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Lanthanum–lithium–sodium double chromates as ionic conductors
Journal of Alloys and Compounds 250 (1997) 520–523
L
Lanthanum–lithium–sodium double chromates as ionic conductors a, b ´ b , J. Santamarıa ´ b , F. Sanchez-Quesada ´ P. Melnikov *, C. Leon a
Institute of Chemistry, UNESP-Araraquara, CP 355, CEP 14800 -900, Araraquara SP, Brazil b ´ ´ Facultad de Ciencias Fısicas , Universidad Complutense de Madrid, 28040, Madrid, Spain
Abstract Lanthanum–lithium–sodium double chromates Li 12x Nax La(CrO 4 ) 2 were prepared and analysed by means of admittance spectroscopy. Their a.c. conductivity parameters are correlated with structural details of high and low temperature forms of pure lanthanum–lithium double chromates. Lithium compounds show the lowest conductivity values and the highest activation energy for ion motion, while the sample Li 0.5 Na 0.5 La(CrO 4 ) 2 exhibits the highest conductivity 10 25 S cm 21 and the lowest activation energy 0.58 eV. Keywords: Lanthanum; Alkali metals; Double chromates; Admittance spectroscopy; Conductivity; Activation energy
1. Introduction A number of inorganic materials have been prepared making substitutions in the matrix Me n Ln m (TO 4 ) 1 , where Me is an alkali metal, Ln is a rare earth element and T is P, As, V, Cr, etc. [1,2]. Recently we described and discussed structural details and relationships in the family of double chromates MeLa(CrO 4 ) 2 , where Me 5 Na–Cs and Ln 5 La–Lu [3]. The existence of less well-known lithium and lithiated sodium phases was also confirmed [4,5]. Structural considerations have been of value in predicting electrical conductivity either of electronic or of ionic types for Na, K and Rb double chromates analysed by means of admittance spectroscopy [2]. The choice and behaviour of mobile cation are, as a rule, decisive factors in the design of chemical sensors. The purpose of the present work is a comparative study of the phases Li n Na 12n La(CrO 4 ) 2 following the standard impedance method and using modulus formalism.
2. Experimental The preparation of the low temperature variety of LiLa(CrO 4 ) 2 was conducted as in Ref. [4] with some modifications. Since the transition into the high temperature form takes months to achieve, this phase was obtained directly using a hydrothermal technique. A mixture of
*Corresponding author. Tel.: (155) 162 32 2022; fax: (155) 162 22 7932. 0925-8388 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0925-8388( 96 )02534-0
Li 2 CO 3 , La 2 O 3 and CrO 3 taken in stoichiometric quantities was dissolved in diluted HCl (1:20) and this solution heated in a sealed glass ampoule at 155 8C for 22 days. LiLa(CrO 4 ) 2 precipitated in the form of fine yellow powder, then was washed with ethyl alcohol and dried at 160 8C. Mixed lithium–sodium chromates were prepared by solid-state reaction between Li 2 CO 3 , Na 2 CO 3 , La 2 O 3 and CrO 3 , also taken in stoichiometric quantities, ground in hexane and after drying at 100 8C heated at 300 8C for 72 h. All starting reagents were Merck products, of analytical grade purity. Phase analysis was carried out by X-ray powder diagrams. Admittance spectroscopy measurements were conducted using an automatically controlled HP 4284A precision LCR meter, frequency ranged from 20 Hz to 1 MHz. To ensure a low noise level, the mean of 16 measurements was used at each frequency. Test specimens were cold pressed cylindrical pellets 5 mm in diameter and approximately 0.5 mm thick, on top of which gold contacts were deposited by evaporation through a mechanical mask. Pellets were mounted in a sealed stainless steel holder through which a high purity dry nitrogen gas was passed to ensure an inert atmosphere and to avoid water condensation. Temperature range was 200–500 K.
