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Protonic conductivity of layered HNbWO6 · 1.5H2O by impedance spectroscopy
Solid State Ionics 76 ( 1995) 337-340 ELSEWIER
Protonic conductivity of layered HNbW06 1.5H20 by impedance spectroscopy l
G. Mangamma *, K. Shahi Department of Physics, Indian Institute of Technology, Kanpur. India Received
15 December
12 December 1994
1993; accepted for publication
Abstract The protonic conductivity of layered HNbWO,. 1.5H20 has been studied by impedance spectroscopy technique. The values of ionic conductivity (cr= 4 X 10m6 S/cm) and diffusion coefficient (D = 4.3 X IO- ” cm*/s) obtained at room temperature are
consistent with the corresponding Keywords: Solid electrolyte;
Impedance
values estimated by an NMR study. spectroscopy;
Layered oxides; Ionic conductivity
- proton
1. Introduction
Layered ionic conductors have, of late, become potential materials for a great deal of study. HNbWO, .I SH,O is one such compound which is an identified proton-containing transition metal oxide belonging to the class of oxides possessing layered structure [ 11. This compound and its analogous compound: HTaWO, .1.5H,O were prepared by exchange of Li+ in LiMWOh (M = Nb or Ta) for proton using dil HCI. The lamellar nature of these oxides was established through the study of ion exchange and intercalation properties. The proposed structure of LiMW06, shown in Fig. 1, contains cation-planes ordered along the c-direction in a rutile network. HNbWO, . 1.5H20 occurs topotactically, the product retaining the structural features of the parent. Accordingly, the protonated oxides consist of NbW06 slabs that can be visualized as being made of Nb/W oxygen octahedra linked
b l
0167-2738/95/$09.50 0 199.5 Elsevier Science B.V. All rights reserved .SSDIO167-2738(94)00302-5
-Li , 0-NblW
.-Oxygen,
0
Fig. I. Structure of (a) layered LiNbWO, HNbWO,. 1.5H,O(Ref. [ 21).
(Ref.
O* Present address: Material Chemistry Division, Chemical Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India.
lb)
(a)
H,
,
H20,
0 - voconcy
1I I); (b) layered
338
G. Mangamma, K. Shahi / Solid State Ianics 76 (1995) 337-340
together by edge and corner. In this compound the water molecules occupy the interlayer region. Since the interlayer attractive forces are weak compared to intralayer binding forces, protons in HNbWO, .l.5H20 are expected on structural considerations to be mobile in two dimensions. For a detailed study of this two dimensional protonic motion in HNbWO, .I .5H20, we have carried out various ‘H NMR expe~men~ like measurement of T,, T,p and T2 (obtained from line narrowing experiment) as a function of temperature [ 2-41. From the analysis of the low temperature ( 110 K) NMR signal, it is seen that two types of protons in HNbWOh viz., the cage protons of the HNbWO~ part of the molecule and protons of the water of crystallization, are distinctly present. Temperature-dependent line narrowing results as well as T,, behavior suggest translational motion of the protons at temperatures above 215 K. The results of the MASS experiments provide evidence for the pa~icipation of the two types of protons in the diffusive motion with a large exchange rate. Further, it has been inferred from these observations [ 2-41 that the mechanism of H + ion-transport in HNbWO, is vacancy-assisted ionic hopping. In this paper we present the results of our impedance spectroscopy study of ~bW0~ 11.5H20 under~en with the specific purpose of obtaining direct information on the true bulk conductivity of the material.
perature for about a few minutes before the readings were taken to ensure equilibration. The measurements were done in both the cooling and heating cycles. The data are found to be reproducible. The impedance measurements were carried out in a temperature range 100 K to 300 K using HP 4192 impedance analyzer. The sample loses the water of ~~stalli~tion above room tem~rat~e and therefore the measurement could not be extended to the region above 300 K.
