Li-content of the Elec- trical Conductivity of Solid and Molten Lithium

The Dependence on the

Li-content of the Elec- trical Conductivity of Solid and Molten Lithium

Sulphate

K J E L L S C H R O E D E R a n d A R N O L D K V I S T

Department of Physics, Chalmers University o f Technology, Göteborg

(Z. Naturforsch. 25 a , 1 3 6 5 — 1 3 6 6 [1970] ; received 29 Juni 1970)

The electrical conductivity of solid and molten lithium sulphate has been measured as a function o f temperature and 6Li-content. The electrical conductivity is higher for

6Li2S04 than for 7Li2S04. F o r f . c . c . lithium sulphate the conductivity is independent o f temperature and Ax/x = 4 . 7 3 % for the isotopically pure salts. F o r the melt

Ax/xl = [4.29 + 1.6 • l O "3 (t - 860)] % (t in°C).

There is, within the experimental errors, n o excess con- ductivity in the mixtures of 6Li2S04 and 7L i 2 S 0 4 .

Some years ago we published results o f conductivity measurements in 6Li2S04 and 7Li2S04 L W e have n o w made a more thorough investigation o f the temperature dependence of this effect b o t h in the solid f. c. c. phase and in the melt up to 1200°C. W e have also measured the electrical conductivity of the equimolar mixture of 6Li2S04 and 7L i 2 S 0 4 .

Experimental

T w o calibrated quartz capillary cells2 with approxi- mately the same cell constants were placed in an air thermo- stat in a furnace. Since the electrical c o n d u c t i v i t y increases b y about 0 . 3 % per °C in cubic Li2S04 it is necessary t o keep the temperature difference between the t w o cells within at least 0.3 °C to get an accuracy o f 0 . 1 % . More than ten independent experiments were performed with small modifications.

The used salts contained 0.0, 50.0 and 95.5 mole %

6Li2S04 and were delivered b y Oak Ridge National L a b o - ratory. T h e y were used without further purification, b u t were very well dried before use. According t o analyses o f the salts (performed b y Oak Ridge National Laboratory) there was n o difference in the chemical composition of the different salts, but we have found that there was a small difference in the corrosion o f our quartz cells b y the 6Li2S04 and 7Li2S04 samples. 6Li2S04 did not even at the highest studied temperatures corrode the glass, while 7Li2S04 also at the melting point caused some corrosion o f the glass.

However, n o time dependence o f the electrical conductivity has been found and since the same c o n d u c t i v i t y results were obtained with increasing and decreasing temperatures, there is no reason to believe that the corrosion has influen- sed the conductivity difference between 6Li2S04 and

7L i2S 04.

The conductivities were measured with a commercial impedance bridge (Radiometer, Copenhagen, t y p e G B 11a)

Reprints request to Dr. A. KVIST, D e p a r t m e n t o f Physics, Chalmers University of Technology,, Falk, S-40220 Göte- borg 5, Sweden.

and the temperatures were measured with Platinel thermo- couples and a Potentiometrie microvoltmeter (Medistor, model A-75).

Results and discussions

Fig. 1 shows the relative conductivity differences be- tween 6Li2S04 and 7Li2S04, and also between the equi- molar mixture and 7Li2S04. The values for 6Li2S04 h a v e been obtained b y linear extrapolation of the results f o r 95.5 mole % enriched 6L i2S 0 4 .

A X de

Fig. 1. T h e relative difference in conductivity between

6Li2S04 and 7Li2S04 (a) and between the equimolar m i x - ture and 7Li2S04 (b). The dotted lines show the maximal

errors.

The results given in the figure are mean values f r o m a great number o f measurements. All measurements fall within the limits given in the figure.

I n the solid n o temperature dependence of the relative c o n d u c t i v i t y difference has been f o u n d and Ax/x = 4 . 7 3 % .

1 A . KVIST, Z. Naturforsch. 21a, 487 [1966].

2 A . KVIST, Z. Naturforsch. 22a, 208 [1967].

This value is somewhat higher than the results reported previously1, b u t the accuracy is somewhat better in this investigation. W e have n o w also used two c o n d u c t i v i t y cells with a v e r y small temperature difference in the same furnace a n d we have thus obtained conductivity values for the t w o cells at the same temperature.

F o r the melt there is a significant increase of Ax/x with temperature:

Axjx = [4.29 + 1.6 • 10-3 (^T _ 860)] (< ^{I N} ° Q . T h e temperature dependence of Ax/x for the melt is similiar to that of molten lithium nitrate and lithium chloride3.

The increase in the relative conductivity difference with temperature is probably due t o a weakening o f the coupling between the cations.

Since great deviations f r o m additivity for binary sulphate s y s t e m s2'4 , 5 have been f o u n d we have also measured the c o n d u c t i v i t y of the equimolar 6Li2S04-7Li2S04 mixture.

A s is seen in Fig. 1, there is within the experimental errors n o excess conductivity in this system.

This work was financially supported b y the Swedish Council for Applied Research.

3 S . J O R D A N , R . L E N K E, a n d A . K L E M M , Z . N a t u r f o r s c h . 2 3 a , 1 5 6 3 [ 1 9 6 8 ] .

4 A . K V I S T . Z . N a t u r f o r s c h . 2 1 A . 1 2 2 1 [ 1 9 6 6 ] .

5 A . K V I S T a n d K . S C H R O E D E R , Z . N a t u r f o r s c h . 2 3 a , 6 7 6 [ 1 9 6 8 ] ,

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