3. Results and discussion The experimental findings indicate clear blocking phenomena at grain boundaries and electrode characteristics of ionic conductors. Fig. 1 shows complex impedance plots for a representative sample Li 0.5 Na 0.5 La(CrO 4 ) 2 at various
P. Melnikov et al. / Journal of Alloys and Compounds 250 (1997) 520 – 523
Fig. 1. Complex impedance plot of a Li 0.5 Na 0.5 La(CrO 4 ) 2 sample at 326, 334 and 350 K.
temperatures. Well developed spurs, characteristic of blocking at electrodes, can be observed at low frequencies, and a second semicircle, overlapped with the bulk one, is associated with the partial blocking of grain boundaries. Measurements have been conducted at low temperatures in order to exclude the effect of blocking mechanisms, since their stronger activated contributions are shifted to lower frequencies. The usage of a standard complex impedance method and modulus formalism offers the advantage of lesser sensitivity to the electrode related processes and provides information about the dynamics of the conductivity relaxation. The dielectric modulus M * 5 1 /´ * is related to the Fourier transform of the decay function through the expression `
3 E
1 df M * (v ) 5 ] 1 2 ]e 2j v t dt ´` dt 0
4
521
Fig. 2(a) and Fig. 2(b) respectively show the real and the imaginary parts of the modulus peak for the same sample Li 0.5 Na 0.5 La(CrO 4 ) 2 . The non-Debye character of the relaxation can be readily seen from the width of the imaginary modulus peak, which is greater than 1.12 decades for FWMH in a Debye relaxation. The reported continuous lines show the fitting to expression in Eq. (1) for b 5 0.43. For the temperature range within which the peak was observable in the available frequency range b remained almost constant, except for the peaks at the edge of the frequency window where changes of about 10% were observed. The non-Debye character of the conductivity relaxation analysed in the time domain using a KWW function can also be observed in the conductivity representation. Fig. 3 shows the real part of the conductivity exhibiting no blocking phenomena in this temperature range and a power behaviour in the form v n at high frequencies, so that the conductivity can be fitted to expressions in the form s 5 s0 1 A(T )v n . This power law dependence has been termed universal response, and the value of the exponent n has been related to correlation
(1)
where ´` is the high frequency permittivity and f (t ) describes the decay of the electric field E (E(t) 5 E(0)f (t)) at constant displacement vector D [6,7]. While for ideal conductors f (t) is an exponential exp(2t /t ), the relaxation being of the Debye kind, for most ionic conductors the decay is best described by a stretched exponential f (t) 5 exp(2t /t ) b . The description of the relaxation using the KKW function is empirical, and the meaning of b remains unclear. It has been claimed that the stretching of the decay function could be due to the existence of a distribution of relaxation times. Most authors believe that it describes a characteristic feature of the relaxation process. In fact, a phenomenological model was developed in which b accounts for the degree of correlation between moving charges [8]. A non-exponential decay of the electric field has been extensively shown in glassy ionic conductors but, to the best of our knowledge, less work has been devoted to the study of the electrical conductivity relaxation in crystalline ionic conductors.
Fig. 2. Real (a) and imaginary (b) parts of the complex modulus as a function of frequency for the Li 0.5 Na 0.5 La(CrO 4 ) 2 sample at 262 (♦), 275 (.), 296 (m), 317 (d) and 336 K (j).
522
P. Melnikov et al. / Journal of Alloys and Compounds 250 (1997) 520 – 523
Fig. 3. A.c. conductivity vs. frequency for a Li 0.5 Na 0.5 La(CrO 4 ) 2 sample at 262 (♦), 267 (.), 275 (m), 2816 (d) and 296 K (j).