3. Results and discussion The ac impedance data for each temperature in each run was obtained manually and all the subsequent analyses were done for an equivalent circuit on an IBM PC using a program developed by Boukamp [ 51. The complex impedance plot at 300 K is shown in Fig. 2. They are in the form of semicircles followed by a tail in the low frequency region. The equivalent circuit assumed to represent the sample is shown in Fig. 3, where C, is the double layer capacitance, Ri is the ionic resistance, C, is the geometric capacitance between electrodes and a Warburglike complex element, apparently related to diffusion 1.2
2. Sample prepa~~on and experimental technique The layered HNbWO, * 1.5H20 was prepared by proton ion-exchange with LiNbWOs which in turn was synthesized as described in Refs. [ 1,2]. LiNbW06 and HNbW06. 1SH,O were analyzed by X-ray powder diffraction (Cu Kar) and confirmed to belong to the layered structure. The lattice constants were determined to be a=4.68 A, c--9.28 A and a=4.71 A, c= 25.7 A for the former and the latter compound respectively. The values obtained for both the compounds are in good agreement with those previously reported [ 1 ] . The sample was finely ground and pelletised at a pressure of 6 ton. The pellet was 1 mm thick with a diameter of 5 mm. Silver paint was applied to the pellet as electrode in order to achieve good electrical contact. Temperature variation was done using an APD closed-cycle liquid helium cryostat with a temperature stability of t_ 1 K. The samples were kept at each tem-
0
1
*. 0
s.’
0.2
’
’
0.4
Cl
Simulotod
data
0
MOOswed
doto
’
’
0.6
’
c
B
Zreal (Mfi) Fig. 2. Impedance.plot of layered HNbWT),. l.SH@ at 300 K.
G. Mangamma,
K. Shahi/Solid
State Ionics 76 (1995) 337-340
g=
Fig. 3. Equivalent
F.
1 1 --7 Ri A
where A is the area of cross section of the specimen in cm* and 1 is its thickness in cm. Fig. 4 shows the log (aT) versus l/T of HNbWO, . l.5H20 for both cooling and heating cycles. This plot displays the two usual regions viz., the high ionic conducting region (300 K> T> 180 K) and rigid-lattice region ( 100 K < T < 180 K) and obeys the Arrhenius equation
circuit.
-4-
E
(2)
”
a u-l
I-
-5
b z
339
_6_
-7-
1000/T
Fig. 4. Plot of log (CT) I .5HZ0.
versus
(K-l) 1000/T
Table I Activation energy of layered HNbWO,. niques Experiment
NMR T, NMR Tz NMR T,, IS IS
for layered
HNbWO,,’
1.5H20 using different tech-
E, (kJ/mol) 13.44 (300>T> 180) 11.72 (300>T>180) 19.2 (300> T> 180) 13.2 (300> T> 180) 2.547 (180>T>lOO)
processes near the electrolyte-electrode interface [ 6-81. A computer simulation representing the assumed equivalent circuit, showed good correlation to the actual impedance results at high frequencies, but did not follow as well at lower frequencies. From the impedance analysis, the intersection of the low frequency semicircle with the Re(Z)-axis represented Ri, the ionits resistance (Fig. 2). The values of Ri corresponding to different temperatures were used to calculate the ionic conductivity (+ from the formula:
An activation energy E, which was calculated from the slope of the graph in the high conduction region is found to be 13.2 W/mole and is quite consistent with the value obtained from the NMR motional narrowing study. But it is lower than the value 19.2 kJ/mole as achieved from the T,p study. Such an anomalous prefactor has been observed in a number of fast ionic conductors [ 9-111, where translational diffusion is the cause of motional narrowing. The reason for this anomaly has been the subject of a large number of investigations and does not concern us here. The activation energy E, from this study is compared with those from the NMR data in Table 1. The diffusion coefficient was estimated using the Nernst-Einstein relation viz., D=
akT N q2’
(3)
where N is the concentration of the charge carrier, q is the charge on the mobile carrier and k is the Boltzmann constant. As mentioned earlier, our pulsed NMR investigations on this compound have shown that the charge transport in HNbWOh .l SH,O is due to the cage protons and water protons of the compound. Therefore, assuming the H+ ions to be mobile charge carriers and taking the volume of a formula unit of HNbWO, .l SH,O to be 143 A’ [ I], N is found to be 25 X lo*’ per cm?. The values of D calculated at room temperature using this value of N turns out to be 4.3 X lo-” cm’/s. The values of o and D at 300 K obtained from this study are of the same order of magnitude as obtained in the NMR motional narrowing study. As far as the
340
G. Mangamma, K. Shahi /Solid State Ionics 76 (I 995) 33 7-340
mechanism of the conductivity enhancement above 200 Kin this system is concerned, NMR measurement indicated that the reorientational dynamics of the water protons seems to decrease the peak separation with temperature till about 200 K when both types of protons are together seen to participate in long range diffusion, the local dynamics thus acting as the precursor to the translational motion. Therefore we associate this enhancement of conductivity which occurs in the same temperature region as shown in Fig. 4 with an onset translational motion of the protons of the water and proton in the HNbW06 part.
