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POINT DEFECTS IN MISCELLANEOUS
SUBSTANCEPRE-TRANSFORMATION
PHENOMENA IN LITHIUM SULPHATE RICH
SULPHATE SYSTEMS AND IN SILVER IODIDE
C.-A. Sjöblom, J. Bowling, B. Heed, B.-E. Mellander, L. Nilsson, A. Lundén
To cite this version:
POINT DEFECTS I N MISCELLANEOUS SUBSTANCE.
PRE-TRANSFORMATION PHENOMENA IN LITHIUM SULPHATE
RICH SULPHATE SYSTEMS AND IN SILVER IODIDE
C.-A. SJOBLOM, J. E. BOWLING, B. HEED, B.-E. MELLANDER, L. NILSSON and A. LUNDEN
Physics Department, Chalmers University of Technology, S 402 20 Gothenburg 5, Sweden
RBsnmB. - fcc Li2S04 et bcc AgI sont quelques exemples de phases ayant une conductivitk klectrique trks grande. 11s sont un peu plastiques (thixotropiques) et ont une chaleur de transfor- mation qui surpasse la chaleur de fusion. On a trouvk que, dans un certain intervalle de tempera- ture, au-dessous de la phase de transformation, plusieurs propriktks physiques ont tendance ii devier des valeurs obtenues par extrapolation h partir de tempkratures plus basses. La capacitk thermique, l'expansion thermique et la conductivite klectrique ont 6tB suivies ii travers la phase de transition, pendant que les propriktks rhkologiques n'ont pu &tre Btudikes que dans la phase de temperature elevke. Les rksultats sont comparks avec les observations faites pr&demment, con- cernant la formation de phases plastiques pour quelques cristaux organiques et le prods de fusion pour diffkrents types de cristaux.
Abstract. - fcc Li2S04 and bcc AgI are examples of phases that have a very high electrical conductivity, are somewhat plastic (thixotropic) and have a heat of transformation that exceeds the heat of melting. It has been found that in a temperature range below the phase transition seve ral physical properties tend to deviate from the values obtained by extrapolating from lower tem- peratures. The heat capacity, thermal expansion and the electrical conductivity have been followed across the phase transition, while the rheological properties could be studied only for the high- temperature phase. The results are compared with previous observations concerning the forma- tion of plastic phases for some organic crystals and the melting process for various types of crystals.
1. Zntroduction. - The energy involved in the transformation between two solid phases is normally small compared with the latent heat of melting. There are, however, cases when the heat of transformation is just about equal to, or even considerably larger than, the heat of melting. In such cases the latter is rather small, and the sum of the latent heats for the solid- solid phase transformation and for melting tend to be about equal to a normal heat of melting.
As expected a large heat of transformation is combined with at least one physical property differing remarkably between the two solid phases, and the high-temperature phases is more liquid-like with respect to these properties. This general description holds for different classes of substances. One case is the fairly large group of organic plastic crystals of which camphor and carbon tetrachloride can be mentioned as typical examples. The properties of plastic crystals have been reviewed recently by Schmid and Wannagat [l]. Another case is that of solid electrolytes or superionic conductors (we prefer to use the first of these names) i. e. solid phases that have an ionic conductivity that otherwise is typical of ionic melts. (This discussion is of course restricted to those salts which undergo a sharp phase transition ; there are also compounds for which the electrical
conductivity gradually changes over a wide tempe- rature range [2].) Typical examples are bcc AgI and fcc Li,SO, with phase transitions at 146 O C and 575 O C , respectively, and with melting points at 560 O C and 860 OC, respectively. These solid electrolytes are actually also plastic crystals [3], although their plas- ticity is far lower than that of a typical organic plastic crystal. There are also other similarities between the inorganic and organic plastic crystals such as the way the heat capacity at constant pressure varies with temperature [4]. Typically the high-temperature phase is cubic while the non-plastic low-temperature phase has a lower symmetry, cf., however, below concern-
ing AgI.
Pre-melting phenomena have been observed for molecular, ionic and metallic crystals [5], i. e. some physical properties can have an anomalous behaviour in a temperature region below the melting point. Similar observations of pre-transformation have been reported for the organic plastic crystals mentioned above, for which properties that change drastically at the phase transition are known to have tail$ down
in the low-temperature phase [l].