between mobile species [9]. The n values determined from non-linear least squares fits to that expression are in the range 0.57–0.62, approximately satisfying b 5 1 2 n as was previously proposed [10]. It is interesting that for the different samples analysed, and irrespective of whatever the conducting ions were (Na, Li or their combinations in different proportions), both b and n showed practically the same values. The b and t values determined from the fits were used to determine the conductivity according to the expressions in Eq. (2): kt l 5 ´ /s and kt l 5 tG (1 /b )b
(2)
where G is the Euler gamma function. Fig. 4 shows conductivity plots against the inverse of
temperature in an Arrhenius fashion for samples with different compositions. It can be seen that the high temperature phase of LiLa(CrO 4 ) 2 unexpectedly exhibits the lowest conductivity values (open triangles). This double chromate belongs to the structural type Na 3 Nd(PO 4 ) 2 [11] and may be envisaged as a result of the following heterovalent substitutions: 3Na 1 1 Nd 31 1 2e 2 → Li 1 1 La 31 and 2PO 432 2 2e 2 → 2CrO 422 . So the carrier motion along the inner spaces may be blocked by La 31 ions trapping Li 1 within the three-dimensional network of this structure. The poor exchange rate of Li 1 for H 1 in the experiments with benzoic acid [4] may be considered as an additional proof of such mobility restrictions. This example is good enough to show that the presence of a small lithium cation in a negatively charged anionic framework is by no means a guarantee for promising ionic conductivity. The low temperature variety of LiLa(CrO 4 ) 2 belongs to a completely different structural type (monoclinic KLu(CrO 4 ) 2 ) where K 1 ions are located in comparatively large tunnels running along the a axis of the lattice [12]. Consequently, when the migration of Li 1 ions becomes easier within these relatively spacious voids the conductivity in a polycrystalline sample of the low temperature LiLa(CrO 4 ) 2 modification is enhanced by more than two orders of magnitude (Fig. 4, solid triangles). As to the sodium-containing phases, the replacement of small amounts of sodium by lithium, e.g. in the composition Li 0.1 Na 0.9 La(CrO 4 ) 2 (Fig. 4, open circles), yields practically the same values of conductivity and activation energy as for pure Na compositions (Fig. 4, solid squares). As soon as some extra lithium is introduced into the structure the conductivity rises drastically, keeping the activation energy unaltered. It could be explained by the fact that cell parameters of NaLa(CrO 4 ) 2 and those of Li 12x Na x La(CrO 4 ) 2 are practically identical [5], so this increment would be due exclusively to the contribution of small guest ions moving more easily through the interstices of the NaLa(CrO 4 ) 2 host network.
Acknowledgments Thanks are due to CNPq, Brazil.
References
Fig. 4. Arrhenius plot of the conductivity for high temperature LiLa(CrO 4 ) 2 (n), low temperature LiLa(CrO 4 ) 2 (m), Li 0.1 Na 0.9 La(CrO 4 ) 2 (s), NaLa(CrO 4 ) 2 (j) and Li 0.5 Na 0.5 La(CrO 4 ) 2 (d).
[1] Sh.B. Ibrahimov, E.G. Zaugolnikova, P.P. Melnikov and L.N. Komissarova, SU Patent 1.71, Cl GO1 N2 /, February 15, 1992. ´ C. Leon, ´ J. Santamarıa ´ and F. [2] P. Melnikov, I. Bueno, E. Moran, ´ Sanchez-Quesada, Solid State Ionics, 63 – 65 (1993) 581. [3] P. Melnikov, J. Chem. Res., (1993) 2939 (microfiche version). [4] P. Melnikov, J. Alloys Compd., 201 (1993) 181. [5] P. Melnikov, J. Alloys Compd., 209 (1994) 269. [6] P.B. Macedo, C.T. Moynihan and R. Bose, Phys. Chem. Glasses, 13 (1972) 171.
P. Melnikov et al. / Journal of Alloys and Compounds 250 (1997) 520 – 523 [7] V. Provenzano, L.I. Boesch, V. Volterra, C.T. Moynihan and P.B. Macedo, J. Am. Ceram. Soc., 55 (1972) 492. ¨ [8] G. Balzer-Jollenbeck, O.K. Lanert, H. Jain and K.L. Nghai, Phys. Rev. B, 9 (9) (1989) 6071. [9] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, 1983.
523
[10] K. Funke, Prog. Solid State Chem., 22 (1993) 111. [11] R. Salmon, C. Parent, M. Vlasse and G. LeFlem, Mater. Res. Bull., 13 (1978) 4395. [12] I. Bueno, C. Parada, A. Monge and C. Ruiz Valero, Proc. Spanish– Latin American Inorganic Chemistry Meet., Santiago de Compostela, Spain, September, 1993, p. 392.