4. Conclusions The complex impedance analysis of HNbW06. 1SH,O indicates that HNbW06. 1.5H20 is a good protonic conductor. The temperature dependence of conductivity data provides a direct evidence for translational motion of the protons from both entities
at and above 215 K which is consistent with the NMR results of the compound. References [I] V. Bhat and J. Gopalakrishnan, Solid State Ionics 26 (1988) 25. [2] G. Mangamma, Ph.D Thesis (Indian Institute of Science, Bangalore, 1992) Chap. 4. [ 31 G. Mangamma, V. Bhat, J. Gopalakrishnan and S.V. Bhat, 7th hit. Conf. on Solid State Ionics (Japan, 1989) Ext. Abst. p, 177. [4] S.V. Bhat and G. Mangamma, 7th Int. Conf. on Solid State Ionics (Japan, 1989) Ext. Abst. p. 176. [5] B.A. Boukamp, Equivalent Circuit Program Manual (Netherlands, 1989). [6] J.R. Macdonald, Impedance Spectroscopy (Wiley, New York, 1989). [7] B.A. Boukamp and R.A. Huggins, Mat. Res. Bull. 13 (1978) 23. [8] B.A. Boukamp, Solid State Ionics 18/19 (1986) 136. [9] P.M. Richards, Solid State Commun. 25 (1978) 1019. [lo] A. Avogadro and M. Villa, J. Chem. Phys. 66 (1978) 2359. [ 1I] P.M. Richards, Physics of Superionic Conductors, Topics in Current Physics, Vol. 15, ed. M.B. Solman (Springer, Berlin, 1979) p. 111.
Protonic conductivity of layered HNbW06 1.5H20 by impedance spectroscopy l
G. Mangamma *, K. Shahi Department of Physics, Indian Institute of Technology, Kanpur. India Received
15 December
12 December 1994
1993; accepted for publication
Abstract The protonic conductivity of layered HNbWO,. 1.5H20 has been studied by impedance spectroscopy technique. The values of ionic conductivity (cr= 4 X 10m6 S/cm) and diffusion coefficient (D = 4.3 X IO- ” cm*/s) obtained at room temperature are
consistent with the corresponding Keywords: Solid electrolyte;
Impedance
values estimated by an NMR study. spectroscopy;
Layered oxides; Ionic conductivity
- proton
1. Introduction
Layered ionic conductors have, of late, become potential materials for a great deal of study. HNbWO, .I SH,O is one such compound which is an identified proton-containing transition metal oxide belonging to the class of oxides possessing layered structure [ 11. This compound and its analogous compound: HTaWO, .1.5H,O were prepared by exchange of Li+ in LiMWOh (M = Nb or Ta) for proton using dil HCI. The lamellar nature of these oxides was established through the study of ion exchange and intercalation properties. The proposed structure of LiMW06, shown in Fig. 1, contains cation-planes ordered along the c-direction in a rutile network. HNbWO, . 1.5H20 occurs topotactically, the product retaining the structural features of the parent. Accordingly, the protonated oxides consist of NbW06 slabs that can be visualized as being made of Nb/W oxygen octahedra linked
b l
0167-2738/95/$09.50 0 199.5 Elsevier Science B.V. All rights reserved .SSDIO167-2738(94)00302-5
-Li , 0-NblW
.-Oxygen,
0
Fig. I. Structure of (a) layered LiNbWO, HNbWO,. 1.5H,O(Ref. [ 21).
(Ref.