Since we have studied for a long time physical properties of solid electrolyte phases (and phas mixtures), we decided t o see to what extent pre-
transformation can be detected for those properties that show a sharp change at the phase transition.
2. Rheological properties. - The appearance of a plastic phase is considered being part of the melting process, i. e. it is a typical pre-melting phenomenon shown by the crystal lattice. For silver iodide, lithium sulphate and some binary sulphate systems the rheo- logical properties have been investigated with a rota- tional viscometer [3] and the plastic phases were found to be thixotropic with a slow regeneration. (It was not possible to extend these studies below the phase transition as we have done with the other pro- perties discussed in this paper.) Thus the characteristic time is 24 hours for silver iodide at 5300C, and 8.4 hours for lithium sulphate at 781 OC. For lithium sulphate the activation volume for flow is 1.2 X 10-25 m3 almost independent of temperature
within the investigated interval (781-848 W), but it decreases to 0.8 X 10-25 m3 if more than 0.3 mole
K2S0, is added. The activation enthalpy for flow is 2.2 eV.
It is not surprising that the plasticity of solid electro- lytes was found to be considerably lower than that of organic plastic crystals. In the solid electrolyte phases discussed here only the anions are fixed to the lattice sites, while a large proportion of the cations is consi- dered to be mobile in the crystal.
3. Heat capacity. - The heat capacities and heats of transformation of lithium sulphate and some lithium sulphate-alkali sulphate mixtures have been measured with an adiabatic calorimeter. In pure Li2S0, the heat of transformation is 41 kJ/mole [4], which includes the heat of pre-transformation, i. e. the deviation of c, in the low-temperature region from the dashed line of figure 1.
The heat capacity is plotted versus temperature in figure 1, where data for reagent grade Li,S04 (Hopkin & Williams, Analar) are given. A similar curve is obtained for suprapure salt (Merck Suprapur). The dashed lines correspond to least-square-fits to an
100
l0
l0
equation suggested by Maier and Kelley 161. The existing deviation from this equation over a wide region below the melting point is typical of a transi- tion to a plastic phase in which molecular rotation is possible [l]. The tail in the c, curve below the transi- tion temperature is a pre-transformation phenomenon, showing that some molecular rotation may take place even before the crystal lattice has changed into the high-temperature modification.
The heat capacity of silver iodide has been studied thoroughly by Perrott and Fletcher [7]. Their c, curves have a lambda-shape both at 1460C and at 430 OC, i. e. at the transformation to the solid elec- trolyte (bcc phase) and at an order-disorder transition within this phase. They obtain similar results at 177 OC and 350 OC for silver sulphide [8]. For both of these compounds the heat capacity of the high temperature phase depends on whether the composition is stoichio- metric. The interpretation is discussed in detail by Perrott and Fletcher [7, 81. For our purpose it is sufficient to note that pre-transformation is detectable in the heat capacity data of these two silver compounds, both when the heat of transition is large and when it is moderate. However, the tail below the transition covers a shorter temperature range for the silver compounds than it does for the lithium sulphate
studied by us.
Differential thermal analysis (DTA) has shown that the sharpness of the phase transition for lithium sul- phate is very sensitive to minute traces of impurities (K. Schroeder, private communication). Thus if analytical grade Li,S04 is used the DTA curve has a rounded bend on the low-temperature side, but it has a sharp break if suprapure grade Li2S0, is used.
C x ~ o - ~ P - J l k g x K
Li,
SO,
-.-*-C.-*-*-.-.-.-
.-.-.A-.-. 2:-
--W-.-*---L.-)*-**- - l I I I I l I *4. Thermal expansion.
-
The thermal expansivity of silver iodide and of lithium sulphate mixtures has been studied in this laboratory by using a quartz push- rod apparatus [g, 101. The thermal expansivity and the heat capacity are connected through the Griineisen equation, and for bcc AgI the above-mentioned order- disorder transition at about 430 OC also shows clearly for the thermal expansion [9] (and in the rheological peasurements too [3]).Clear indications of pre-transformation are found in the monoclinic (low temperature) phase of lithium sulphate for thermal expansion. For suprapure Li2S0, a large number of measurements have been made, starting at room temperature. If a polynomial of the second grade is used to correlate the length increase
Aljl with the temperature, it is found that both the
coefficient of the t 2 term and the standard deviation increase gradually as the upper limit of the chosen temperature range is extended towards higher tempe- ratures. If the deviation from such a polynomial of the second grade is considered as due to pre-transfor- mation, this can be traced some 100 to 150 degrees below the transformation point, similar to the range for the heat capacity.
