Electrical-conductivity behavior in crystalline cerium (IV) phosphates
N. M. SHASH(1) and H. M. ALY(2)
(1) Physics Department, Faculty of Science - Benha, Egypt
(2) Chemistry Department, Faculty of Science - Benha, Egypt
(ricevuto il 13 Febbraio 1996; revisionato il 5 Agosto 1996; approvato il 27 Gennaio 1997)
Summary. — Cerium (IV) phosphate compounds with a different crystalline form had been prepared. The temperature dependence of the electrical conductivity, s , showed an anomalous behavior in a temperature range from 30 to 220 7C. As the temperature is increased to a characteristic value, T * , s attenuates to a minimum value where it activates with temperature. The results are discussed according to the diminishing of protonic conduction as the temperature is raised at relatively low temperature range. The activation energies of conduction are found to be of strong dependence on the lattice water content. The temperature dependences of the activation energies around T * are also discussed. The dielectric loss measurements give a direct evidence to the effect of lattice water on the barrier heights for charge carrier jumping.
PACS 72.15 – Electronic conduction in metals and alloys.
1. – Introduction
In recent years, there has been an intensification of research aimed to discovering new proton conductors and studying the mechanisms of conduction in these compounds. This renewed activity has been driven by the potential use of such compound in fuel cells, sensors, water electrolysis units and other electrochemical devices.
Cerium (IV) phosphates are of interest because some of them exhibit ion exchange behaviors. However, there is a storage of information in the electrical properties of these materials. Cerium phosphates as ion exchangers have been prepared under different conditions [1]. The composition, stability, degree of crystallinity and structure are strongly dependent on the experimental conditions of preparation [2, 3].
Cerium phosphates constitute a complicated system of compounds of unknown structure and interesting behavior. Emphasis has been done on the kinetic behavior of these compounds as well as the ion exchange properties [4]. The present study was undertaken to clarify and extend the knowledge of the electrical conduction behavior of crystalline cerium (IV) phosphate compounds.
N.M.SHASHandH.M.ALY 930
2. – Experimental
The chemicals were analytical grade reagent and used without farther purification. Cerium phosphate has been prepared as previously described [1]. The initial ratios of PO23
4 /Ce in solution, reflux time and water content are given in table I.
TABLEI. – Formulas, water content, ion exchange capacity, PO23
4 /Ce ratio and reflux time (in
days), for the different compounds of crystalline cerium (IV) phosphates.
Sample Formula H2O (%) IEC(*) (meq Og) Ratio PO23 4 /Ce Reflux time (days) N1 Ce( OH )0.375( PO4)0.375 [ ( NH4)0.09H10.16( PO4)1.25] Q 0.25H2O 2.10 0.92 2.68 6.50 N2 Ce( OH )0.45( PO4)0.45( HPO4)1.1Q 0.33H2O 3.36 1.50 2.00 7.90 N3 Ce( OH )0.7( PO4)1.1Q 0.5H2O 4.20 1.38 5.00 5.90 N4 Ce( HO )0.27( PO4)0.27( HPO4)1.46Q 0.55H2O 10.00 7.63 5.00 6.60 N5 Ce( OH )1.62( NH4HPO4)0.35( H2PO4)0.68 ( PO4)0.45Q 0.6H2O 10.60 7.82 3.75 8.00
(*) Ion exchange capacity.
The electrical conductivity measurements have been carried out on cerium phosphate pellets of A 10 mm diameter and A 3 mm thickness compressed at 33103
kgm Ocm2. The pellets were coated with silver using a conducting silver paint which showed Ohmic contact with the samples. However, the fused calcium chloride was placed at the bottom of the sample holder to protect the samples from moisture. The DC conductivity measurements were carried out using two electrode methods and the circulating current was measured using a Keithly 617 programmable electrometer. The noise was reduced by using coaxial cables, metal shielding and common group loops.
The isothermal studies were achieved using an U10 type ultrathermostate with a precision 62 7C over all the range of temperature. The sample temperature was measured using a Cu-constantane thermocouple.
