiPO3 increases, along with subtle increase for
peaks corresponding to Li(MoP2O8). Thus the first (at low temperature)
and second (at high temperature) crystallizations may correspond to
major precipitation of Li(MoP2O8) and LiPO3 compounds, respectively.
3.2. Differential scanning calorimetry (DSC)
Further, to confirm the glassy nature of as prepared melt-quenched
samples, differential scanning calorimetry (DSC) scans (10 °C/min)
were performed on LP, 5MLP and 15MLP samples (Fig. 3). The appearance of endothermic smooth dip followed by exothermic peak corresponds to the glass transition temperature (Tg) and crystallization
temperatures (Tc), respectively.
In case of LP glassy sample (Fig. 3a), the Tg appears at 320 °C followed
by a board exothermic peak with onset at 365 °C. As investigated previously, this peak may correspond to massive crystallization of LiPO3[24].
Whereas in case of 5MLP and 15MLP (Fig. 3 b and c) the crystallization
peaks are apparently merged-up, board and spanning over ~ 60 and
90 °C, respectively. These merged up peaks suggest at least two crystallizations. The first and second exothermic peaks may correspond to major
precipitation of (i) Li(MoP2O8) and (ii) LiPO3, respectively as suggested
Fig. 2. XRD patterns for the 15MLP glassy sample after annealing at (a) 400 °C and
(b) 500 °C.
M. Rathore, A. Dalvi / Solid State Ionics 263 (2014) 119–124
Fig. 3. DSC scans at a heating rate of 10 °C/min for the glassy samples.
by XRD results. Interestingly, the Tg and Tc values shift towards higher
temperatures which further suggests improved thermal stability with
MoO3 addition.
3.3. Electrical transport
To understand the effect of MoO3 addition on electrical transport in a
complete temperature range, the σ–T cycles (1 kHz) (Fig. 4) were
carried out on glass as well as glass-ceramic samples, firstly at a controlled heating rate of 1 °C/min essentially to observe thermal events.
Since in the whole region of temperature, the conductivity at 1 kHz
lies in the plateau region of σ–ω plot [8], it is assumed as dc conductivity
Fig. 4. Electrical conductivity–temperature cycles at a heating rate of 1 °C/min for the
samples' symbols denoting: (●) glass (G) and (○) glass-ceramics (GC) samples.
121
exhibiting bulk response. Fig. 4a, c and e represents the σ–T cycles for
pristine glassy compositions. In the thermally stable region (i.e. below
Tg), the conductivity shows reversible Arrhenius behavior for all the
three compositions. Thus the samples are thermally stable at least up
to ~330 °C. Whereas, in the thermally unstable region (T ≥ Tg), a deviation from linearity at Tg is observed that may be due to an increase in
free volume of the glass matrix, as also seen in Ag+ and Li+ ion oxide
glasses [25]. Such a deviation is more prominent for low MoO3 content
samples. On further heating, as the temperature approaches Tc, crystallization begins and the σ–T cycles exhibit two kinks as shown in Fig. 4.
Firstly, the fall in the conductivity is more prominent for second kink
(Tc2) which may correspond to crystallization of LiPO3. Secondly, such
a fall in the conductivity at Tc2 is more apparent and massive in low
MoO3 content sample and gradually becomes less prominent for samples with high MoO3 content (15MLP). Thus it is once again evident
that MoO3 stabilizes the glassy phase effectively suppressing the crystallization of LiPO3.
It may be further suggested that during crystallization also the sample
remains predominantly ionic. In a previous study, on a novel salt free
V2O5–P2O5 glassy system [12], a sudden rise in the conductivity was
witnessed during the crystallization due to predominately electronic
conduction.
Fig. 4b, d and f represents the σ–T cycles obtained for the corresponding glass-ceramic samples. The conductivity of these samples
exhibits an Arrhenius behavior with no thermal events in the whole
temperature range, except neat Tc2 above which the conductivity increases with a little higher activation energy. Such behavior further suggested better thermal stability of glass-ceramics. It may be emphasized
here that the conductivity of the glass-ceramics as compared to glasses
at 100 °C is low only by few factors, whereas, in our previous investigation on Li2SO4–Li2O–P2O5[8] system the conductivity in glass-ceramics
was found to be significantly low (at least ~ 2 orders of magnitude)
than that of the corresponding glasses.
