Ionic Conduction in Superionic System Li2O:P2O5:WO3

Measurement of electrical conductivity (σ) and Seebeck coefficient (S) of the studied superionic system Li2O: P2O5: WO3 have been reported in the temperature range 415K to the melting point 1110K of the solid. All the measurement have been performed on solidified melt. The studied system shows a clear superionic phase. In the temperature range 950K to 1110K, it shows high ionic conductivity (σi ~ 90Ω-1 m-1 at 1050K) with small (0.38eV) activation energy and almost negligible electronic conductivity. Molar magnetic susceptibility (XM) and dielectric constant (K) have also been measured against temperature to confirm anomalies observed in σ and S results.

was then melted, shaken thoroughly and allowed to cool slowly to get solidified melt. X-ray diffraction shows that no unreacted starting material was left and single phase compound is formed. The measurement on electrical conductivity (σ), Seebeck coefficient (S) and dielectric constant (K) were performed on solidified melt whereas molar magnetic susceptibility (X M ) was performed on powdered sample from 400K to the melting point of the solid. The details of sample holder and measurements have been described elsewhere [21][22][23] .

RESULTS AND DISCUSSION
The electrical conductivity (σ ac ) as a function of frequency (50Hz, 100Hz, 1kHZ, 3kHz and 10kHz) has been measurement at constant temperature. The results are not shown. For comparison σ dc values at corresponding temperature are also given in same figure. It is seen from this figure that there is no difference between σ dc values at different frequencies. Also σ dc is almost same as σ dc . This result indicates that grain-boundary effects are not very significant in this solid. To investigate the nature of charge carrier σ dc has been measured as a function of time at a constant temperature. This study has been preformed at six widely separated temperatures. The results are shown in Fig. 1. It is seen form this figure that σ dc decreases with time and tends to attain a constant value after a long time. This time of constancy becomes small at lower temperature. This indicates that the compound is essentially mixed conductor. The value extrapolated to t=0 gives total (electronic + ionic) conductivity (σ) where as the value obtained for t→∞ gives electronic part of total conductivity. Using he plot of σ dc vs T -1 s values s e have been obtained. The ratio (r) is then calculated using equation-1 at five to six widely separated temperature. The plot of log r with temperature is shown in Fig.2. It is seen from this figure that r increases with temperature becomes very fast for T>890K and at higher temperature T>1000K it tends to decrease with temperature.
σ and S measurements on three to four solidified melts from different lots have been done as a function of temperature. No hysteresis was observed and obtained σ and S values are almost in both heating and cooling cycles. The results are shown as log σT vs T -1 plots in Fig. 3. It is seen from this figure that s values is high being 1.85 × 10 -2 at 50K. The material is hygroscopic. The log σT vs T -1 plot is linear between temperature range 415 K to 765K after that there is step and continuous increase of log σ and then conductivity increase slowly till the melting point. 1110K of the solid. The linear region of the plot can be represented by the equation 2.
The values of Ea and C are 0.37 ev and 1.87 × 10 3 Ω -1 m -1 K and 0.45 ev and 4.40 × 10 5 Ω -1 m -1 K for temperature range 415 K to 765K and 765 K to 1110K respectively. The S vs T -1 plot is also linear in the temperature interval 500K to 760K and 770K to 1110K. The linear region can be represented by the equation 3. The ratio of ionic electronic conductivities at different temperatures have been evaluated from time dependence study of σ dc . Using these values we have separated ionic (σ i ) and electronic (σ e ) parts of electrical conductivity. Plot of log σ i T vs T -1 is shown in Fig. 3. by dotted line. It is seen from this figure that there is almost no difference between σ i and σ at higher temperature (T>800K). However, they differ to some extent in lower temperature range. In the second region σ is very high (~100Ω -1 m -1 ). The ratio r is of the order of 10 3 and essentially σ i =σ. Thus this compound is pure ionic conductor. Thermo electric power in this range is negative indicating cations to be the mobile charge carrier. Thus from the value of σ and ionic nature of conductivity we can say that the compound is a good solid electrolyte. The span of superionic phase is 160K (950-1110K). The transition to superionic phase takes place at 950K. The linear regions of log σ T and S vs T -1 plots can be represented by the relation. Such as result is predicted by Extened latic gas model. In the lower temperature range of electronic contribution is small (from 20 to 10 present). Thus there is dominance of ionic conductivity. The sign of S in this temperature range is positive. Hence current carries are negatively charged. This can happpen when either cation vacancies of anions are mobile in this temperature range. Motion of anions at lower temperature is less probable. Hence the conduction seemes to be dominated by cation vacancies or Schottky type defects.
The log σ e T vs T -1 plot (Fig. 3.) is similar to the plot of log σ i T vs T -1 . σ e jumps by several order of magnitude around particular temperature. The linear region can be represented by the equation 6. The value of σ 0 and W are 3.08 × 10³ Ω -1 m -1 and 0.51 eV.
The X M and K studies are performed on two to these samples prepared in different lots. The X M and log K plots are shown in Fig. 4. It is seen from that X M increases very slowly up to 770K, shows a peak at 950K then decreases wiht increase of temperature. The value of X M is throughout negative. Similar trend of variation is observed in log K vs T. The value of K is 55 at 500K. It increase with temperature up to 950K then decreases slowly indicating an anormaly at 945K as observed in log σT vs T -1 plot. The studied superionic system is ionic compound and contain no magnetic ions. Thus it is area diamagnetic as has been confirmed by the negative value of X M . Their diamagnetism depends upon its bonding configuration. The onset of disordering at phase transition disrupts the bonding configuration and leads to an abrupt change in X M . Thus the temperature around which anomaly appears is also confirmed by K and σ plots.