The dielectric parameters were measured, in a frequency range from 50 Hz to 100 kHz, by using a Philips PM 6304 programmable automatic RLC meter. Two fine wires of copper were attached to the area electrodes by using silver dag.
3. – Results and discussion
3.1. Temperature dependence of DC conductivity. – The five crystalline forms of cerium (IV) phosphates under investigation were given symbols N1, N2, N3, N4 and N5. The formula, water content, ion exchange capacity (IEC), ratio of PO23
4 /Ce and the reflux time are given in table I. From this table it is clear that as the water percent increases, the IEC increases. This is in agreement with that results obtained by Aly [4].
Figure 1 shows the temperature dependence of the electrical conductivity, sDC, of different samples of cerium (IV) phosphate compounds in temperature range between room temperature and 220 7C. The electrical conductivity decreases as the temperature increases, in the relatively low temperature range. The attenuation in sDC
Fig. 1. – The temperature dependence of the electrical conductivity of crystalline cerium (IV) phosphates compounds.
N.M.SHASHandH.M.ALY 932
is extended to the characteristic temperature T * , then an increase of the electrical conductivity with increasing temperature takes place. In addition, as the water percent in different cerium (IV) phosphate samples increases, the conductivity increases.
The relatively low temperature conductivity values are of the same order of that of water. Therefore, the water content of the compounds plays the dominant rule in the conductivity behavior and the origin of the charge carriers under the influence of the applied electric field [5-8].
Generally, the electrical conduction in such compounds can be attributed to two mechanisms; one is based on the attenuation of s with increasing temperature below
T * which may be attributed to the effect of dehydration of the samples. The variation of
the electrical conductivity per unit temperature, in the relatively low temperature range, is strongly dependent on the degree of hydration of these compounds. However, the presence of such H2O molecules in cerium phosphates results in protonic conduction which contributes with remarkable component in conduction process [9]. Chowdhry et al. [8] have attributed conduction in hydrated metal oxide materials, at low temperature up to 100 7C, to the protonic conduction. Herman and Clearfield [2] have studied the thermogravimetric and I.R. analysis for different compounds of cerium (IV) phosphates. They found that in all the investigated samples a gradual loss of weight of samples was recorded up to 200 7C which has been attributed mainly to the continuous split out of lattice water. This was also confirmed by the presence of weak absorption band at 1606–1613 cm21.
The attenuated part in s-103/T relation may be discussed according to the following Arrhenius relation [10]:
s 4BT exp [DE 8 /KT] ,
(1)
where B is a temperature-independent pre-exponential parameter, DE 8 is an activation energy term concerning the attenuation in conduction process and K is Boltzmann’s constant. The values of D E 8 which fit relation (1) are obtained and listed in table II. The second mechanism is the activation of the electrical conductivity above T * , which may be attributed to the thermal activation of charge carriers in the host matrix of crystalline cerium (IV) phosphates. This activated part in s-103/T relation could be discussed according to the following relation [11]:
sT 4A exp [2DE/KT] ,
(2)
where DE is the activation energy of the conduction, and A is a temperature-independent parameter. The values of D E are obtained and listed in table II.
TABLE II. – The activation energy values for the investigated samples as a function of lattice
water content. Sample H2O content (%) D E 8( eV )6S.E. D E( eV )6S.E. N1 2.10 0.72060.148 0.96360.130 N2 3.36 0.69160.019 0.71860.170 N3 4.20 0.59660.044 0.70360.071 N4 10.00 0.51460.061 0.51060.104 N5 10.60 0.41060.130 0.48760.037
Fig. 2. – The lattice water content dependence of the activation energy of crystalline cerium (IV) phosphates compounds.
The competition of these two mechanisms in the two temperature ranges results in the appearance of the minimum in the electrical conductivity values at the characteristic temperature T*.
From table II and fig. 2, it is noticed that as the water content in these compounds increases, the activation energy decreases which agrees with the conclusion obtained by Clearfield [12].