Further, the conductivity increases with the addition of MoO3 for
both glasses as well as glass-ceramic samples, and as expected opposite
Fig. 5. Activation energy and conductivity as function of MoO3 content for (a) glasses and
(b) glass-ceramics.
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M. Rathore, A. Dalvi / Solid State Ionics 263 (2014) 119–124
trend is seen in the activation energy (Fig. 5). For x ≥ 15 mol% conductivity drops that may be due to partially crystalline nature of the samples.
Results are in good agreement with a previous study on xMoO3–(1-x)
[0.5Li2O–P2O5] glassy system [19]. However, in the present case Li+ ion
content is fixed and MoO3 substitutes P2O5. Thus the thermally stable
ionic system is obtained, without compromising the Li+ ion content.
As suggested earlier [19] the enhancement in conductivity in the
glassy phase may be attributed to substitution of a relatively bigger
ion (Mo in place P) that, in turn, leads to an increase in free volume of
the glass matrix. The reason for conductivity enhancement in glassceramic due to MoO3 substitution is further scrutinized. To understand
the glass-ceramic formation and its correlation with electrical transport,
scanning electron microscopy (SEM) is performed on the samples 5MLP
and 15MLP annealed well above crystallization temperature and the results are shown in Fig. 6. In both the samples, glass-ceramic formation is
evident. The low MoO3 content sample (5MLP) is evidently a dense
glass-ceramic with relatively larger average grain size of ~500 nm due
to which massive crystallization in this sample is observed in σ–T cycle.
On the other hand, the 15MLP sample is found to be consisted of well separated, relatively smaller grains of average size ~50–100 nm surrounded
by connective glass tissues. The SEM results may be correlated with electrical transport in the glass-ceramics. It may be suggested that the massive fall of the conductivity during the crystallization seen in case of
5MLP may be attributed due to unavailability of connective glass tissues
due to dense glass-ceramic formation, schematically also shown in
Fig. 6c. Thus a relatively poor conductivity of crystallites results into a
drastic fall during crystallization [26]. Such a fall is not prominent for
15MLP due to available glass tissue due to smaller crystallite size. As
shown in Fig. 6d, ions can bypass the crystallites and move comfortably
through these tissues during electrical transport. These connective tissues
are stable and thus 15MLP glass-ceramic is more glass-like and exhibits
higher conductivity. Thus the SEM results are in good agreement with
those of the σ–T cycles and also compliment the X-ray results.
It is relevant to mention here that in some phosphate and silicate
glasses [27] there is a possibility of proton conduction especially when
measurements are done in open atmosphere. Since these samples are
prepared by melt-quenching route and measurements are performed
above 150 °C, such possibility is not expected in the present case. To estimate the contribution of ions in total electrical transport impedance
spectroscopy measurements (42 Hz–5 MHz) were carried out at different temperatures and for different compositions. The Nyquist or cole–
cole plots (Z″ vs Z′) obtained for 15MLP glassy sample are shown in
Fig. 7a. Further, the same plot is also shown for different compositions
at temperature 250 °C (Fig. 7b). In both cases, the plot exhibits depression semicircles followed by an inclined line that may be represented by
parallel combination of resistance (R) and constant phase element
(CPE1) for bulk in series with another element CPE2 (interface). The
low frequency inclined line is most likely due to polarization at the
electrode–electrolyte interface and strongly suggests predominantly
ionic nature of the samples. The depressed semicircles represent the
bulk response and suggest distribution of relaxation times.
To further estimate the electronic contribution to the conductivity, a
dc potential (1 V) was applied to the sample and transient current (It) as
function of time was measured (Fig. 8). As apparent, initially the transient current drops rapidly and saturates subsequently to a notably
small value (Is). For all the samples, probably due to faster response of
Li+ ions to the applied electric field, it was not possible to measure Io
(current at exactly t = 0) accurately. Nevertheless, for glass as well as
glass-ceramic samples apparently a drastic fall in the transient current
confirms their predominant ionic nature. In the present case (Fig. 8),
the saturation current (Is) is essentially due to electronic contribution.
Thus the electronic conductivity for the sample is obtained by the following relation:
σ¼
lI s
V pA
ð1Þ
where Vp is the applied voltage, l the thickness and A is cross-sectional
area of the sample. To examine temperature dependence of the electronic transport, the electronic conductivity was carefully measured in
the thermally stable region. For one of the samples i.e. 15MLP the temperature dependence of conductivity is shown for glass (Fig. 9a) and its
Fig. 6. SEM images (2 μm resolution) of glass-ceramic samples for (a) 5MLP and (b) 15MLP. Schematic representation of possible motion of ions in: (c) 5MLP and (d) 15MLP. The arrow
indicates likely pathways for ions after crystallization.
