Ionic conductivity of molten GdCl3NaCl and GdCl3KCl systems

ELSEVIER

Journal of Alloys and Compounds 245 (1906) 66-69

Ionic conductivity

of molten GdCl,-NaCi K. Fukushima,

M. Hayakawa,

and GdCl,-KC1

systems

Y. Iwadate*

Abstracd Electrical conductivity K values of the molten GdCl,-NaCI and GdCI,-KCI systems were measured by a conventional a.c. technique. The molar conductivity A values were evaluated using the data of conductivity and molar volume. The it values of molten GdCl 1suggested that the existence of dimeric or more polymeric complex anions might be presumed in the melt. as was the case in a series of molten rare earth chlorides. Additionaily. the A values appeared to be affected qualitatively by the cationic radii and the forms of linkage, that is corner- or edge-sharing of octahedral units. Kemork

Gdolinium

chloride: Alkali

chloride: Melt: Molar

conductivity

1. Introduction Since conductivity reflects the average dynamical structure of a molten salt, it has been a very objective and reliable indication for better understanding of transport phenomena of ionic melts, especially for the melts containing trivalent ions, as well as of structural features. As a sample melt of trivalent ions resulting in a huge coulomb interaction, scientific attention and interest have recently been focused on rare earth trichloride melts. There have been several sets of data on the electrical conductivity of molten rare earth trichlorides (RCl,) [l-8]. The series of molar conductivity of rare earth chlorides suggested that the octahedral complex anion RCIi- exists in the melts; this was examined by means of X-ray diffraction [9111, neutron diffraction 1121and Raman spectroscopy [13-151. In contrast, the solids of rare earth chlorides can be classified largely into hexagonal and monoclinic structures 1161. It was reported that dimers with corner-sharing of octahedral units existed in molten LaCI,, CeCl,, FrCl,, and NdCl, [9,10] which possess the former type of structure: those with edge-sharing in molten YCI, [12], ErCl, [ll] and DyCl, [17] belong to the latter type in the solid state. The results so far

examined indicated that the A values of the hexagonal type of solid were larger than those of monoclinic type, and the order of the values was the same as the

-*Correspondingauthor. oY25-838%/%/$15.000 19% ElscvierScience S.A. All rights reserved PI1 SO925-%388(96)02485-l

cationic radii. As for GdCl,. whit! has the smallest cationic radius in hexagonal-type rare earth chlorides, the structure has been reported in Refs. [14,18-201 as having the octahedral complex anions, GdC1’:-, as the basic structural units, and as forming some clusters of distorted corner-sharing octahedral units in the melt. In this work the conductivities of molten GdCl,-NaCl and GdCI,-KCl systems were measured and the molar conductivities were discussed and compared with those of the other molten rare earth chlorides.

2. Experimental

GdCI, was prepared by heating a mixture of high purity Gd,O, and 2.5 times the theoretical amount of NH,CI at 623 K for 3 h. Actually 5Og of GdCI, and 110 g of NH,Cl were used for the reaction. Excess NH,Cl was removed by heating at 1173 K. GdCl, thus obtained was purified by sublimation at 1273 K under reduced pressure, so as to remove small amounts of water, oxychloride, unreacted NH,CI and oxide [4]. The chemicals NaCl, KC1 and KNO, of analytical reagent grade were dried under reduced pressure at about SOK below the respective melting temperatures for 8 h. The electrical conductivities of molten GdCI,NaCl and GdCl,-KCI systems were .neasured as functions of mole fraction of GdCI, and temperature by a conventional a-c. technique. The polarization-free resistance of the melt was attained by varying the

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molten GdCI,-NaCI and GdCI,-KCI systems were almost linear. the ‘4 values were parameterized into the conventional Arrhenius-type equation /I = Aexp(-E,,IRT)

(3)

The resuhs of the frequency factor A and the activation energy E,, are listed in Table 3. ,4 values increased with increasing temperature (see Fig. 1). Fig.

Fig. 1. Molar GdCI,~-NaCI tern (GdCI,

(f)

62%

(GdCI, (f’)

conductivity (m)

variation

and GdCI,-KCI

(0)

with

rempcrarurc

2 shows the molar conductivity isotherms at 10% K. .9 values of the molten GdCI,-NaCl and GdCI,-KC1 systems increased steeply with decreasing GdCl, concentration in the range of .r ~0.25. These results suggest that for low concentrations of GdCI,, the formation of dimeric or more polymeric complex anions composed of GdCli--type octahedra as structural units might be restrained, and discrete GdCIzmight increase as described in the X-ray diffraction I18.191 and Raman spectroscopic studies of the GdCl,-NaCI and GdCI,-KC1 systems [14,20]. Also, the concentrations of the free Na’, K’ and Cl-, which were not ligands of the Gd-complex, increased with decreasing GdCl, concentration; consequently, the conductivities of the systems tended to rise. These trends have been observed for the other rare earth chloride systems [3-71. Matsuura et al. 1261reported that the internal mobilities of K’ in the molten DyCI,-KCI system decrease with increasing DyCI, concentration. A similar phenomenon might also occur in the molten GdCI,-KCI system. The A value of molten GdCI, was 2.110X 10-j S m’mol-’ at I150 K. As shown in Fig. 3. this value was situated between NdCI, and YCI, in a series of molten rare earth chlorides. The data for LaCI, [S], CeCI, 111. PrCI, 131, NdCl, [4]. YCI, 161, and ErCI, 171 were taken from the literature. -Gda, has the s:?me hexa-

in molten

systems. GdCI,-NaCI

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equations in Table 3.

Fig. 3. Molar conductivily at I150 K for pure molten salts.

gonat structure in the solid skate as the above first four chlorides, and :he corner-shared dimeric species of octahedral units misled in these mehs. However. the YCI, and ErCl, solids possess the monuclinic structure. different from GdCI, in the solid state. and the edge-shared species occurred on melting. The results of this work support the previous suggestion [7] that the A values are affected qualitatively by the cationic radii and the types of linkage Considering Ref. 1271. in which the main cationic electrically conducting species wem conjectured to he non-associatedtrivalent cations in ell the rare earth chloride melts, although their lifetimes are presumably very short. it may be reasonable to suppose that corner-shared octahedral units exist more than edge-shared units in these systems: thus. the electrically conducting species arc not necessarily the same as the species existing in the molten mixtures. 4 Conclusions In a series of mohen rare earth chlorides with or without alkali metal chlorides, the mo!ar conductivities are affected qualitatively hy the cationic radii and the short-range ordering around the cations. In the GdCI,-NaCI and GdCI,I-KCI systems. the molar condudivities decreased steeply with increasing GdCI,\ concentration in the range I
References II

1H.R. Brunslem. fJ 1lYh2) 44.

AS. Dwukin

and M.A. BrediF. J PIws. Clwn..