In fact, the obtained values of both DE 8 and DE are valid only in the relatively low-and high-temperature ranges, respectively. In the narrow range of temperatures around T * , the temperature dependence of the activation energy, in these two mentioned ranges of temperatures, are found to obey the following relations:
E 8(T) 4DE 82a8(T *2T)1b 8 for T ET * , (3)
E(T) 4DE2a8(T2T *)1b for T DT * ,
(38)
where E 8(T) and E(T) are the temperature dependence activation energies below and beyond T *, respectively and a 8, b 8, a and b are the fitting parameters. The obtained values of these fitting parameters are summarized in table III. From this table, it is clear that the values of a 8[4dE 8(T)/dT] decrease with increasing water percent in cerium phosphate structures. This is expected because of the slight effect on protonic
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TABLEIII. – The fitting parameters a 8, b 8, a and b for different samples.
Sample Attenuated Activated
a 8 b 8 a b N1 0.024 0.852 0.012 0.983 N2 0.021 0.994 0.019 0.617 N3 0.015 0.647 0.009 0.669 N4 0.011 0.302 0.014 0.487 N5 0.010 0.392 0.021 0.390
conduction in the high water content in such structure. In the lower water content up to 4.2% the values of b 8 are found to be higher than DE 8, whereas they are lower than D E 8 for the higher water content (10 and 10.6%). Actually, the energy parameter, b 8, must be lower than D E 8 at any temperature below T *. Therefore, the unexpected higher values of this parameter could be attributed to different mechanisms which affect the conduction process, such as the early accumulated hydroxyl groups of the framework of cerium phosphate structures.
On the other hand, beyond T * , beside the protonic conduction due to the water content, there is a contribution of accumulated hydroxyl groups as well as the thermal generation of electrons in conduction process. Therefore, the irregular variation of a with water percent could be attributed to the domination of any of the mentioned components in conduction process.
3.2. Isothermal annealing studies. – The isothermal annealing studies were accomplished between room temperature and 90 7C for all investigated samples. Figure 3, as a representative one, illustrates semilogarithmic plots of the electrical conductivity, s , against the time of annealing, t . It is noticed that s decreases as the annealing time increases up to a stationary value, ss. The attenuation in s with increasing time of annealing is the resultant of carrier generation and carrier recombination and Oor capturing up to the equilibrium state. The s-t behavior may be discussed according to the following relation [13, 14]:
[ (ln st2 ln ss) /(ln s02 ln ss) ] Aexp [2Rt] , (4)
where, st, ss and s0 are the conductivities at time t , the stationary value and at t 40 respectively, and R is the rate constant. The deduced values of R are listed in table IV. From this table it is clear that the rate process of conduction is temperature dependent process, however, R(T) may be discussed according to the following relation:
R(T) Aexp [DEr/KT] , (5)
where D Er is an activation energy concerning carriers recombination and Oor carrier annihilation. The values of D Er are estimated using the least square method and are given in table IV. The activation energy values of the rate constant R calculated according to eq. (5) suggest that the recombination process occurs at trapping levels between 0.244 and 0.566 eV.
Fig. 3. – Time dependence of the electrical conductivity at different ambient temperatures for sample N5.
3.3. Dielectric measurements. – Figure 4, as a representative one, illustrates the frequency dependence of the dielectric loss, e 9, for all the investigated samples at different temperatures. It is clear that the general behavior is the decrease of the dielectric loss with increasing frequency. In addition, as the temperature increases, in the relatively low temperature range, e 9 decreases. This may be attributed to the presence of H2O molecules which contribute with the remarkable component in the
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TABLE IV. – The rate constant and activation energy at different temperatures for all
investigated samples. Temperature (7C) N1 N2 N3 N4 N5 30 0.0625 0.0625 0.0316 0.0473 0.0360 50 0.0110 0.0540 0.0038 0.0277 0.0484 70 0.0094 0.0096 0.0021 0.0210 0.0169 90 0.0088 0.0144 — 0.0015 0.0070 Activation energy D E (eV) 0.294 0.290 0.566 0.494 0.244
dielectric loss. Therefore, it is expected that the values of the dielectric loss are shifted to the lower values as the temperature is raised at this temperature range. As the temperature is raised, above T * , the contributions of host material in dielectric loss dominate and the values of e 9 are shifted to higher values. No systematic trend in the temperature dependence of e 9 is observed.