L
Lanthanum–lithium–sodium double chromates as ionic conductors a, b ´ b , J. Santamarıa ´ b , F. Sanchez-Quesada ´ P. Melnikov *, C. Leon a
Institute of Chemistry, UNESP-Araraquara, CP 355, CEP 14800 -900, Araraquara SP, Brazil b ´ ´ Facultad de Ciencias Fısicas , Universidad Complutense de Madrid, 28040, Madrid, Spain
Abstract Lanthanum–lithium–sodium double chromates Li 12x Nax La(CrO 4 ) 2 were prepared and analysed by means of admittance spectroscopy. Their a.c. conductivity parameters are correlated with structural details of high and low temperature forms of pure lanthanum–lithium double chromates. Lithium compounds show the lowest conductivity values and the highest activation energy for ion motion, while the sample Li 0.5 Na 0.5 La(CrO 4 ) 2 exhibits the highest conductivity 10 25 S cm 21 and the lowest activation energy 0.58 eV. Keywords: Lanthanum; Alkali metals; Double chromates; Admittance spectroscopy; Conductivity; Activation energy
1. Introduction A number of inorganic materials have been prepared making substitutions in the matrix Me n Ln m (TO 4 ) 1 , where Me is an alkali metal, Ln is a rare earth element and T is P, As, V, Cr, etc. [1,2]. Recently we described and discussed structural details and relationships in the family of double chromates MeLa(CrO 4 ) 2 , where Me 5 Na–Cs and Ln 5 La–Lu [3]. The existence of less well-known lithium and lithiated sodium phases was also confirmed [4,5]. Structural considerations have been of value in predicting electrical conductivity either of electronic or of ionic types for Na, K and Rb double chromates analysed by means of admittance spectroscopy [2]. The choice and behaviour of mobile cation are, as a rule, decisive factors in the design of chemical sensors. The purpose of the present work is a comparative study of the phases Li n Na 12n La(CrO 4 ) 2 following the standard impedance method and using modulus formalism.
2. Experimental The preparation of the low temperature variety of LiLa(CrO 4 ) 2 was conducted as in Ref. [4] with some modifications. Since the transition into the high temperature form takes months to achieve, this phase was obtained directly using a hydrothermal technique. A mixture of
*Corresponding author. Tel.: (155) 162 32 2022; fax: (155) 162 22 7932. 0925-8388 / 97 / $17.00 1997 Elsevier Science B.V. All rights reserved PII S0925-8388( 96 )02534-0
Li 2 CO 3 , La 2 O 3 and CrO 3 taken in stoichiometric quantities was dissolved in diluted HCl (1:20) and this solution heated in a sealed glass ampoule at 155 8C for 22 days. LiLa(CrO 4 ) 2 precipitated in the form of fine yellow powder, then was washed with ethyl alcohol and dried at 160 8C. Mixed lithium–sodium chromates were prepared by solid-state reaction between Li 2 CO 3 , Na 2 CO 3 , La 2 O 3 and CrO 3 , also taken in stoichiometric quantities, ground in hexane and after drying at 100 8C heated at 300 8C for 72 h. All starting reagents were Merck products, of analytical grade purity. Phase analysis was carried out by X-ray powder diagrams. Admittance spectroscopy measurements were conducted using an automatically controlled HP 4284A precision LCR meter, frequency ranged from 20 Hz to 1 MHz. To ensure a low noise level, the mean of 16 measurements was used at each frequency. Test specimens were cold pressed cylindrical pellets 5 mm in diameter and approximately 0.5 mm thick, on top of which gold contacts were deposited by evaporation through a mechanical mask. Pellets were mounted in a sealed stainless steel holder through which a high purity dry nitrogen gas was passed to ensure an inert atmosphere and to avoid water condensation. Temperature range was 200–500 K.