200 LOO 600 eoo *c
PRE-TRANSFORMATION IN LITHIUM SULPHATE AND SILVER IODIDE C7-461
High temperature X-ray is used to study the tempe- rature dependence of the lattice parameters a, b, and c and the angle
P
for the monoclinic phase of lithium sulphate. The calculated volume change AV/V is compared with that obtained from the measured Al/l. The two curves coincide within experimental error, but if there is any deviation between them, the one obtained from the X-ray data tends to be less depen- dent on the temperature, as is to be expected since the dilatometric expansion includes both changes in the lattice coefficients and an increase in the number of voids.5. Electrical conductivity. - Pre-melting effects have been observed by previous workers for the fcc phases of silver chloride and bromide [5], and Friauf has recently shown that all the observations regarding conductivity and diffusion in the region close below the melting point can be explained as being due to an anomalous rise in defect concentration caused by a general softening of the lattice [l I]. For silver iodide the situation is complicated by the fact that at ordinary pressures the salt consists of a mixture of stable 8-AgI and metastable y-AgI up to the transition to the bcc phase. However, at high pressures AgI also has the fcc structure at room temperature. We have studied the electrical conductivity of fcc AgI as a function of pressure and temperature 1121, and when passing from the fcc to the bcc phase at 4 and 5 kbar (at 129 OC and 157 OC, respectively), an excess increase in conductivity occurs, similar to the above- mentioned observations for AgI and AgBr.
The electrical conductivity of Li,S04 has been measured using different ac techniques in the interval 50-1 000 Hz. The sample has been a pressed pellet with integral electrode layers consisting of a fine mixture of Li2S04 and graphite or silver powder. Also a capillary cell with silver wire electrodes and filled with molten and later solidified Li2S0, has been used. The results show a change in conductivity at the phase transition at 575 OC of about two orders of magnitude. Most of this change happens in the immediate vicinity of the transition with a well defined upper limit at 575 oC. However, a clearly visible tail of gradually diminishing excess conductivity extends at least 30 OC down into the low temperature region, when reagent grade Li2S04 is used [13], while this tail is smaller when suprapure Li,S04 is used, figure 2.
L 00 500 6 0 0 7 O O 0 c log 1;fR I
1.5 1.4 1.3 1.2 1,1 1.0
1 0 0 0 1 T
FIG. 2. - Electrical conductance of lithium sulphate, reagent
grade (dotted line) and suprapure (solid line). The dashed lines are linear extrapolations.
6. Discussion. - Our studies of the temperature dependence of a number of physical properties in the temperature region below the transition to a solid- electrolyte phase show deviations that indicate that pre-transformation occurs for silver iodide as well as for lithium sulphate. Although for some properties the evidence is not clear, all the different observations support each other.
Up to the present date most studies of pre-trans- formation have been on pre-melting, concerning which there, however, is much confusion in the litera- ture [ 5 ] , since two-phase effects sometimes are included. Studies of plastic crystals by other workers [l], as well as ours of solid electrolytes indicate that homophase pre-transformation occurs frequently.
The differences found by us between suprapure and reagent grade lithium sulphate show that such effects can be enhanced by small traces of impurities. The fact that the anion is polyatomic for the sulphates might be of importance for existing differences bet- ween lithium sulphate and silver iodide.
Acknowledgments.
-
This investigation has been supported by the Swedish Natural Sciences Re- search Council, The Swedish Board of Technical Development and Car1 Tryggers Stiftelse for vetens- kaplig forskning.References
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[2] O'KEEFFE, M., Superionic Conductors, Ed. G . D. Mahan and J. Kubat.
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L61 MAIER, C. G. and KELLEY, K. K., J. ,4111. Chem. Soc. 54 [l01 JANSSON, B. and SJOBLOM, C.-A., Z. Naturforsch. 28a (1973)
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DISCUSSION