The frequency dependence of dielectric loss can be expressed according to the
Fig. 4. – Frequency dependence of dielectric loss, e 9, at different ambient temperatures for sample N4.
TABLE V. – The calculated values of Wm(eV) at different temperatures for the samples under investigation. Temperature (7C) N1 N2 N3 N4 N5 20 0.198 0.112 0.168 0.117 0.116 35 0.193 0.119 0.141 0.122 0.119 50 0.210 0.125 0.155 0.135 0.130 75 0.231 0.137 0.663 0.174 0.151 100 — 0.141 0.727 0.207 0.175 120 — 0.156 0.623 0.237 0.188 160 0.203 0.158 0.673 0.250 0.214 200 0.193 0.188 0.613 0.258 0.238 following relation [15]: e 9(v) 4 (e02 eQ) 2 p 2N(ne2/e 0)3KTtm0Wm24vm, (6)
where m 44KT/Wm, n is the number of carriers that hop, N is the concentration of
localized states and Wm is the energy required to move the charge carrier (proton
and Oor electron) from one site to infinite.
Accordingly, eq. (6) could be reformulated as follows:
e 9AAv2m. (7)
The plots of log e 9 against logv suggests straight lines, fig. 4, with slopes equal to m. The values of Wm can be calculated, for all the investigated samples at different
temperatures, and are given in table V. In general, it is clear that, as the temperature increases, the values of Wmincrease in all the studied temperature range. This may be
attributed to the discontinuity in water chains due to the continuous split out of lattice water which results in increasing the distance between tow adjacent water molecules. Therefore, the jump distance or the energy required for jumping is increased. The observed irregular variation of Wm, in the relatively high temperature range, for
samples N1 and N3 may be due to the presence of ammonium ion in compound N1 and also the condensation of hydroxyl groups in N3. However, this confirmed by the TG and I.R. analysis is in agreement with that obtained by Herman and Clearfield [2].
* * *
Thanks are due to Dr. M. K. EL-MANSYfor fruitful discussion and suggestions.
R E F E R E N C E S
[1] CLEARFIELDA., Prog. Crystal Growth Charact., 21 (1990) 1.
[2] HERMANR. G. and CLEARFIELDA., J. Inorg. Nucl. Chem., 37 (1975) 1697. [3] HERMANR. G. and CLEARFIELDA., J. Inorg. Nucl. Chem., 38 (1976) 853. [4] ALYH. M., Solvent Extr. Ion Exch., 14 (1996) 171.
[5] TENNAKONEK., J. Solid State Chem., 50 (1983) 112. [6] TENNAKONEK., J. Solid State Chem., 52 (1984) 217.
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[7] EL-MANSYM. K., DIEFALLAHE. M. and SHASHN. M., Radiat. Phys. Chem., 45 (1995) 151. [8] CHOWDHRYU., BARKLEYJ. R., ENGLANDA. D. and SLEIGHTA. W., Mat. Res. Bull., 17 (1982) 917. [9] ENGLANDW. A., CROSSM. G., HAMNETTA., WISEMAN P. J. and GOODENOUGHJ. B., Solid
State Ionic, 1 (1980) 231.
[10] KLAFFKYR. W., ROSE B. H., GOLANDA. N. and DIVEVESG. J., Phys. Rev. B, 12 (1980) 3610. [11] AUSTINand MOTTN. F., Adv. Phys., 18 (1969) 41.
[12] CLEARFIELDA., Chem. Rev., 88 (1988) 125.
[13] DIEFALLAHE. M., EL-MANSYM. K., MOUSAM. A., HASSANM. K. and EL-GAHAMYM. A., Rad.
Phys. Chem., 26 (1985) 619.
[14] ONSAGERL., Phys. Rev., 54 (1938) 554.
[15] GIUNTINIJ. C., ZANCHETTAJ. V., JULLIEND., EHOLIER. and HOUENOU P., J. Non-Crystal