3. Results and discussion The experimental findings indicate clear blocking phenomena at grain boundaries and electrode characteristics of ionic conductors. Fig. 1 shows complex impedance plots for a representative sample Li 0.5 Na 0.5 La(CrO 4 ) 2 at various
P. Melnikov et al. / Journal of Alloys and Compounds 250 (1997) 520 – 523
Fig. 1. Complex impedance plot of a Li 0.5 Na 0.5 La(CrO 4 ) 2 sample at 326, 334 and 350 K.
temperatures. Well developed spurs, characteristic of blocking at electrodes, can be observed at low frequencies, and a second semicircle, overlapped with the bulk one, is associated with the partial blocking of grain boundaries. Measurements have been conducted at low temperatures in order to exclude the effect of blocking mechanisms, since their stronger activated contributions are shifted to lower frequencies. The usage of a standard complex impedance method and modulus formalism offers the advantage of lesser sensitivity to the electrode related processes and provides information about the dynamics of the conductivity relaxation. The dielectric modulus M * 5 1 /´ * is related to the Fourier transform of the decay function through the expression `
3 E
1 df M * (v ) 5 ] 1 2 ]e 2j v t dt ´` dt 0
4
521
Fig. 2(a) and Fig. 2(b) respectively show the real and the imaginary parts of the modulus peak for the same sample Li 0.5 Na 0.5 La(CrO 4 ) 2 . The non-Debye character of the relaxation can be readily seen from the width of the imaginary modulus peak, which is greater than 1.12 decades for FWMH in a Debye relaxation. The reported continuous lines show the fitting to expression in Eq. (1) for b 5 0.43. For the temperature range within which the peak was observable in the available frequency range b remained almost constant, except for the peaks at the edge of the frequency window where changes of about 10% were observed. The non-Debye character of the conductivity relaxation analysed in the time domain using a KWW function can also be observed in the conductivity representation. Fig. 3 shows the real part of the conductivity exhibiting no blocking phenomena in this temperature range and a power behaviour in the form v n at high frequencies, so that the conductivity can be fitted to expressions in the form s 5 s0 1 A(T )v n . This power law dependence has been termed universal response, and the value of the exponent n has been related to correlation
(1)
where ´` is the high frequency permittivity and f (t ) describes the decay of the electric field E (E(t) 5 E(0)f (t)) at constant displacement vector D [6,7]. While for ideal conductors f (t) is an exponential exp(2t /t ), the relaxation being of the Debye kind, for most ionic conductors the decay is best described by a stretched exponential f (t) 5 exp(2t /t ) b . The description of the relaxation using the KKW function is empirical, and the meaning of b remains unclear. It has been claimed that the stretching of the decay function could be due to the existence of a distribution of relaxation times. Most authors believe that it describes a characteristic feature of the relaxation process. In fact, a phenomenological model was developed in which b accounts for the degree of correlation between moving charges [8]. A non-exponential decay of the electric field has been extensively shown in glassy ionic conductors but, to the best of our knowledge, less work has been devoted to the study of the electrical conductivity relaxation in crystalline ionic conductors.
Fig. 2. Real (a) and imaginary (b) parts of the complex modulus as a function of frequency for the Li 0.5 Na 0.5 La(CrO 4 ) 2 sample at 262 (♦), 275 (.), 296 (m), 317 (d) and 336 K (j).
522
P. Melnikov et al. / Journal of Alloys and Compounds 250 (1997) 520 – 523
Fig. 3. A.c. conductivity vs. frequency for a Li 0.5 Na 0.5 La(CrO 4 ) 2 sample at 262 (♦), 267 (.), 275 (m), 2816 (d) and 296 K (j).