M. Rathore, A. Dalvi / Solid State Ionics 263 (2014) 119–124
123
Fig. 9. Total and electronic conductivity vs inverse of temperature for 15MLP in the
thermally stable region for (a) glass and (b) glass-ceramic.
Fig. 7. Nyquist plots for (a) 15MLP glassy sample at three different temperatures and
(b) for different compositions at a fixed temperature.
corresponding glass-ceramic (Fig. 9b) sample. Interestingly, the conductivity increases exponentially with temperature for both cases.
The electronic transport in disordered solids can be explained using
a theory proposed by Mott [28,29]. According to this model which was
essentially proposed for semiconducting glasses, for very low temperatures T ≪ θD (θD = debye temperature) the contribution of phonons
is essentially suppressed and electronic transport follows the well
Fig. 8. Transient current (It) as function of time for (a) glass and (b) glass-ceramic samples.
known Mott's T1/4 law which is also known as variable range hopping
mechanism.
On the other hand, when temperature exceeds θD, significant phonon contribution is predominant and conductivity follows Arrhenius
dependence of temperature according to the following equation:
E
− kTp
σ ¼ σ 0e
:
ð2Þ
The activation energy (Ep) in the above expression corresponds to
polaron hopping and is mainly due to two important contributions,
viz., (i) the binding energy for polaron formation and (ii) the energy difference in the initial and final states due to the difference in the ion
coordination.
Arrhenius type conductivity behavior over a wide range of temperature in the present case strongly suggests phonon assisted polaron hopping as also suggested in earlier investigations [14,16] in the glass and
glass-ceramics.
It may be emphasized that in similar to previous investigations [19]
MoO3 addition increases the electronic conductivity notably (Fig. 9).
The electrons are introduced to the system from MoO3 during annealing
of the sample in its molten state. These electrons contribute to electrical
transport through hopping conduction from a site near Mo+5 to a Mo+6
ion. Thus electrons behave like a charge placed in polarizable medium
and the induced polarization surrounding the electrons accompanies
electrons during transport. The activation energy for polaron hopping,
calculated from the slope, is found to be slightly higher for glassceramic (0.61 eV) than that of glassy sample (0.50 eV) for 15MLP. This
further suggests that the glassy state appears to be more favorable for
the polaron hopping. Furthermore, the nano-crystallites that precipitated during crystallization hardly facilitate electronic conduction. It
should also be emphasized that the total activation energy (Fig. 5)
which is predominantly due to ionic contribution is found comparable
to that of polaron hopping. This further suggests that the transport of
Li+ ions and polaron is equally facilitated in the matrix.
Previously in 15Li2O–15P2O5–70MoO3 system [18], predominately
electronic nature was seen due to high MoO3 content. In the present
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M. Rathore, A. Dalvi / Solid State Ionics 263 (2014) 119–124
content samples. The glass and glass-ceramic samples are predominately ionic. The CV measurements confirm an appreciable stability even at
higher temperatures and compliment the ionic nature of these samples.
The present investigation reveals that highly conducting glass-ceramic
with prominent ionic nature can be obtained using conditional glass
former substitution as well.
Acknowledgments
This work is supported by DST-FIST (SR/FST/PSI-150/2010)
and UGC Special Assistance Programme (F-530/3/DRS/2009/SAP-I),
Government of India. Authors would like to thank the late Professor
Suresh Chandra of Banaras Hindu University, India for his suggestions
and fruitful discussions.
References
Fig. 10. Cyclic voltammograms for the glassy samples for two compositions at 200 °C
(a) 10MLP and (b) 15MLP.
work, due to low MoO3 content ionic transport predominates and electronic conductivity is notably low.
3.4. Cyclic voltammetry
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4. Conclusions
Addition of MoO3 in the Li2O–P2O5 glass matrix reveals interesting
results. Such a substitution in place of P2O5 improves the thermal stability as well as ionic conductivity, in turn confirmed by SEM, XRD, DSC
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is essentially due to suppressed crystallization that leads to availability
of connective glass tissues, as more evidently seen for higher MoO3
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