O* Present address: Material Chemistry Division, Chemical Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India.
lb)
(a)
H,
,
H20,
0 - voconcy
1I I); (b) layered
338
G. Mangamma, K. Shahi / Solid State Ianics 76 (1995) 337-340
together by edge and corner. In this compound the water molecules occupy the interlayer region. Since the interlayer attractive forces are weak compared to intralayer binding forces, protons in HNbWO, .l.5H20 are expected on structural considerations to be mobile in two dimensions. For a detailed study of this two dimensional protonic motion in HNbWO, .I .5H20, we have carried out various ‘H NMR expe~men~ like measurement of T,, T,p and T2 (obtained from line narrowing experiment) as a function of temperature [ 2-41. From the analysis of the low temperature ( 110 K) NMR signal, it is seen that two types of protons in HNbWOh viz., the cage protons of the HNbWO~ part of the molecule and protons of the water of crystallization, are distinctly present. Temperature-dependent line narrowing results as well as T,, behavior suggest translational motion of the protons at temperatures above 215 K. The results of the MASS experiments provide evidence for the pa~icipation of the two types of protons in the diffusive motion with a large exchange rate. Further, it has been inferred from these observations [ 2-41 that the mechanism of H + ion-transport in HNbWO, is vacancy-assisted ionic hopping. In this paper we present the results of our impedance spectroscopy study of ~bW0~ 11.5H20 under~en with the specific purpose of obtaining direct information on the true bulk conductivity of the material.
perature for about a few minutes before the readings were taken to ensure equilibration. The measurements were done in both the cooling and heating cycles. The data are found to be reproducible. The impedance measurements were carried out in a temperature range 100 K to 300 K using HP 4192 impedance analyzer. The sample loses the water of ~~stalli~tion above room tem~rat~e and therefore the measurement could not be extended to the region above 300 K.
3. Results and discussion The ac impedance data for each temperature in each run was obtained manually and all the subsequent analyses were done for an equivalent circuit on an IBM PC using a program developed by Boukamp [ 51. The complex impedance plot at 300 K is shown in Fig. 2. They are in the form of semicircles followed by a tail in the low frequency region. The equivalent circuit assumed to represent the sample is shown in Fig. 3, where C, is the double layer capacitance, Ri is the ionic resistance, C, is the geometric capacitance between electrodes and a Warburglike complex element, apparently related to diffusion 1.2
2. Sample prepa~~on and experimental technique The layered HNbWO, * 1.5H20 was prepared by proton ion-exchange with LiNbWOs which in turn was synthesized as described in Refs. [ 1,2]. LiNbW06 and HNbW06. 1SH,O were analyzed by X-ray powder diffraction (Cu Kar) and confirmed to belong to the layered structure. The lattice constants were determined to be a=4.68 A, c--9.28 A and a=4.71 A, c= 25.7 A for the former and the latter compound respectively. The values obtained for both the compounds are in good agreement with those previously reported [ 1 ] . The sample was finely ground and pelletised at a pressure of 6 ton. The pellet was 1 mm thick with a diameter of 5 mm. Silver paint was applied to the pellet as electrode in order to achieve good electrical contact. Temperature variation was done using an APD closed-cycle liquid helium cryostat with a temperature stability of t_ 1 K. The samples were kept at each tem-
0
1
*. 0
s.’
0.2
’
’
0.4
Cl
Simulotod
data
0
MOOswed
doto
’
’
0.6
’
c
B
Zreal (Mfi) Fig. 2. Impedance.plot of layered HNbWT),. l.SH@ at 300 K.
G. Mangamma,
K. Shahi/Solid
State Ionics 76 (1995) 337-340
g=
Fig. 3. Equivalent
F.
1 1 --7 Ri A
where A is the area of cross section of the specimen in cm* and 1 is its thickness in cm. Fig. 4 shows the log (aT) versus l/T of HNbWO, . l.5H20 for both cooling and heating cycles. This plot displays the two usual regions viz., the high ionic conducting region (300 K> T> 180 K) and rigid-lattice region ( 100 K < T < 180 K) and obeys the Arrhenius equation
circuit.