between mobile species [9]. The n values determined from non-linear least squares fits to that expression are in the range 0.57–0.62, approximately satisfying b 5 1 2 n as was previously proposed [10]. It is interesting that for the different samples analysed, and irrespective of whatever the conducting ions were (Na, Li or their combinations in different proportions), both b and n showed practically the same values. The b and t values determined from the fits were used to determine the conductivity according to the expressions in Eq. (2): kt l 5 ´ /s and kt l 5 tG (1 /b )b
(2)
where G is the Euler gamma function. Fig. 4 shows conductivity plots against the inverse of
temperature in an Arrhenius fashion for samples with different compositions. It can be seen that the high temperature phase of LiLa(CrO 4 ) 2 unexpectedly exhibits the lowest conductivity values (open triangles). This double chromate belongs to the structural type Na 3 Nd(PO 4 ) 2 [11] and may be envisaged as a result of the following heterovalent substitutions: 3Na 1 1 Nd 31 1 2e 2 → Li 1 1 La 31 and 2PO 432 2 2e 2 → 2CrO 422 . So the carrier motion along the inner spaces may be blocked by La 31 ions trapping Li 1 within the three-dimensional network of this structure. The poor exchange rate of Li 1 for H 1 in the experiments with benzoic acid [4] may be considered as an additional proof of such mobility restrictions. This example is good enough to show that the presence of a small lithium cation in a negatively charged anionic framework is by no means a guarantee for promising ionic conductivity. The low temperature variety of LiLa(CrO 4 ) 2 belongs to a completely different structural type (monoclinic KLu(CrO 4 ) 2 ) where K 1 ions are located in comparatively large tunnels running along the a axis of the lattice [12]. Consequently, when the migration of Li 1 ions becomes easier within these relatively spacious voids the conductivity in a polycrystalline sample of the low temperature LiLa(CrO 4 ) 2 modification is enhanced by more than two orders of magnitude (Fig. 4, solid triangles). As to the sodium-containing phases, the replacement of small amounts of sodium by lithium, e.g. in the composition Li 0.1 Na 0.9 La(CrO 4 ) 2 (Fig. 4, open circles), yields practically the same values of conductivity and activation energy as for pure Na compositions (Fig. 4, solid squares). As soon as some extra lithium is introduced into the structure the conductivity rises drastically, keeping the activation energy unaltered. It could be explained by the fact that cell parameters of NaLa(CrO 4 ) 2 and those of Li 12x Na x La(CrO 4 ) 2 are practically identical [5], so this increment would be due exclusively to the contribution of small guest ions moving more easily through the interstices of the NaLa(CrO 4 ) 2 host network.
Acknowledgments Thanks are due to CNPq, Brazil.
References
Fig. 4. Arrhenius plot of the conductivity for high temperature LiLa(CrO 4 ) 2 (n), low temperature LiLa(CrO 4 ) 2 (m), Li 0.1 Na 0.9 La(CrO 4 ) 2 (s), NaLa(CrO 4 ) 2 (j) and Li 0.5 Na 0.5 La(CrO 4 ) 2 (d).
[1] Sh.B. Ibrahimov, E.G. Zaugolnikova, P.P. Melnikov and L.N. Komissarova, SU Patent 1.71, Cl GO1 N2 /, February 15, 1992. ´ C. Leon, ´ J. Santamarıa ´ and F. [2] P. Melnikov, I. Bueno, E. Moran, ´ Sanchez-Quesada, Solid State Ionics, 63 – 65 (1993) 581. [3] P. Melnikov, J. Chem. Res., (1993) 2939 (microfiche version). [4] P. Melnikov, J. Alloys Compd., 201 (1993) 181. [5] P. Melnikov, J. Alloys Compd., 209 (1994) 269. [6] P.B. Macedo, C.T. Moynihan and R. Bose, Phys. Chem. Glasses, 13 (1972) 171.
P. Melnikov et al. / Journal of Alloys and Compounds 250 (1997) 520 – 523 [7] V. Provenzano, L.I. Boesch, V. Volterra, C.T. Moynihan and P.B. Macedo, J. Am. Ceram. Soc., 55 (1972) 492. ¨ [8] G. Balzer-Jollenbeck, O.K. Lanert, H. Jain and K.L. Nghai, Phys. Rev. B, 9 (9) (1989) 6071. [9] A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric Press, London, 1983.
523
[10] K. Funke, Prog. Solid State Chem., 22 (1993) 111. [11] R. Salmon, C. Parent, M. Vlasse and G. LeFlem, Mater. Res. Bull., 13 (1978) 4395. [12] I. Bueno, C. Parada, A. Monge and C. Ruiz Valero, Proc. Spanish– Latin American Inorganic Chemistry Meet., Santiago de Compostela, Spain, September, 1993, p. 392.