-4-
E
(2)
”
a u-l
I-
-5
b z
339
_6_
-7-
1000/T
Fig. 4. Plot of log (CT) I .5HZ0.
versus
(K-l) 1000/T
Table I Activation energy of layered HNbWO,. niques Experiment
NMR T, NMR Tz NMR T,, IS IS
for layered
HNbWO,,’
1.5H20 using different tech-
E, (kJ/mol) 13.44 (300>T> 180) 11.72 (300>T>180) 19.2 (300> T> 180) 13.2 (300> T> 180) 2.547 (180>T>lOO)
processes near the electrolyte-electrode interface [ 6-81. A computer simulation representing the assumed equivalent circuit, showed good correlation to the actual impedance results at high frequencies, but did not follow as well at lower frequencies. From the impedance analysis, the intersection of the low frequency semicircle with the Re(Z)-axis represented Ri, the ionits resistance (Fig. 2). The values of Ri corresponding to different temperatures were used to calculate the ionic conductivity (+ from the formula:
An activation energy E, which was calculated from the slope of the graph in the high conduction region is found to be 13.2 W/mole and is quite consistent with the value obtained from the NMR motional narrowing study. But it is lower than the value 19.2 kJ/mole as achieved from the T,p study. Such an anomalous prefactor has been observed in a number of fast ionic conductors [ 9-111, where translational diffusion is the cause of motional narrowing. The reason for this anomaly has been the subject of a large number of investigations and does not concern us here. The activation energy E, from this study is compared with those from the NMR data in Table 1. The diffusion coefficient was estimated using the Nernst-Einstein relation viz., D=
akT N q2’
(3)
where N is the concentration of the charge carrier, q is the charge on the mobile carrier and k is the Boltzmann constant. As mentioned earlier, our pulsed NMR investigations on this compound have shown that the charge transport in HNbWOh .l SH,O is due to the cage protons and water protons of the compound. Therefore, assuming the H+ ions to be mobile charge carriers and taking the volume of a formula unit of HNbWO, .l SH,O to be 143 A’ [ I], N is found to be 25 X lo*’ per cm?. The values of D calculated at room temperature using this value of N turns out to be 4.3 X lo-” cm’/s. The values of o and D at 300 K obtained from this study are of the same order of magnitude as obtained in the NMR motional narrowing study. As far as the
340
G. Mangamma, K. Shahi /Solid State Ionics 76 (I 995) 33 7-340
mechanism of the conductivity enhancement above 200 Kin this system is concerned, NMR measurement indicated that the reorientational dynamics of the water protons seems to decrease the peak separation with temperature till about 200 K when both types of protons are together seen to participate in long range diffusion, the local dynamics thus acting as the precursor to the translational motion. Therefore we associate this enhancement of conductivity which occurs in the same temperature region as shown in Fig. 4 with an onset translational motion of the protons of the water and proton in the HNbW06 part.
4. Conclusions The complex impedance analysis of HNbW06. 1SH,O indicates that HNbW06. 1.5H20 is a good protonic conductor. The temperature dependence of conductivity data provides a direct evidence for translational motion of the protons from both entities
at and above 215 K which is consistent with the NMR results of the compound. References [I] V. Bhat and J. Gopalakrishnan, Solid State Ionics 26 (1988) 25. [2] G. Mangamma, Ph.D Thesis (Indian Institute of Science, Bangalore, 1992) Chap. 4. [ 31 G. Mangamma, V. Bhat, J. Gopalakrishnan and S.V. Bhat, 7th hit. Conf. on Solid State Ionics (Japan, 1989) Ext. Abst. p, 177. [4] S.V. Bhat and G. Mangamma, 7th Int. Conf. on Solid State Ionics (Japan, 1989) Ext. Abst. p. 176. [5] B.A. Boukamp, Equivalent Circuit Program Manual (Netherlands, 1989). [6] J.R. Macdonald, Impedance Spectroscopy (Wiley, New York, 1989). [7] B.A. Boukamp and R.A. Huggins, Mat. Res. Bull. 13 (1978) 23. [8] B.A. Boukamp, Solid State Ionics 18/19 (1986) 136. [9] P.M. Richards, Solid State Commun. 25 (1978) 1019. [lo] A. Avogadro and M. Villa, J. Chem. Phys. 66 (1978) 2359. [ 1I] P.M. Richards, Physics of Superionic Conductors, Topics in Current Physics, Vol. 15, ed. M.B. Solman (Springer, Berlin, 1979) p. 111.